QgsMapLayerRenderer class

Base class for utility classes that encapsulate information necessary for rendering of map layers.

Contents

The rendering is typically done in a background thread, so it is necessary to keep all structures required for rendering away from the original map layer because it may change any time.

Because the data needs to be copied (to avoid the need for locking), it is highly desirable to use copy-on-write where possible. This way, the overhead of copying (both memory and CPU) will be kept low. Qt containers and various Qt classes use implicit sharing.

The scenario will be:

  1. renderer job (doing preparation in the GUI thread) calls QgsMapLayer::createMapRenderer() and gets instance of this class. The instance is initialized at that point and should not need additional calls to QgsVectorLayer.
  2. renderer job (still in GUI thread) stores the renderer for later use.
  3. renderer job (in worker thread) calls QgsMapLayerRenderer::render()
  4. renderer job (again in GUI thread) will check errors() and report them

Derived classes

class QgsMeshLayerRenderer
Implementation of threaded rendering for mesh layers.
class QgsRasterLayerRenderer
Implementation of threaded rendering for raster layers.
class QgsVectorLayerRenderer
Implementation of threaded rendering for vector layers.

Public functions

auto errors() const -> QStringList
Returns list of errors (problems) that happened during the rendering.
auto feedback() const -> QgsFeedback* virtual
Access to feedback object of the layer renderer (may be null)
auto layerId() const -> QString
Gets access to the ID of the layer rendered by this class.
auto render() -> bool pure virtual
Do the rendering (based on data stored in the class)

Function documentation

QgsFeedback* QgsMapLayerRenderer::feedback() const virtual

Access to feedback object of the layer renderer (may be null)