[FEATURE] Add QgsFeatureListComboBox with live-filter-capabilities

python/core/core_auto.sip

@@ -311,6 +311,7 @@
 %Include qgsfieldmodel.sip
 %Include qgsfieldproxymodel.sip
 %Include qgsfiledownloader.sip
+%Include qgsfeaturefiltermodel.sip
 %Include qgsgeometryvalidator.sip
 %Include qgsgml.sip
 %Include qgsgmlschema.sip

python/core/qgsfeaturefiltermodel.sip

@@ -0,0 +1,118 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/qgsfeaturefiltermodel.h                                     *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+
+
+class QgsFeatureFilterModel : QAbstractItemModel
+{
+%Docstring
+ Provides a list of features based on filter conditions.
+ Features are fetched asynchronously.
+%End
+
+%TypeHeaderCode
+#include "qgsfeaturefiltermodel.h"
+%End
+  public:
+    enum Role
+    {
+      IdentifierValueRole,
+      ValueRole
+    };
+
+    QgsFeatureFilterModel( QObject *parent = 0 );
+    ~QgsFeatureFilterModel();
+
+    QgsVectorLayer *sourceLayer() const;
+%Docstring
+ :rtype: QgsVectorLayer
+%End
+    void setSourceLayer( QgsVectorLayer *sourceLayer );
+
+    QString displayExpression() const;
+%Docstring
+ :rtype: str
+%End
+    void setDisplayExpression( const QString &displayExpression );
+
+    QString filterValue() const;
+%Docstring
+ :rtype: str
+%End
+    void setFilterValue( const QString &filterValue );
+
+    virtual QModelIndex index( int row, int column, const QModelIndex &parent ) const;
+    virtual QModelIndex parent( const QModelIndex &child ) const;
+    virtual int rowCount( const QModelIndex &parent ) const;
+    virtual int columnCount( const QModelIndex &parent ) const;
+    virtual QVariant data( const QModelIndex &index, int role ) const;
+
+    QString filterExpression() const;
+%Docstring
+ An additional filter expression to apply, next to the filterValue.
+ Can be used for spatial filtering etc.
+ :rtype: str
+%End
+
+    void setFilterExpression( const QString &filterExpression );
+%Docstring
+ An additional filter expression to apply, next to the filterValue.
+ Can be used for spatial filtering etc.
+%End
+
+    bool isLoading() const;
+%Docstring
+ :rtype: bool
+%End
+
+    QString identifierField() const;
+%Docstring
+ :rtype: str
+%End
+    void setIdentifierField( const QString &identifierField );
+
+    QVariant extraIdentifierValue() const;
+%Docstring
+ :rtype: QVariant
+%End
+    void setExtraIdentifierValue( const QVariant &extraIdentifierValue );
+
+    int extraIdentifierValueIndex() const;
+%Docstring
+ :rtype: int
+%End
+
+    bool extraValueDoesNotExist() const;
+%Docstring
+ :rtype: bool
+%End
+
+  signals:
+    void sourceLayerChanged();
+    void displayExpressionChanged();
+    void filterValueChanged();
+    void filterExpressionChanged();
+    void isLoadingChanged();
+    void identifierFieldChanged();
+    void filterJobCompleted();
+    void extraIdentifierValueChanged();
+    void extraIdentifierValueIndexChanged( int index );
+    void extraValueDoesNotExistChanged();
+    void beginUpdate();
+    void endUpdate();
+
+};
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/qgsfeaturefiltermodel.h                                     *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

python/gui/gui_auto.sip

@@ -108,6 +108,7 @@
 %Include qgsfeatureselectiondlg.sip
 %Include qgsfieldcombobox.sip
 %Include qgsfieldexpressionwidget.sip
+%Include qgsfeaturelistcombobox.sip
 %Include qgsfieldvalidator.sip
 %Include qgsfieldvalueslineedit.sip
 %Include qgsfilewidget.sip

python/gui/qgsfeaturelistcombobox.sip

@@ -0,0 +1,104 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/gui/qgsfeaturelistcombobox.h                                     *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+
+
+class QgsFeatureListComboBox : QComboBox
+{
+%Docstring
+*************************************************************************
+qgsfieldlistcombobox.h - QgsFieldListComboBox
+
+---------------------
+begin                : 10.3.2017
+copyright            : (C) 2017 by Matthias Kuhn
+email                : matthias@opengis.ch
+**************************************************************************
+                                                                         *
+   This program is free software; you can redistribute it and/or modify  *
+   it under the terms of the GNU General Public License as published by  *
+   the Free Software Foundation; either version 2 of the License, or     *
+   (at your option) any later version.                                   *
+                                                                         *
+**************************************************************************
+%End
+
+%TypeHeaderCode
+#include "qgsfeaturelistcombobox.h"
+%End
+  public:
+    QgsFeatureListComboBox( QWidget *parent = 0 );
+
+    QgsVectorLayer *sourceLayer() const;
+%Docstring
+ :rtype: QgsVectorLayer
+%End
+    void setSourceLayer( QgsVectorLayer *sourceLayer );
+
+    QString displayExpression() const;
+%Docstring
+ :rtype: str
+%End
+    void setDisplayExpression( const QString &displayExpression );
+
+    QString filterExpression() const;
+%Docstring
+ :rtype: str
+%End
+    void setFilterExpression( const QString &filterExpression );
+
+    QVariant identifierValue() const;
+%Docstring
+ :rtype: QVariant
+%End
+    void setIdentifierValue( const QVariant &identifierValue );
+
+    QgsFeatureRequest currentFeatureRequest() const;
+%Docstring
+ :rtype: QgsFeatureRequest
+%End
+
+    bool allowNull() const;
+%Docstring
+ :rtype: bool
+%End
+    void setAllowNull( bool allowNull );
+
+    QString identifierField() const;
+%Docstring
+ :rtype: str
+%End
+    void setIdentifierField( const QString &identifierField );
+
+    QModelIndex currentModelIndex() const;
+%Docstring
+ :rtype: QModelIndex
+%End
+
+    virtual void focusOutEvent( QFocusEvent *event );
+
+    virtual void keyPressEvent( QKeyEvent *event );
+
+  signals:
+    void sourceLayerChanged();
+    void displayExpressionChanged();
+    void filterExpressionChanged();
+    void identifierValueChanged();
+    void identifierFieldChanged();
+    void allowNullChanged();
+
+};
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/gui/qgsfeaturelistcombobox.h                                     *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

