132e76a596
This adds a new input type for expression inputs. Expression inputs can be linked to a parent layer so that the builder shows the correct fields and layer variables. It's designed for two use cases: 1. to be used when an algorithm specifically requires an expression, eg Select by Expression and Extract by Expression. 2. to be potentially used as a replacement input instead of string or number literals in algorithms. Eg - if the simplify algorithm tolerance parameter was replaced with an expression paremeter, then this expression would be evaluated for every feature before simplifying that feature. It would allow parameters to be calculated per feature, as opposed to the current approach of calculating a parameter once before running the algorithm. It would also mean algorithms like "variable distance buffer" would no longer be needed, as a single "buffer" algorithm could then be used for either a fixed distance, field based, or expression based distance.
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QGIS is an Open Source Geographic Information System. The project was born in May of 2002 and was established as a project on SourceForge in June of the same year. We've worked hard to make GIS software (which is traditionally expensive commercial software) a viable prospect for anyone with basic access to a Personal Computer. QGIS currently runs on most Unix platforms (macOS/OS X included) and Windows. QGIS is developed using the Qt toolkit (http://qt.io) and C++. This means that QGIS feels snappy to use and has a pleasing, easy to use graphical user interface.
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