Binding

`GObject` instance (or source) and another property on another `GObject` instance (or target).

Whenever the source property changes, the same value is applied to the target property; for instance, the following binding:

```c g_object_bind_property (object1, "property-a", object2, "property-b", G_BINDING_DEFAULT); ```

will cause the property named "property-b" of object2 to be updated every time [method@GObject.set] or the specific accessor changes the value of the property "property-a" of object1.

It is possible to create a bidirectional binding between two properties of two `GObject` instances, so that if either property changes, the other is updated as well, for instance:

```c g_object_bind_property (object1, "property-a", object2, "property-b", G_BINDING_BIDIRECTIONAL); ```

will keep the two properties in sync.

It is also possible to set a custom transformation function (in both directions, in case of a bidirectional binding) to apply a custom transformation from the source value to the target value before applying it; for instance, the following binding:

```c g_object_bind_property_full (adjustment1, "value", adjustment2, "value", G_BINDING_BIDIRECTIONAL, celsius_to_fahrenheit, fahrenheit_to_celsius, NULL, NULL); ```

will keep the "value" property of the two adjustments in sync; the celsius_to_fahrenheit function will be called whenever the "value" property of adjustment1 changes and will transform the current value of the property before applying it to the "value" property of adjustment2.

Vice versa, the fahrenheit_to_celsius function will be called whenever the "value" property of adjustment2 changes, and will transform the current value of the property before applying it to the "value" property of adjustment1.

Note that Binding does not resolve cycles by itself; a cycle like

``` object1GBinding:propertyA -> object2GBinding:propertyB object2GBinding:propertyB -> object3 GBinding:propertyC object3GBinding:propertyC -> object1GBinding:propertyA ```

might lead to an infinite loop. The loop, in this particular case, can be avoided if the objects emit the `GObject:GBinding:notify ` signal only if the value has effectively been changed. A binding is implemented using the `GObject:GBinding:notify` signal, so it is susceptible to all the various ways of blocking a signal emission, like [func@GObject.signal_stop_emission] or [ func@GObject.signal_handler_block].

A binding will be severed, and the resources it allocates freed, whenever either one of the `GObject` instances it refers to are finalized, or when the Binding instance loses its last reference.

Bindings for languages with garbage collection can use [method@GObject.Binding.unbind] to explicitly release a binding between the source and target properties, instead of relying on the last reference on the binding, source, and target instances to drop.

Example: Property bindings:

public class ObjectA : Object {
	public int property_a { get; set; }
	public int property_b { get; set; }
	public int property_c { get; set; }
	public bool property_d { get; set; }
	public int property_e { get; set; }
}

public class ObjectB : Object {
	public int property_a { get; set; }
	public int property_b { get; set; }
	public int property_c { get; set; }
	public bool property_d { get; set; }
	public int property_e { get; set; }
}

public static int main (string[] args) {
	// Objects:
	ObjectA obja = new ObjectA ();
	ObjectB objb = new ObjectB ();


	//
	// BindingFlags.DEFAULT:
	//

	print ("BindingFlags.DEFAULT:\n");

	obja.property_a = 50;
	objb.property_a = 60;

	obja.bind_property ("property-a", objb, "property-a", BindingFlags.DEFAULT);

	// Output: ``50 - 60``
	print (" %d - %d\n", obja.property_a, objb.property_a);

	// Output: ``10 - 10``
	obja.property_a = 10;
	print (" %d - %d\n", obja.property_a, objb.property_a);

	// Output: ``10 - 15``
	objb.property_a = 15;
	print (" %d - %d\n", obja.property_a, objb.property_a);


	//
	// BindingFlags.BIDIRECTIONAL:
	//

	print ("BindingFlags.BIDIRECTIONAL:\n");

	obja.property_b = 50;
	objb.property_b = 60;

	obja.bind_property ("property-b", objb, "property-b", BindingFlags.BIDIRECTIONAL);

	// Output: ``50 - 60``
	print (" %d - %d\n", obja.property_b, objb.property_b);

	// Output: ``10 - 10``
	obja.property_b = 10;
	print (" %d - %d\n", obja.property_b, objb.property_b);

	// Output: ``15 - 15``
	objb.property_b = 15;
	print (" %d - %d\n", obja.property_b, objb.property_b);


	//
	// BindingFlags.SYNC_CREATE | BindingFlags.BIDIRECTIONAL:
	//

	print ("BindingFlags.SYNC_CREATE | BindingFlags.BIDIRECTIONAL:\n");

	obja.property_c = 50;
	objb.property_c = 60;

	obja.bind_property ("property-c", objb, "property-c", BindingFlags.SYNC_CREATE | BindingFlags.BIDIRECTIONAL);

	// Output: ``50 - 50``
	print (" %d - %d\n", obja.property_c, objb.property_c);

	// Output: ``10 - 10``
	obja.property_c = 10;
	print (" %d - %d\n", obja.property_c, objb.property_c);

	// Output: ``20 - 20``
	objb.property_c = 20;
	print (" %d - %d\n", obja.property_c, objb.property_c);


	//
	// BindingFlags.INVERT_BOOLEAN:
	//

	print ("BindingFlags.INVERT_BOOLEAN:\n");

	obja.property_d = false;
	objb.property_d = true;

	obja.bind_property ("property-d", objb, "property-d", BindingFlags.INVERT_BOOLEAN);

	// Output: ``true - false``
	obja.property_d = true;
	print (" %s - %s\n", obja.property_d.to_string (), objb.property_d.to_string ());

	// Output: ``false - true``
	obja.property_d = false;
	print (" %s - %s\n", obja.property_d.to_string (), objb.property_d.to_string ());


	//
	// Transformer:
	//

	print ("Transformer:\n");

	obja.bind_property ("property-e", objb, "property-e", BindingFlags.SYNC_CREATE | BindingFlags.BIDIRECTIONAL, (binding, srcval, ref targetval) => {
		int src = (int) srcval;
		targetval.set_int (src * 2);
		return true;
	}, (binding, srcval, ref targetval) => {
		int src = (int) srcval;
		targetval.set_int (src / 2);
		return true;
	});


	obja.property_e = 50;
	objb.property_e = 60;

	// Output: ``30 - 60``
	print (" %d - %d\n", obja.property_e, objb.property_e);

	// Output: ``10 - 20``
	obja.property_e = 10;
	print (" %d - %d\n", obja.property_e, objb.property_e);

	// Output: ``20 - 40``
	objb.property_e = 40;
	print (" %d - %d\n", obja.property_e, objb.property_e);

	return 0;
}

valac --pkg gobject-2.0 GLib.Object.bind_property.vala