src/core/CMakeLists.txt

@@ -175,6 +175,7 @@ SET(QGIS_CORE_SRCS
   qgsfeaturesink.cpp
   qgsfeaturesource.cpp
   qgsfeaturestore.cpp
+  qgsfeaturefiltermodel.cpp
   qgsfield.cpp
   qgsfieldconstraints.cpp
   qgsfieldformatter.cpp
@@ -590,6 +591,8 @@ SET(QGIS_CORE_MOC_HDRS
   qgsfieldmodel.h
   qgsfieldproxymodel.h
   qgsfiledownloader.h
+  qgsfeaturefiltermodel.h
+  qgsfeaturefiltermodel_p.h
   qgsgeometryvalidator.h
   qgsgml.h
   qgsgmlschema.h

src/core/qgsfeaturefiltermodel.cpp

@@ -0,0 +1,412 @@
+/***************************************************************************
+  qgsfeaturefiltermodel.cpp - QgsFeatureFilterModel
+
+ ---------------------
+ begin                : 10.3.2017
+ copyright            : (C) 2017 by Matthias Kuhn
+ email                : matthias@opengis.ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#include "qgsfeaturefiltermodel.h"
+#include "qgsfeaturefiltermodel_p.h"
+
+QgsFeatureFilterModel::QgsFeatureFilterModel( QObject *parent )
+  : QAbstractItemModel( parent )
+  , mSourceLayer( nullptr )
+{
+  mReloadTimer.setInterval( 100 );
+  mReloadTimer.setSingleShot( true );
+  connect( &mReloadTimer, &QTimer::timeout, this, &QgsFeatureFilterModel::scheduledReload );
+  setExtraIdentifierValueUnguarded( QVariant() );
+}
+
+QgsFeatureFilterModel::~QgsFeatureFilterModel()
+{
+  if ( mGatherer )
+    connect( mGatherer, &QgsFieldExpressionValuesGatherer::finished, mGatherer, &QgsFieldExpressionValuesGatherer::deleteLater );
+}
+
+QgsVectorLayer *QgsFeatureFilterModel::sourceLayer() const
+{
+  return mSourceLayer;
+}
+
+void QgsFeatureFilterModel::setSourceLayer( QgsVectorLayer *sourceLayer )
+{
+  if ( mSourceLayer == sourceLayer )
+    return;
+
+  mSourceLayer = sourceLayer;
+  reload();
+  emit sourceLayerChanged();
+}
+
+QString QgsFeatureFilterModel::displayExpression() const
+{
+  return mDisplayExpression;
+}
+
+void QgsFeatureFilterModel::setDisplayExpression( const QString &displayExpression )
+{
+  if ( mDisplayExpression == displayExpression )
+    return;
+
+  mDisplayExpression = displayExpression;
+  reload();
+  emit displayExpressionChanged();
+}
+
+QString QgsFeatureFilterModel::filterValue() const
+{
+  return mFilterValue;
+}
+
+void QgsFeatureFilterModel::setFilterValue( const QString &filterValue )
+{
+  if ( mFilterValue == filterValue )
+    return;
+
+  mFilterValue = filterValue;
+  reload();
+  emit filterValueChanged();
+}
+
+QString QgsFeatureFilterModel::filterExpression() const
+{
+  return mFilterExpression;
+}
+
+void QgsFeatureFilterModel::setFilterExpression( const QString &filterExpression )
+{
+  if ( mFilterExpression == filterExpression )
+    return;
+
+  mFilterExpression = filterExpression;
+  reload();
+  emit filterExpressionChanged();
+}
+
+bool QgsFeatureFilterModel::isLoading() const
+{
+  return mGatherer;
+}
+
+QModelIndex QgsFeatureFilterModel::index( int row, int column, const QModelIndex &parent ) const
+{
+  Q_UNUSED( parent )
+  return createIndex( row, column, nullptr );
+}
+
+QModelIndex QgsFeatureFilterModel::parent( const QModelIndex &child ) const
+{
+  Q_UNUSED( child )
+  return QModelIndex();
+}
+
+int QgsFeatureFilterModel::rowCount( const QModelIndex &parent ) const
+{
+  Q_UNUSED( parent );
+
+  return mEntries.size();
+}
+
+int QgsFeatureFilterModel::columnCount( const QModelIndex &parent ) const
+{
+  Q_UNUSED( parent )
+  return 1;
+}
+
+QVariant QgsFeatureFilterModel::data( const QModelIndex &index, int role ) const
+{
+  if ( !index.isValid() )
+    return QVariant();
+
+  switch ( role )
+  {
+    case Qt::DisplayRole:
+    case Qt::EditRole:
+    case ValueRole:
+      return mEntries.value( index.row() ).value;
+
+    case IdentifierValueRole:
+      return mEntries.value( index.row() ).identifierValue;
+  }
+
+  return QVariant();
+}
+
+void QgsFeatureFilterModel::updateCompleter()
+{
+  emit beginUpdate();
+  QVector<Entry> entries = mGatherer->entries();
+
+  if ( mExtraIdentifierValueIndex == -1 )
+    setExtraIdentifierValueUnguarded( QVariant() );
+
+  // Only reloading the current entry?
+  if ( mGatherer->data().toBool() )
+  {
+    if ( !entries.isEmpty() )
+    {
+      mEntries.replace( mExtraIdentifierValueIndex, entries.at( 0 ) );
+      emit dataChanged( index( mExtraIdentifierValueIndex, 0, QModelIndex() ), index( mExtraIdentifierValueIndex, 0, QModelIndex() ) );
+      mShouldReloadCurrentFeature = false;
+      setExtraValueDoesNotExist( false );
+    }
+    else
+    {
+      setExtraValueDoesNotExist( true );
+    }
+
+    mShouldReloadCurrentFeature = false;
+  }
+  else
+  {
+    // We got strings for a filter selection
+    std::sort( entries.begin(), entries.end(), []( const Entry & a, const Entry & b ) { return a.value.localeAwareCompare( b.value ) < 0; } );
+
+    int newEntriesSize = entries.size();
+
+    // Find the index of the extra entry in the new list
+    int currentEntryInNewList = -1;
+    if ( mExtraIdentifierValueIndex != -1 )
+    {
+      for ( int i = 0; i < newEntriesSize; ++i )
+      {
+        if ( entries.at( i ).identifierValue == mExtraIdentifierValue )
+        {
+          currentEntryInNewList = i;
+          mEntries.replace( mExtraIdentifierValueIndex, entries.at( i ) );
+          emit dataChanged( index( mExtraIdentifierValueIndex, 0, QModelIndex() ), index( mExtraIdentifierValueIndex, 0, QModelIndex() ) );
+          setExtraValueDoesNotExist( false );
+          break;
+        }
+      }
+    }
+    else
+    {
+      Q_ASSERT_X( false, "QgsFeatureFilterModel::updateCompleter", "No extra identifier value generated. Should not get here." );
+    }
+
+    int firstRow = 0;
+
+    // Move the extra entry to the first position
+    if ( mExtraIdentifierValueIndex != -1 )
+    {
+      if ( mExtraIdentifierValueIndex != 0 )
+      {
+        beginMoveRows( QModelIndex(), mExtraIdentifierValueIndex, mExtraIdentifierValueIndex, QModelIndex(), 0 );
+        mEntries.move( mExtraIdentifierValueIndex, 0 );
+        endMoveRows();
+      }
+      firstRow = 1;
+    }
+
