Backwards Incompatible Changes

Table of Contents

• QGIS 2.4
  • Multi-threaded Rendering
• QGIS 2.6
  • Legend Refactoring

The API of QGIS libraries is allowed to be changed just between major versions of QGIS. For example, there are various planned backwards incompatible changes between QGIS 1.8 and 2.0 because the version 2.0 is a new major version. After a release of a major version of QGIS (e.g. 2.0) the developer team is committed to maintain stable API for all subsequent minor releases (2.2, 2.4, ...). That roughly means we do not rename classes and methods, remove them nor change their semantics. Existing code should keep working when the user updates QGIS to another minor version (e.g. from 2.0 to 2.2), so all extensions of existing classes should be done in a manner that third party developers do not need to adjust their code to work properly with newer QGIS releases.

Sometimes, however, we may need to break the API as a result of some code changes. These cases should be only exceptions and they should happen only after consideration and agreement of the development team. Backwards incompatible changes with too big impact should be deferred to a major version release.

This page tries to maintain a list with incompatible changes that happened in previous releases.

QGIS 2.4

Multi-threaded Rendering

• QgsMapCanvas::refresh() only schedules a map refresh (in near feature) and returns immediately - before it would do the drawing immediately and return when the map is redrawn. A call to refresh() will have no effect if there is already a scheduled pending refresh. When map canvas does actual rendering, it will first emit renderStarting() signal, once done it will emit mapCanvasRefreshed(). The client code doing refresh() in order to later save map image should be updated to use new QgsMapRendererJob API which is better suited for such task.
• QgsPluginLayer::draw() is now run in a background thread. It is recommended to implement newly added QgsPluginLayer::createMapRenderer() method instead of using QgsPluginLayer::draw().

QGIS 2.6

Legend Refactoring

• QgsComposerLegend::model() - not being used anymore. The model was replaced by one based on QgsLayerTreeModel class and is available in QgsComposerLegend::modelV2()