GBinding: GObject Reference Manual

## GBinding GBinding — Bind two object properties |

## Functions

GObject * | g_binding_get_source ()
GObject * | g_binding_dup_source ()
const gchar * | g_binding_get_source_property ()
GObject * | g_binding_get_target ()
GObject * | g_binding_dup_target ()
const gchar * | g_binding_get_target_property ()
GBindingFlags | g_binding_get_flags ()
void | g_binding_unbind ()
GBinding * | g_object_bind_property ()
gboolean | (*GBindingTransformFunc) ()
GBinding * | g_object_bind_property_full ()
GBinding * | g_object_bind_property_with_closures ()

## Properties

GBindingFlags | flags | Read / Write / Construct Only
GObject * | source | Read / Write / Construct Only
char * | source-property | Read / Write / Construct Only
GObject * | target | Read / Write / Construct Only
char * | target-property | Read / Write / Construct Only

## Types and Values

| GBinding
enum | GBindingFlags

## Object Hierarchy

        GFlags
        ╰── GBindingFlags
        GObject
        ╰── GBinding

## Includes

    #include <glib-object.h>
    #include <gobject/gvaluecollector.h>

## Description

GBinding is the representation of a binding between a property on a 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:

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    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 `g_object_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:

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    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:

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    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 GBinding does not resolve cycles by itself; a cycle like

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    object1:propertyA -> object2:propertyB
    object2:propertyB -> object3:propertyC
    object3:propertyC -> object1:propertyA

might lead to an infinite loop. The loop, in this particular case, can be avoided if the objects emit the “notify” signal only if the value has effectively been changed. A binding is implemented using the “notify” signal, so it is susceptible to all the various ways of blocking a signal emission, like `g_signal_stop_emission()` or `g_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 GBinding instance loses its last reference.

Bindings for languages with garbage collection can use `g_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.

GBinding is available since GObject 2.26

## Functions

### g_binding_get_source ()

    GObject *
    g_binding_get_source (GBinding *binding);

`g_binding_get_source` has been deprecated since version 2.68 and should not be used in newly-written code.

Use `g_binding_dup_source()` for a safer version of this function.

Retrieves the GObject instance used as the source of the binding.

A GBinding can outlive the source GObject as the binding does not hold a strong reference to the source. If the source is destroyed before the binding then this function will return `NULL`.

Use `g_binding_dup_source()` if the source or binding are used from different threads as otherwise the pointer returned from this function might become invalid if the source is finalized from another thread in the meantime.

#### Parameters

binding | a GBinding |

#### Returns

the source GObject, or `NULL` if the source does not exist any more.

[transfer none][nullable]

Since: 2.26

### g_binding_dup_source ()

    GObject *
    g_binding_dup_source (GBinding *binding);

Retrieves the GObject instance used as the source of the binding.

A GBinding can outlive the source GObject as the binding does not hold a strong reference to the source. If the source is destroyed before the binding then this function will return `NULL`.

#### Parameters

binding | a GBinding |

#### Returns

the source GObject, or `NULL` if the source does not exist any more.

[transfer full][nullable]

Since: 2.68

### g_binding_get_source_property ()

    const gchar *
    g_binding_get_source_property (GBinding *binding);

Retrieves the name of the property of “source” used as the source of the binding.

#### Parameters

binding | a GBinding |

#### Returns

the name of the source property

Since: 2.26

### g_binding_get_target ()

    GObject *
    g_binding_get_target (GBinding *binding);

`g_binding_get_target` has been deprecated since version 2.68 and should not be used in newly-written code.

Use `g_binding_dup_target()` for a safer version of this function.

Retrieves the GObject instance used as the target of the binding.

A GBinding can outlive the target GObject as the binding does not hold a strong reference to the target. If the target is destroyed before the binding then this function will return `NULL`.

Use `g_binding_dup_target()` if the target or binding are used from different threads as otherwise the pointer returned from this function might become invalid if the target is finalized from another thread in the meantime.

#### Parameters

binding | a GBinding |

#### Returns

the target GObject, or `NULL` if the target does not exist any more.

[transfer none][nullable]

Since: 2.26

### g_binding_dup_target ()

    GObject *
    g_binding_dup_target (GBinding *binding);

Retrieves the GObject instance used as the target of the binding.

A GBinding can outlive the target GObject as the binding does not hold a strong reference to the target. If the target is destroyed before the binding then this function will return `NULL`.