+    // Remove all entries (except for extra entry if existant)
+    beginRemoveRows( QModelIndex(), firstRow, mEntries.size() - firstRow );
+    mEntries.remove( firstRow, mEntries.size() - firstRow );
+    endRemoveRows();
+
+    if ( currentEntryInNewList == -1 )
+    {
+      beginInsertRows( QModelIndex(), 1, entries.size() + 1 );
+      mEntries += entries;
+      endInsertRows();
+      setExtraIdentifierValueIndex( 0 );
+    }
+    else
+    {
+      if ( currentEntryInNewList != 0 )
+      {
+        beginInsertRows( QModelIndex(), 0, currentEntryInNewList - 1 );
+        mEntries = entries.mid( 0, currentEntryInNewList );
+        endInsertRows();
+      }
+      else
+      {
+        mEntries.replace( 0, entries.at( 0 ) );
+      }
+
+      emit dataChanged( index( currentEntryInNewList, 0, QModelIndex() ), index( currentEntryInNewList, 0, QModelIndex() ) );
+
+      beginInsertRows( QModelIndex(), currentEntryInNewList + 1, newEntriesSize - currentEntryInNewList - 1 );
+      mEntries += entries.mid( currentEntryInNewList + 1 );
+      endInsertRows();
+      setExtraIdentifierValueIndex( currentEntryInNewList );
+    }
+
+    emit filterJobCompleted();
+  }
+  emit endUpdate();
+}
+
+void QgsFeatureFilterModel::gathererThreadFinished()
+{
+  delete mGatherer;
+  mGatherer = nullptr;
+  emit isLoadingChanged();
+}
+
+void QgsFeatureFilterModel::scheduledReload()
+{
+  if ( !mSourceLayer )
+    return;
+
+  bool wasLoading = false;
+
+  if ( mGatherer )
+  {
+    // Send the gatherer thread to the graveyard:
+    //   forget about it, tell it to stop and delete when finished
+    disconnect( mGatherer, &QgsFieldExpressionValuesGatherer::collectedValues, this, &QgsFeatureFilterModel::updateCompleter );
+    disconnect( mGatherer, &QgsFieldExpressionValuesGatherer::finished, this, &QgsFeatureFilterModel::gathererThreadFinished );
+    connect( mGatherer, &QgsFieldExpressionValuesGatherer::finished, mGatherer, &QgsFieldExpressionValuesGatherer::deleteLater );
+    mGatherer->stop();
+    wasLoading = true;
+  }
+
+  QgsFeatureRequest request;
+
+  if ( mShouldReloadCurrentFeature )
+  {
+    request.setFilterExpression( QStringLiteral( "%1 = %2" ).arg( QgsExpression::quotedColumnRef( mIdentifierField ), QgsExpression::quotedValue( mExtraIdentifierValue ) ) );
+  }
+  else
+  {
+    QString filterClause;
+
+    if ( mFilterValue.isEmpty() && !mFilterExpression.isEmpty() )
+      filterClause = mFilterExpression;
+    else if ( mFilterExpression.isEmpty() && !mFilterValue.isEmpty() )
+      filterClause = QStringLiteral( "(%1) ILIKE '\%%2\%'" ).arg( mDisplayExpression, mFilterValue );
+    else if ( !mFilterExpression.isEmpty() && !mFilterValue.isEmpty() )
+      filterClause = QStringLiteral( "(%1) AND ((%2) ILIKE '\%%3\%')" ).arg( mFilterExpression, mDisplayExpression, mFilterValue );
+
+    request.setFilterExpression( filterClause );
+  }
+
+  request.setLimit( 100 );
+
+  mGatherer = new QgsFieldExpressionValuesGatherer( mSourceLayer, mDisplayExpression, mIdentifierField, request );
+  mGatherer->setData( mShouldReloadCurrentFeature );
+
+  connect( mGatherer, &QgsFieldExpressionValuesGatherer::collectedValues, this, &QgsFeatureFilterModel::updateCompleter );
+  connect( mGatherer, &QgsFieldExpressionValuesGatherer::finished, this, &QgsFeatureFilterModel::gathererThreadFinished );
+
+  mGatherer->start();
+  if ( !wasLoading )
+    emit isLoadingChanged();
+}
+
+void QgsFeatureFilterModel::setExtraIdentifierValueIndex( int index )
+{
+  if ( mExtraIdentifierValueIndex == index )
+    return;
+
+  mExtraIdentifierValueIndex = index;
+  emit extraIdentifierValueIndexChanged( index );
+}
+
+void QgsFeatureFilterModel::reloadCurrentFeature()
+{
+  mShouldReloadCurrentFeature = true;
+  mReloadTimer.start();
+}
+
+void QgsFeatureFilterModel::setExtraIdentifierValueUnguarded( const QVariant &extraIdentifierValue )
+{
+  const QVector<Entry> entries = mEntries;
+
+  int index = 0;
+  for ( const Entry &entry : entries )
+  {
+    if ( entry.identifierValue == extraIdentifierValue )
+    {
+      setExtraIdentifierValueIndex( index );
+      break;
+    }
+
+    index++;
+  }
+
+  // Value not found in current entries
+  if ( mExtraIdentifierValueIndex != index )
+  {
+    beginInsertRows( QModelIndex(), 0, 0 );
+    mEntries.prepend( Entry( extraIdentifierValue, QStringLiteral( "(%1)" ).arg( extraIdentifierValue.toString() ) ) );
+    endInsertRows();
+    setExtraIdentifierValueIndex( 0 );
+
+    reloadCurrentFeature();
+  }
+}
+
+bool QgsFeatureFilterModel::extraValueDoesNotExist() const
+{
+  return mExtraValueDoesNotExist;
+}
+
+void QgsFeatureFilterModel::setExtraValueDoesNotExist( bool extraValueDoesNotExist )
+{
+  if ( mExtraValueDoesNotExist == extraValueDoesNotExist )
+    return;
+
+  mExtraValueDoesNotExist = extraValueDoesNotExist;
+  emit extraValueDoesNotExistChanged();
+}
+
+int QgsFeatureFilterModel::extraIdentifierValueIndex() const
+{
+  return mExtraIdentifierValueIndex;
+}
+
+QString QgsFeatureFilterModel::identifierField() const
+{
+  return mIdentifierField;
+}
+
+void QgsFeatureFilterModel::setIdentifierField( const QString &identifierField )
+{
+  if ( mIdentifierField == identifierField )
+    return;
+
+  mIdentifierField = identifierField;
+  emit identifierFieldChanged();
+}
+
+void QgsFeatureFilterModel::reload()
+{
+  mReloadTimer.start();
+}
+
+QVariant QgsFeatureFilterModel::extraIdentifierValue() const
+{
+  return mExtraIdentifierValue;
+}
+
+void QgsFeatureFilterModel::setExtraIdentifierValue( const QVariant &extraIdentifierValue )
+{
+  if ( extraIdentifierValue == mExtraIdentifierValue && extraIdentifierValue.isNull() == mExtraIdentifierValue.isNull() )
+    return;
+
+  setExtraIdentifierValueUnguarded( extraIdentifierValue );
+
+  mExtraIdentifierValue = extraIdentifierValue;
+  emit extraIdentifierValueChanged();
+}
+
+QVariant QgsFieldExpressionValuesGatherer::data() const
+{
+  return mData;
+}
+
+void QgsFieldExpressionValuesGatherer::setData( const QVariant &data )
+{
+  mData = data;
+}

