Basics - PyGObject

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# Basics

Hint

In this example, we will use GTK widgets to demonstrate GObject capabilities.

## GObject Initialization

GObjects are initialized like any other Python class.

    label = Gtk.Label()

## Properties

GObject has a powerful properties system.

Properties describe the configuration and state of a gobject. Each gobject has its own particular set of properties. For example, a GTK button has the property `label` which contains the text of the label widget inside the button.

You can specify the name and value of any number of properties as keyword arguments when creating an instance of a gobject. To create a label aligned to the right with the text “Hello World”, use:

    label = Gtk.Label(label='Hello World', halign=Gtk.Align.END)

which is equivalent to

    label = Gtk.Label()
    label.set_label('Hello World')
    label.set_halign(Gtk.Align.END)

There are various ways of interacting with a gobject properties from Python, we already have seen the two first ways, these are setting them on initialization or using the getters and setters functions that the gobject might provide.

Other option is to use `GObject.Object` builtin methods `GObject.Object.get_property()` and `GObject.Object.set_property()`. Using these methods is more common when you have created a `GObject.Object` subclass and you don’t have getters and setters functions.

    label = Gtk.Label()
    label.set_property('label', 'Hello World')
    label.set_property('halign', Gtk.Align.END)
    print(label.get_property('label'))

Instead of using getters and setters you can also get and set the gobject properties through the `props` property such as `label.props.label = 'Hello World'`. This is equivalent to the more verbose methods that we saw before, and it’s a more pythonic way of interacting with properties.

To see which properties are available for a gobject you can `dir` the `props` property:

    widget = Gtk.Box()
    print(dir(widget.props))

This will print to the console the list of properties that a `Gtk.Box` has.

### Property Bindings

GObject provides a practical way to bind properties of two different gobjects. This is done using the `GObject.Object.bind_property()` method.

The behavior of this binding can be controlled by passing a `GObject.BindingFlags` of choice. `GObject.BindingFlags.DEFAULT` will update the target property every time the source property changes. `GObject.BindingFlags.BIDIRECTIONAL` creates a bidirectional binding; if either the property of the source or the property of the target changes, the other is updated. `GObject.BindingFlags.SYNC_CREATE` is similar to `DEFAULT` but it will also synchronize the values of the source and target properties when creating the binding. `GObject.BindingFlags.INVERT_BOOLEAN` works only for boolean properties and setting one property to `True` will result in the other being set to `False` and vice versa (this flag cannot be used when passing custom transformation functions to `GObject.Object.bind_property()`).

    entry = Gtk.Entry()
    label = Gtk.Label()

    entry.bind_property('text', label, 'label', GObject.BindingFlags.DEFAULT)

In this example entry is our source object and `text` the source property to bind. label is the target object and the namesake property `label` is the target property. Every time someone changes the `text` property of the entry the label `label` will be updated as well with the same value.

### Property Bindings with Transformations

Sometimes you may want to bind two properties that are incompatible, or you simply need to apply some transformation between these values. For these scenarios `GObject.Object.bind_property()` also accepts custom transformation functions that serve to this purpose.

The transformation functions take as first argument the `GObject.Binding` instance for this binding and as second argument the property value depending on the direction of the transformation. For `transform_to` this will be the value of the source property and for `transform_from` the value of the target property. Each function should return the value to be set in the other object’s property, with the correct type.

In this example we’ll do an `int` to `bool` type conversion between two objects:

    def transform_to(_binding, value):
       return bool(value)  # Return int converted to a bool

    def transform_from(_binding, value):
        return int(value)  # Return bool converted to a int

    source.bind_property(
        'int_prop',
        target,
        'bool_prop',
        GObject.BindingFlags.BIDIRECTIONAL,
        transform_to,
        transform_from
    )

## Signals

GObject signals are a system for registering callbacks for specific events.

A generic example is:

    handler_id = gobject.connect('event', callback, data)

Firstly, gobject is an instance of a gobject we created earlier. Next, the event we are interested in. Each gobject has its own particular events which can occur. This means that when the gobject emits the event, the signal is issued. Thirdly, the callback argument is the name of the callback function. It contains the code which runs when signals of the specified type are issued. Finally, the data argument includes any data which should be passed when the signal is issued. However, this argument is completely optional and can be left out if not required.

The function returns a number that identifies this particular signal-callback pair. It is required to disconnect from a signal such that the callback function will not be called during any future or currently ongoing emissions of the signal it has been connected to:

    gobject.disconnect(handler_id)

When creating the callback function for a signal, the arguments it accepts will depend on the specific signal, but for a signal with no arguments it will look like this:

    def on_event(gobject, data):
        ...

    my_object.connect('event', on_event, data)

Where `gobject` is the object that triggered the signal and `data` is the additional data that we previously passed to the `GObject.Object.connect()` method. If the signal had arguments, they will come before the optional data argument.

### The `notify` signal

When any of a GObject’s properties change, it will emit the `notify` signal. This is a “detailed” signal, meaning that you can listen to a subset of the signal, in this case a specific property. For example, you can connect to the signal in the form of `notify::property-name`:

    def callback(label, _pspec):
        print(f'The label prop changed to {label.props.label}')

    label = Gtk.Label()
    label.connect('notify::label', callback)