#### Parameters

binding | a GBinding |

#### Returns

the target GObject, or `NULL` if the target does not exist any more.

[transfer full][nullable]

Since: 2.68

### g_binding_get_target_property ()

    const gchar *
    g_binding_get_target_property (GBinding *binding);

Retrieves the name of the property of “target” used as the target of the binding.

#### Parameters

binding | a GBinding |

#### Returns

the name of the target property

Since: 2.26

### g_binding_get_flags ()

    GBindingFlags
    g_binding_get_flags (GBinding *binding);

Retrieves the flags passed when constructing the GBinding.

#### Parameters

binding | a GBinding |

#### Returns

the GBindingFlags used by the GBinding

Since: 2.26

### g_binding_unbind ()

    void
    g_binding_unbind (GBinding *binding);

Explicitly releases the binding between the source and the target property expressed by `binding` .

This function will release the reference that is being held on the `binding` instance if the binding is still bound; if you want to hold on to the GBinding instance after calling `g_binding_unbind()`, you will need to hold a reference to it.

Note however that this function does not take ownership of `binding` , it only unrefs the reference that was initially created by `g_object_bind_property()` and is owned by the binding.

#### Parameters

binding | a GBinding |

Since: 2.38

### g_object_bind_property ()

    GBinding *
    g_object_bind_property (gpointer source,
                            const gchar *source_property,
                            gpointer target,
                            const gchar *target_property,
                            GBindingFlags flags);

Creates a binding between `source_property` on `source` and `target_property` on `target` .

Whenever the `source_property` is changed the `target_property` is updated using the same value. For instance:

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    g_object_bind_property (action, "active", widget, "sensitive", 0);

Will result in the "sensitive" property of the widget GObject instance to be updated with the same value of the "active" property of the action GObject instance.

If `flags` contains `G_BINDING_BIDIRECTIONAL` then the binding will be mutual: if `target_property` on `target` changes then the `source_property` on `source` will be updated as well.

The binding will automatically be removed when either the `source` or the `target` instances are finalized. To remove the binding without affecting the `source` and the `target` you can just call `g_object_unref()` on the returned GBinding instance.

Removing the binding by calling `g_object_unref()` on it must only be done if the binding, `source` and `target` are only used from a single thread and it is clear that both `source` and `target` outlive the binding. Especially it is not safe to rely on this if the binding, `source` or `target` can be finalized from different threads. Keep another reference to the binding and use `g_binding_unbind()` instead to be on the safe side.

A GObject can have multiple bindings.

#### Parameters

source | the source GObject. | [type GObject.Object]
source_property | the property on `source` to bind |
target | the target GObject. | [type GObject.Object]
target_property | the property on `target` to bind |
flags | flags to pass to GBinding |

#### Returns

the GBinding instance representing the binding between the two GObject instances. The binding is released whenever the GBinding reference count reaches zero.

[transfer none]

Since: 2.26

### GBindingTransformFunc ()

    gboolean
    (*GBindingTransformFunc) (GBinding *binding,
                              const GValue *from_value,
                              GValue *to_value,
                              gpointer user_data);

A function to be called to transform `from_value` to `to_value` .

If this is the `transform_to` function of a binding, then `from_value` is the `source_property` on the `source` object, and `to_value` is the `target_property` on the `target` object. If this is the `transform_from` function of a `G_BINDING_BIDIRECTIONAL` binding, then those roles are reversed.

#### Parameters

binding | a GBinding |
from_value | the GValue containing the value to transform |
to_value | the GValue in which to store the transformed value |
user_data | data passed to the transform function |

#### Returns

`TRUE` if the transformation was successful, and `FALSE` otherwise

Since: 2.26

### g_object_bind_property_full ()

    GBinding *
    g_object_bind_property_full (gpointer source,
                                 const gchar *source_property,
                                 gpointer target,
                                 const gchar *target_property,
                                 GBindingFlags flags,
                                 GBindingTransformFunc transform_to,
                                 GBindingTransformFunc transform_from,
                                 gpointer user_data,
                                 GDestroyNotify notify);

Complete version of `g_object_bind_property()`.

Creates a binding between `source_property` on `source` and `target_property` on `target` , allowing you to set the transformation functions to be used by the binding.

If `flags` contains `G_BINDING_BIDIRECTIONAL` then the binding will be mutual: if `target_property` on `target` changes then the `source_property` on `source` will be updated as well. The `transform_from` function is only used in case of bidirectional bindings, otherwise it will be ignored

The binding will automatically be removed when either the `source` or the `target` instances are finalized. This will release the reference that is being held on the GBinding instance; if you want to hold on to the GBinding instance, you will need to hold a reference to it.