src/core/qgsfeaturefiltermodel.h

@@ -0,0 +1,163 @@
+/***************************************************************************
+  qgsfeaturefiltermodel.h - QgsFeatureFilterModel
+
+ ---------------------
+ begin                : 10.3.2017
+ copyright            : (C) 2017 by Matthias Kuhn
+ email                : matthias@opengis.ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#ifndef QGSFEATUREFILTERMODEL_H
+#define QGSFEATUREFILTERMODEL_H
+
+#include <QAbstractItemModel>
+
+#include "qgsvectorlayer.h"
+
+class QgsFieldExpressionValuesGatherer;
+
+/**
+ * Provides a list of features based on filter conditions.
+ * Features are fetched asynchronously.
+ */
+class CORE_EXPORT QgsFeatureFilterModel : public QAbstractItemModel
+{
+    Q_OBJECT
+
+    Q_PROPERTY( QgsVectorLayer *sourceLayer READ sourceLayer WRITE setSourceLayer NOTIFY sourceLayerChanged )
+    Q_PROPERTY( QString displayExpression READ displayExpression WRITE setDisplayExpression NOTIFY displayExpressionChanged )
+    Q_PROPERTY( QString filterValue READ filterValue WRITE setFilterValue NOTIFY filterValueChanged )
+    Q_PROPERTY( QString filterExpression READ filterExpression WRITE setFilterExpression NOTIFY filterExpressionChanged )
+    Q_PROPERTY( bool isLoading READ isLoading NOTIFY isLoadingChanged )
+
+    /**
+     * A field of sourceLayer that is unique and should be used to identify features.
+     * Normally the primary key field.
+     * Needs to match the identifierValue.
+     */
+    Q_PROPERTY( QString identifierField READ identifierField WRITE setIdentifierField NOTIFY identifierFieldChanged )
+
+    /**
+     * The value that identifies the current feature.
+     */
+    Q_PROPERTY( QVariant extraIdentifierValue READ extraIdentifierValue WRITE setExtraIdentifierValue NOTIFY extraIdentifierValueChanged )
+
+    Q_PROPERTY( int extraIdentifierValueIndex READ extraIdentifierValueIndex NOTIFY extraIdentifierValueIndexChanged )
+
+  public:
+    enum Role
+    {
+      IdentifierValueRole = Qt::UserRole,
+      ValueRole
+    };
+
+    QgsFeatureFilterModel( QObject *parent = nullptr );
+    ~QgsFeatureFilterModel();
+
+    QgsVectorLayer *sourceLayer() const;
+    void setSourceLayer( QgsVectorLayer *sourceLayer );
+
+    QString displayExpression() const;
+    void setDisplayExpression( const QString &displayExpression );
+
+    QString filterValue() const;
+    void setFilterValue( const QString &filterValue );
+
+    virtual QModelIndex index( int row, int column, const QModelIndex &parent ) const override;
+    virtual QModelIndex parent( const QModelIndex &child ) const override;
+    virtual int rowCount( const QModelIndex &parent ) const override;
+    virtual int columnCount( const QModelIndex &parent ) const override;
+    virtual QVariant data( const QModelIndex &index, int role ) const override;
+
+    /**
+     * An additional filter expression to apply, next to the filterValue.
+     * Can be used for spatial filtering etc.
+     */
+    QString filterExpression() const;
+
+    /**
+     * An additional filter expression to apply, next to the filterValue.
+     * Can be used for spatial filtering etc.
+     */
+    void setFilterExpression( const QString &filterExpression );
+
+    bool isLoading() const;
+
+    QString identifierField() const;
+    void setIdentifierField( const QString &identifierField );
+
+    QVariant extraIdentifierValue() const;
+    void setExtraIdentifierValue( const QVariant &extraIdentifierValue );
+
+    int extraIdentifierValueIndex() const;
+
+    bool extraValueDoesNotExist() const;
+
+  signals:
+    void sourceLayerChanged();
+    void displayExpressionChanged();
+    void filterValueChanged();
+    void filterExpressionChanged();
+    void isLoadingChanged();
+    void identifierFieldChanged();
+    void filterJobCompleted();
+    void extraIdentifierValueChanged();
+    void extraIdentifierValueIndexChanged( int index );
+    void extraValueDoesNotExistChanged();
+    void beginUpdate();
+    void endUpdate();
+
+  private slots:
+    void updateCompleter();
+    void gathererThreadFinished();
+    void scheduledReload();
+
+  private:
+    void setExtraIdentifierValueIndex( int index );
+    void setExtraValueDoesNotExist( bool extraValueDoesNotExist );
+    void reload();
+    void reloadCurrentFeature();
+    void setExtraIdentifierValueUnguarded( const QVariant &extraIdentifierValue );
+    struct Entry
+    {
+      Entry()
+      {}
+
+      Entry( QVariant _identifierValue, const QString &_value )
+        : identifierValue( _identifierValue )
+        , value( _value )
+      {}
+
+      QVariant identifierValue;
+      QString value;
+
+      bool operator()( const Entry &lhs, const Entry &rhs ) const;
+    };
+
+    QgsVectorLayer *mSourceLayer;
+    QString mDisplayExpression;
+    QString mFilterValue;
+    QString mFilterExpression;
+
+    QVector<Entry> mEntries;
+    QgsFieldExpressionValuesGatherer *mGatherer = nullptr;
+    QTimer mReloadTimer;
+    bool mShouldReloadCurrentFeature;
+    bool mExtraValueDoesNotExist = false;
+
+    QString mIdentifierField;
+
+    QVariant mExtraIdentifierValue;
+
+    int mExtraIdentifierValueIndex = -1;
+
+    friend class QgsFieldExpressionValuesGatherer;
+};
+
+#endif // QGSFEATUREFILTERMODEL_H

