Instellingen lezen en opslaan¶

Vaak is het voor een plug-in nuttig om enkele variabelen op te slaan zodat de gebruiker ze niet opnieuw hoeft in te voeren of te selecteren als de plug-in een volgende keer wordt uitgevoerd.

Deze variabelen kunnen worden opgeslagen en weer worden opgehaald met de hulp van Qt en de API van QGIS. Voor elke variabele zou u een sleutel moeten kiezen die kan worden gebruikt om toegang te verkrijgen tot de variabele — voor de favoriete kleur van de gebruiker zou u de sleutel “favourite_color” kunnen gebruiken of elke andere tekenreeks met betekenis. Het wordt aanbevolen enige structuur aan te brengen in het benoemen van sleutels.

We can make difference between several types of settings:

• global settings — they are bound to the user at particular machine. QGIS itself stores a lot of global settings, for example, main window size or default snapping tolerance. This functionality is provided directly by Qt framework by the means of QSettings class. By default, this class stores settings in system’s “native” way of storing settings, that is — registry (on Windows), .plist file (on macOS) or .ini file (on Unix). The QSettings documentation is comprehensive, so we will provide just a simple example

  def store():
    s = QSettings()
    s.setValue("myplugin/mytext", "hello world")
    s.setValue("myplugin/myint",  10)
    s.setValue("myplugin/myreal", 3.14)
  
  def read():
    s = QSettings()
    mytext = s.value("myplugin/mytext", "default text")
    myint  = s.value("myplugin/myint", 123)
    myreal = s.value("myplugin/myreal", 2.71)
  

  The second parameter of the value() method is optional and specifies the default value if there is no previous value set for the passed setting name.

• project settings — vary between different projects and therefore they are connected with a project file. Map canvas background color or destination coordinate reference system (CRS) are examples — white background and WGS84 might be suitable for one project, while yellow background and UTM projection are better for another one. An example of usage follows

  proj = QgsProject.instance()
  
  # store values
  proj.writeEntry("myplugin", "mytext", "hello world")
  proj.writeEntry("myplugin", "myint", 10)
  proj.writeEntry("myplugin", "mydouble", 0.01)
  proj.writeEntry("myplugin", "mybool", True)
  
  # read values
  mytext = proj.readEntry("myplugin", "mytext", "default text")[0]
  myint = proj.readNumEntry("myplugin", "myint", 123)[0]
  

  As you can see, the writeEntry() method is used for all data types, but several methods exist for reading the setting value back, and the corresponding one has to be selected for each data type.

• map layer settings — these settings are related to a particular instance of a map layer with a project. They are not connected with underlying data source of a layer, so if you create two map layer instances of one shapefile, they will not share the settings. The settings are stored in project file, so if the user opens the project again, the layer-related settings will be there again. This functionality has been added in QGIS v1.4. The API is similar to QSettings — it takes and returns QVariant instances

  # save a value
  layer.setCustomProperty("mytext", "hello world")
  
  # read the value again
  mytext = layer.customProperty("mytext", "default text")