To remove the binding, call `g_binding_unbind()`.

A GObject can have multiple bindings.

The same `user_data` parameter will be used for both `transform_to` and `transform_from` transformation functions; the `notify` function will be called once, when the binding is removed. If you need different data for each transformation function, please use `g_object_bind_property_with_closures()` instead.

#### Parameters

source | the source GObject. | [type GObject.Object]
source_property | the property on `source` to bind |
target | the target GObject. | [type GObject.Object]
target_property | the property on `target` to bind |
flags | flags to pass to GBinding |
transform_to | the transformation function from the `source` to the `target` , or `NULL` to use the default. | [scope notified][nullable]
transform_from | the transformation function from the `target` to the `source` , or `NULL` to use the default. | [scope notified][nullable]
user_data | custom data to be passed to the transformation functions, or `NULL` |
notify | a function to call when disposing the binding, to free resources used by the transformation functions, or `NULL` if not required. | [nullable]

#### Returns

the GBinding instance representing the binding between the two GObject instances. The binding is released whenever the GBinding reference count reaches zero.

[transfer none]

Since: 2.26

### g_object_bind_property_with_closures ()

    GBinding *
    g_object_bind_property_with_closures (gpointer source,
                                          const gchar *source_property,
                                          gpointer target,
                                          const gchar *target_property,
                                          GBindingFlags flags,
                                          GClosure *transform_to,
                                          GClosure *transform_from);

Creates a binding between `source_property` on `source` and `target_property` on `target` , allowing you to set the transformation functions to be used by the binding.

This function is the language bindings friendly version of `g_object_bind_property_full()`, using GClosures instead of function pointers.

[rename-to g_object_bind_property_full]

#### Parameters

source | the source GObject. | [type GObject.Object]
source_property | the property on `source` to bind |
target | the target GObject. | [type GObject.Object]
target_property | the property on `target` to bind |
flags | flags to pass to GBinding |
transform_to | a GClosure wrapping the transformation function from the `source` to the `target` , or `NULL` to use the default |
transform_from | a GClosure wrapping the transformation function from the `target` to the `source` , or `NULL` to use the default |

#### Returns

the GBinding instance representing the binding between the two GObject instances. The binding is released whenever the GBinding reference count reaches zero.

[transfer none]

Since: 2.26

## Types and Values

### GBinding

    typedef struct _GBinding GBinding;

GBinding is an opaque structure whose members cannot be accessed directly.

Since: 2.26

### enum GBindingFlags

Flags to be passed to `g_object_bind_property()` or `g_object_bind_property_full()`.

This enumeration can be extended at later date.

#### Members

G_BINDING_DEFAULT | The default binding; if the source property changes, the target property is updated with its value. |
G_BINDING_BIDIRECTIONAL | Bidirectional binding; if either the property of the source or the property of the target changes, the other is updated. |
G_BINDING_SYNC_CREATE | Synchronize the values of the source and target properties when creating the binding; the direction of the synchronization is always from the source to the target. |
G_BINDING_INVERT_BOOLEAN | If the two properties being bound are booleans, setting one to `TRUE` will result in the other being set to `FALSE` and vice versa. This flag will only work for boolean properties, and cannot be used when passing custom transformation functions to `g_object_bind_property_full()`. |

Since: 2.26

## Property Details

### The `“flags”` property

      “flags”                    GBindingFlags

Flags to be used to control the GBinding

Owner: GBinding

Flags: Read / Write / Construct Only

Since: 2.26

### The `“source”` property

      “source”                   GObject *

The GObject that should be used as the source of the binding

Owner: GBinding

Flags: Read / Write / Construct Only

Since: 2.26

### The `“source-property”` property

      “source-property”          char *

The name of the property of “source” that should be used as the source of the binding.

This should be in canonical form to get the best performance.

Owner: GBinding

Flags: Read / Write / Construct Only

Default value: NULL

Since: 2.26

### The `“target”` property

      “target”                   GObject *

The GObject that should be used as the target of the binding

Owner: GBinding

Flags: Read / Write / Construct Only

Since: 2.26

### The `“target-property”` property

      “target-property”          char *

The name of the property of “target” that should be used as the target of the binding.

This should be in canonical form to get the best performance.

Owner: GBinding

Flags: Read / Write / Construct Only

Default value: NULL

Since: 2.26