src/core/qgsfeaturefiltermodel_p.h

@@ -0,0 +1,131 @@
+/***************************************************************************
+  qgsfeaturefiltermodel_p - QgsFieldExpressionValuesGatherer
+
+ ---------------------
+ begin                : 10.3.2017
+ copyright            : (C) 2017 by Matthias Kuhn
+ email                : matthias@opengis.ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#ifndef QGSFEATUREFILTERMODEL_P_H
+#define QGSFEATUREFILTERMODEL_P_H
+
+#include <QThread>
+#include "qgsvectorlayer.h"
+#include "qgsfeaturefiltermodel.h"
+#include "qgslogger.h"
+#include "qgsvectorlayerfeatureiterator.h"
+
+#define SIP_NO_FILE
+
+// just internal guff - definitely not for exposing to public API!
+///@cond PRIVATE
+
+/**
+ * \class QgsFieldExpressionValuesGatherer
+ * Gathers features with substring matching on an expression.
+ *
+ * \since QGIS 3.0
+ */
+class QgsFieldExpressionValuesGatherer: public QThread
+{
+    Q_OBJECT
+
+  public:
+    QgsFieldExpressionValuesGatherer( QgsVectorLayer *layer, const QString &displayExpression, const QString &identifierField, const QgsFeatureRequest &request = QgsFeatureRequest() )
+      : mSource( new QgsVectorLayerFeatureSource( layer ) )
+      , mDisplayExpression( displayExpression )
+      , mRequest( request )
+      , mWasCanceled( false )
+      , mIdentifierField( identifierField )
+    {
+    }
+
+    ~QgsFieldExpressionValuesGatherer()
+    {
+    }
+
+    virtual void run() override
+    {
+      mWasCanceled = false;
+
+      mIterator = mSource->getFeatures( mRequest );
+
+      QgsDebugMsg( QStringLiteral( "New gatherer: %1" ).arg( mRequest.filterExpression()->expression() ) );
+      mDisplayExpression.prepare( &mExpressionContext );
+
+      QgsFeature feat;
+      int attribute = mSource->fields().indexOf( mIdentifierField );
+
+      while ( mIterator.nextFeature( feat ) )
+      {
+        mExpressionContext.setFeature( feat );
+        mEntries.append( QgsFeatureFilterModel::Entry( feat.attribute( attribute ), mDisplayExpression.evaluate( &mExpressionContext ).toString() ) );
+
+        if ( mWasCanceled )
+          return;
+      }
+
+      emit collectedValues();
+    }
+
+    //! Informs the gatherer to immediately stop collecting values
+    void stop()
+    {
+      mWasCanceled = true;
+    }
+
+    //! Returns true if collection was canceled before completion
+    bool wasCanceled() const { return mWasCanceled; }
+
+    QVector<QgsFeatureFilterModel::Entry> entries() const
+    {
+      return mEntries;
+    }
+
+    QgsFeatureRequest request() const
+    {
+      return mRequest;
+    }
+
+    /**
+     * Internal data, use for whatever you want. Defaults to -1.
+     */
+    QVariant data() const;
+
+    /**
+     * Internal data, use for whatever you want. Defaults to -1.
+     */
+    void setData( const QVariant &data ); // TODO: Do we still need this???
+
+  signals:
+
+    /**
+     * Emitted when values have been collected
+     * @param values list of unique matching string values
+     */
+    void collectedValues();
+
+  private:
+
+    std::unique_ptr<QgsVectorLayerFeatureSource> mSource;
+    QgsExpression mDisplayExpression;
+    QgsExpressionContext mExpressionContext;
+    QgsFeatureRequest mRequest;
+    QgsFeatureIterator mIterator;
+    bool mWasCanceled;
+    QVector<QgsFeatureFilterModel::Entry> mEntries;
+    QString mIdentifierField;
+    QVariant mData;
+};
+
+///@endcond
+
+
+#endif // QGSFEATUREFILTERMODEL_P_H

src/gui/CMakeLists.txt

@@ -241,6 +241,7 @@ SET(QGIS_GUI_SRCS
   qgsfeatureselectiondlg.cpp
   qgsfieldcombobox.cpp
   qgsfieldexpressionwidget.cpp
+  qgsfeaturelistcombobox.cpp
   qgsfieldvalidator.cpp
   qgsfieldvalueslineedit.cpp
   qgsfilewidget.cpp
@@ -412,6 +413,7 @@ SET(QGIS_GUI_MOC_HDRS
   qgsfeatureselectiondlg.h
   qgsfieldcombobox.h
   qgsfieldexpressionwidget.h
+  qgsfeaturelistcombobox.h
   qgsfieldvalidator.h
   qgsfieldvalueslineedit.h
   qgsfilewidget.h

