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Leggere E Memorizzare Impostazioni

Molte volte è utile per un plugin salvare alcune variabili in modo tale che l’utente non debba inserirle o selezionarle di nuovo la volta successiva che il plugin è eseguito.

Questa variabili possono essere salvate e recuperate con l’aiuto delle API di Qt e QGIS. Per ogni variabile, dovresti scegliere una chiave che sarà usata per accedere alla variabile — per il colore preferito dell’utente potresti usare la chiave “favourite_color” o una qualunque altra stringa significativa. È raccomandabile dare una qualche struttura alla denominazione delle chiavi.

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")