src/gui/qgsfeaturelistcombobox.cpp

@@ -0,0 +1,217 @@
+/***************************************************************************
+  qgsfieldlistcombobox.cpp - QgsFieldListComboBox
+
+ ---------------------
+ begin                : 10.3.2017
+ copyright            : (C) 2017 by Matthias Kuhn
+ email                : matthias@opengis.ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#include "qgsfeaturelistcombobox.h"
+
+#include "qgsfeaturefiltermodel.h"
+#include "qgsanimatedicon.h"
+#include "qgsfilterlineedit.h"
+#include "qgslogger.h"
+
+#include <QCompleter>
+#include <QLineEdit>
+#include <QKeyEvent>
+
+QgsFeatureListComboBox::QgsFeatureListComboBox( QWidget *parent )
+  : QComboBox( parent )
+  , mModel( new QgsFeatureFilterModel( this ) )
+  , mCompleter( new QCompleter( mModel ) )
+{
+  mCompleter->setCaseSensitivity( Qt::CaseInsensitive );
+  mCompleter->setFilterMode( Qt::MatchContains );
+  setCompleter( mCompleter );
+  mCompleter->setWidget( this );
+  connect( mModel, &QgsFeatureFilterModel::sourceLayerChanged, this, &QgsFeatureListComboBox::sourceLayerChanged );
+  connect( mModel, &QgsFeatureFilterModel::displayExpressionChanged, this, &QgsFeatureListComboBox::displayExpressionChanged );
+  connect( mModel, &QgsFeatureFilterModel::filterExpressionChanged, this, &QgsFeatureListComboBox::filterExpressionChanged );
+  connect( mModel, &QgsFeatureFilterModel::isLoadingChanged, this, &QgsFeatureListComboBox::onLoadingChanged );
+  connect( mModel, &QgsFeatureFilterModel::filterJobCompleted, this, &QgsFeatureListComboBox::onFilterUpdateCompleted );
+  connect( mModel, &QgsFeatureFilterModel::extraIdentifierValueChanged, this, &QgsFeatureListComboBox::identifierValueChanged );
+  connect( mModel, &QgsFeatureFilterModel::extraIdentifierValueIndexChanged, this, &QgsFeatureListComboBox::setCurrentIndex );
+  connect( mModel, &QgsFeatureFilterModel::identifierFieldChanged, this, &QgsFeatureListComboBox::identifierFieldChanged );
+  connect( mCompleter, static_cast<void( QCompleter::* )( const QModelIndex & )>( &QCompleter::highlighted ), this, &QgsFeatureListComboBox::onItemSelected );
+  connect( mCompleter, static_cast<void( QCompleter::* )( const QModelIndex & )>( &QCompleter::activated ), this, &QgsFeatureListComboBox::onActivated );
+  connect( mModel, &QgsFeatureFilterModel::beginUpdate, this, &QgsFeatureListComboBox::storeLineEditState );
+  connect( mModel, &QgsFeatureFilterModel::endUpdate, this, &QgsFeatureListComboBox::restoreLineEditState );
+
+  connect( this, static_cast<void( QgsFeatureListComboBox::* )( int )>( &QgsFeatureListComboBox::currentIndexChanged ), this, &QgsFeatureListComboBox::onCurrentIndexChanged );
+
+  mLineEdit = new QgsFilterLineEdit();
+  setEditable( true );
+  setLineEdit( mLineEdit );
+  setModel( mModel );
+
+  connect( mLineEdit, &QgsFilterLineEdit::textEdited, this, &QgsFeatureListComboBox::onCurrentTextChanged );
+
+  connect( mLineEdit, &QgsFilterLineEdit::textChanged, this, []( const QString & text )
+  {
+    QgsDebugMsg( QStringLiteral( "Edit text changed to %1" ).arg( text ) );
+  } );
+}
+
+QgsVectorLayer *QgsFeatureListComboBox::sourceLayer() const
+{
+  return mModel->sourceLayer();
+}
+
+void QgsFeatureListComboBox::setSourceLayer( QgsVectorLayer *sourceLayer )
+{
+  mModel->setSourceLayer( sourceLayer );
+}
+
+QString QgsFeatureListComboBox::displayExpression() const
+{
+  return mModel->displayExpression();
+}
+
+void QgsFeatureListComboBox::setDisplayExpression( const QString &expression )
+{
+  mModel->setDisplayExpression( expression );
+}
+
+void QgsFeatureListComboBox::onCurrentTextChanged( const QString &text )
+{
+  mPopupRequested = true;
+  mModel->setFilterValue( text );
+}
+
+void QgsFeatureListComboBox::onFilterUpdateCompleted()
+{
+  if ( mPopupRequested )
+    mCompleter->complete();
+
+  mPopupRequested = false;
+}
+
+void QgsFeatureListComboBox::onLoadingChanged()
+{
+  mLineEdit->setShowSpinner( mModel->isLoading() );
+}
+
+void QgsFeatureListComboBox::onItemSelected( const QModelIndex &index )
+{
+  setCurrentIndex( index.row() );
+}
+
+void QgsFeatureListComboBox::onCurrentIndexChanged( int i )
+{
+  QModelIndex modelIndex = mModel->index( i, 0, QModelIndex() );
+  mModel->setExtraIdentifierValue( mModel->data( modelIndex, QgsFeatureFilterModel::IdentifierValueRole ) );
+  mLineEdit->setText( mModel->data( modelIndex, QgsFeatureFilterModel::ValueRole ).toString() );
+}
+
+void QgsFeatureListComboBox::onActivated( QModelIndex modelIndex )
+{
+  setIdentifierValue( mModel->data( modelIndex, QgsFeatureFilterModel::IdentifierValueRole ) );
+  QgsDebugMsg( QStringLiteral( "Activated index" ) );
+  QgsDebugMsg( QStringLiteral( "%1 %2" ).arg( QString::number( modelIndex.row() ), mModel->data( modelIndex, QgsFeatureFilterModel::ValueRole ).toString() ) );
+  mLineEdit->setText( mModel->data( modelIndex, QgsFeatureFilterModel::ValueRole ).toString() );
+}
+
+void QgsFeatureListComboBox::storeLineEditState()
+{
+  mLineEditState.store( mLineEdit );
+}
+
+void QgsFeatureListComboBox::restoreLineEditState()
+{
+  mLineEditState.restore( mLineEdit );
+}
+
+QString QgsFeatureListComboBox::identifierField() const
+{
+  return mModel->identifierField();
+}
+
+void QgsFeatureListComboBox::setIdentifierField( const QString &identifierField )
+{
+  mModel->setIdentifierField( identifierField );
+}
+
+QModelIndex QgsFeatureListComboBox::currentModelIndex() const
+{
+  return mModel->index( mModel->extraIdentifierValueIndex(), 0, QModelIndex() );
+}
+
+void QgsFeatureListComboBox::focusOutEvent( QFocusEvent *event )
+{
+  Q_UNUSED( event )
+  QComboBox::focusOutEvent( event );
+  mLineEdit->setText( mModel->data( currentModelIndex(), QgsFeatureFilterModel::ValueRole ).toString() );
+}
+
+void QgsFeatureListComboBox::keyPressEvent( QKeyEvent *event )
+{
+  if ( event->key() == Qt::Key_Escape )
+  {
+    mLineEdit->setText( mModel->data( currentModelIndex(), QgsFeatureFilterModel::ValueRole ).toString() );
+  }
+  QComboBox::keyReleaseEvent( event );
+}
+
+bool QgsFeatureListComboBox::allowNull() const
+{
+  return mAllowNull;
+}
+
+void QgsFeatureListComboBox::setAllowNull( bool allowNull )
+{
+  if ( mAllowNull == allowNull )
+    return;
+
+  mAllowNull = allowNull;
+  emit allowNullChanged();
+}
+
+QVariant QgsFeatureListComboBox::identifierValue() const
+{
+  return mModel->extraIdentifierValue();
+}
+
+void QgsFeatureListComboBox::setIdentifierValue( const QVariant &identifierValue )
+{
+  mModel->setExtraIdentifierValue( identifierValue );
+}
+
+QgsFeatureRequest QgsFeatureListComboBox::currentFeatureRequest() const
+{
+  return QgsFeatureRequest().setFilterExpression( QStringLiteral( "%1 = %2" ).arg( QgsExpression::quotedColumnRef( mModel->identifierField() ), QgsExpression::quotedValue( mModel->extraIdentifierValue() ) ) );
+}
+
+QString QgsFeatureListComboBox::filterExpression() const
+{
+  return mModel->filterExpression();
+}
+
+void QgsFeatureListComboBox::setFilterExpression( const QString &filterExpression )
+{
+  mModel->setFilterExpression( filterExpression );
+}
+
+void QgsFeatureListComboBox::LineEditState::store( QLineEdit *lineEdit )
+{
+  text = lineEdit->text();
+  selectionStart = lineEdit->selectionStart();
+  selectionLength = lineEdit->selectedText().length();
+  cursorPosition = lineEdit->cursorPosition();
+
+}
+
+void QgsFeatureListComboBox::LineEditState::restore( QLineEdit *lineEdit ) const
+{
+  lineEdit->setText( text );
+  lineEdit->setCursorPosition( cursorPosition );
+  lineEdit->setSelection( selectionStart, selectionLength );
+}

src/gui/qgsfeaturelistcombobox.h

@@ -0,0 +1,110 @@
+/***************************************************************************
+  qgsfieldlistcombobox.h - QgsFieldListComboBox
+
+ ---------------------
+ begin                : 10.3.2017
+ copyright            : (C) 2017 by Matthias Kuhn
+ email                : matthias@opengis.ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#ifndef QGSFIELDLISTCOMBOBOX_H
+#define QGSFIELDLISTCOMBOBOX_H
+
+#include <QComboBox>
+
+#include "qgsfeature.h"
+#include "qgsfeaturerequest.h"
+#include "qgis_gui.h"
+
+class QgsVectorLayer;
+class QgsFeatureFilterModel;
+class QgsAnimatedIcon;
+class QgsFilterLineEdit;
+
+class GUI_EXPORT QgsFeatureListComboBox : public QComboBox
+{
+    Q_OBJECT
+
+    Q_PROPERTY( QgsVectorLayer *sourceLayer READ sourceLayer WRITE setSourceLayer NOTIFY sourceLayerChanged )
+    Q_PROPERTY( QString displayExpression READ displayExpression WRITE setDisplayExpression NOTIFY displayExpressionChanged )
+    Q_PROPERTY( QString filterExpression READ filterExpression WRITE setFilterExpression NOTIFY filterExpressionChanged )
+    Q_PROPERTY( QVariant identifierValue READ identifierValue WRITE setIdentifierValue NOTIFY identifierValueChanged )
+    Q_PROPERTY( QString identifierField READ identifierField WRITE setIdentifierField NOTIFY identifierFieldChanged )
+    Q_PROPERTY( bool allowNull READ allowNull WRITE setAllowNull NOTIFY allowNullChanged )
+
+  public:
+    QgsFeatureListComboBox( QWidget *parent = nullptr );
+
+    QgsVectorLayer *sourceLayer() const;
+    void setSourceLayer( QgsVectorLayer *sourceLayer );
+
+    QString displayExpression() const;
+    void setDisplayExpression( const QString &displayExpression );
+
+    QString filterExpression() const;
+    void setFilterExpression( const QString &filterExpression );
+
+    QVariant identifierValue() const;
+    void setIdentifierValue( const QVariant &identifierValue );
+
+    QgsFeatureRequest currentFeatureRequest() const;
+
+    bool allowNull() const;
+    void setAllowNull( bool allowNull );
+
+    QString identifierField() const;
+    void setIdentifierField( const QString &identifierField );
+
+    QModelIndex currentModelIndex() const;
+
+    virtual void focusOutEvent( QFocusEvent *event ) override;
+
+    virtual void keyPressEvent( QKeyEvent *event ) override;
+
+  signals:
+    void sourceLayerChanged();
+    void displayExpressionChanged();
+    void filterExpressionChanged();
+    void identifierValueChanged();
+    void identifierFieldChanged();
+    void allowNullChanged();
+
+  private slots:
+    void onCurrentTextChanged( const QString &text );
+    void onFilterUpdateCompleted();
+    void onLoadingChanged();
+    void onItemSelected( const QModelIndex &index );
+    void onCurrentIndexChanged( int i );
+    void onActivated( QModelIndex index );
+    void storeLineEditState();
+    void restoreLineEditState();
+
+  private:
+    struct LineEditState
+    {
+      void store( QLineEdit *lineEdit );
+      void restore( QLineEdit *lineEdit ) const;
+
+      QString text;
+      int selectionStart;
+      int selectionLength;
+      int cursorPosition;
+    };
+
+    QgsFeatureFilterModel *mModel;
+    QCompleter *mCompleter;
+    QString mDisplayExpression;
+    QgsFilterLineEdit *mLineEdit;
+    bool mAllowNull = true;
+    bool mPopupRequested = false;
+    bool mIsCurrentlyEdited = false;
+    LineEditState mLineEditState;
+};
+
+#endif // QGSFIELDLISTCOMBOBOX_H

src/gui/qgsfeaturelistcombobox_p.h

@@ -0,0 +1,19 @@
+/***************************************************************************
+  qgsfeaturelistcombobox_p.h
+
+ ---------------------
+ begin                : 24.10.2017
+ copyright            : (C) 2017 by Matthias Kuhn
+ email                : matthias@opengis.ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#ifndef QGSFEATURELISTCOMBOBOX_P_H
+#define QGSFEATURELISTCOMBOBOX_P_H
+
+#endif // QGSFEATURELISTCOMBOBOX_P_H