Package ch.bailu.gtk.gtk

Class Expression

java.lang.Object
ch.bailu.gtk.type.Type
ch.bailu.gtk.type.Pointer
ch.bailu.gtk.gtk.Expression
All Implemented Interfaces:
PointerInterface
Direct Known Subclasses:
CClosureExpression, ClosureExpression, ConstantExpression, ObjectExpression, PropertyExpression
public class Expression extends Pointer
`GtkExpression` provides a way to describe references to values.

An important aspect of expressions is that the value can be obtained
from a source that is several steps away. For example, an expression
may describe ‘the value of property A of `object1`, which is itself the
value of a property of `object2`’. And `object1` may not even exist yet
at the time that the expression is created. This is contrast to `GObject`
property bindings, which can only create direct connections between
the properties of two objects that must both exist for the duration
of the binding.

An expression needs to be "evaluated" to obtain the value that it currently
refers to. An evaluation always happens in the context of a current object
called `this` (it mirrors the behavior of object-oriented languages),
which may or may not influence the result of the evaluation. Use
[method@Gtk.Expression.evaluate] for evaluating an expression.

Various methods for defining expressions exist, from simple constants via
[ctor@Gtk.ConstantExpression.new] to looking up properties in a `GObject`
(even recursively) via [ctor@Gtk.PropertyExpression.new] or providing
custom functions to transform and combine expressions via
[ctor@Gtk.ClosureExpression.new].

Here is an example of a complex expression:

```c
color_expr = gtk_property_expression_new (GTK_TYPE_LIST_ITEM,
NULL, "item");
expression = gtk_property_expression_new (GTK_TYPE_COLOR,
color_expr, "name");
```

when evaluated with `this` being a `GtkListItem`, it will obtain the
"item" property from the `GtkListItem`, and then obtain the "name" property
from the resulting object (which is assumed to be of type `GTK_TYPE_COLOR`).

A more concise way to describe this would be

```
this->item->name
```

The most likely place where you will encounter expressions is in the context
of list models and list widgets using them. For example, `GtkDropDown` is
evaluating a `GtkExpression` to obtain strings from the items in its model
that it can then use to match against the contents of its search entry.
`GtkStringFilter` is using a `GtkExpression` for similar reasons.

By default, expressions are not paying attention to changes and evaluation is
just a snapshot of the current state at a given time. To get informed about
changes, an expression needs to be "watched" via a [struct@Gtk.ExpressionWatch],
which will cause a callback to be called whenever the value of the expression may
have changed; [method@Gtk.Expression.watch] starts watching an expression, and
[method@Gtk.ExpressionWatch.unwatch] stops.

Watches can be created for automatically updating the property of an object,
similar to GObject's `GBinding` mechanism, by using [method@Gtk.Expression.bind].

## GtkExpression in GObject properties

In order to use a `GtkExpression` as a `GObject` property, you must use the
[id@gtk_param_spec_expression] when creating a `GParamSpec` to install in the
`GObject` class being defined; for instance:

```c
obj_props[PROP_EXPRESSION] =
gtk_param_spec_expression ("expression",
"Expression",
"The expression used by the widget",
G_PARAM_READWRITE |
G_PARAM_STATIC_STRINGS |
G_PARAM_EXPLICIT_NOTIFY);
```

When implementing the `GObjectClass.set_property` and `GObjectClass.get_property`
virtual functions, you must use [id@gtk_value_get_expression], to retrieve the
stored `GtkExpression` from the `GValue` container, and [id@gtk_value_set_expression],
to store the `GtkExpression` into the `GValue`; for instance:

```c
// in set_property()...
case PROP_EXPRESSION:
foo_widget_set_expression (foo, gtk_value_get_expression (value));
break;

// in get_property()...
case PROP_EXPRESSION:
gtk_value_set_expression (value, foo->expression);
break;
```

## GtkExpression in .ui files

`GtkBuilder` has support for creating expressions. The syntax here can be used where
a `GtkExpression` object is needed like in a `<property>` tag for an expression
property, or in a `<binding name="property">` tag to bind a property to an expression.

To create a property expression, use the `<lookup>` element. It can have a `type`
attribute to specify the object type, and a `name` attribute to specify the property
to look up. The content of `<lookup>` can either be an element specfiying the expression
to use the object, or a string that specifies the name of the object to use.

Example:

```xml
<lookup name='search'>string_filter</lookup>
```

To create a constant expression, use the `<constant>` element. If the type attribute
is specified, the element content is interpreted as a value of that type. Otherwise,
it is assumed to be an object. For instance:

```xml
<constant>string_filter</constant>
<constant type='gchararray'>Hello, world</constant>
```

To create a closure expression, use the `<closure>` element. The `type` and `function`
attributes specify what function to use for the closure, the content of the element
contains the expressions for the parameters. For instance:

```xml
<closure type='gchararray' function='combine_args_somehow'>
<constant type='gchararray'>File size:</constant>
<lookup type='GFile' name='size'>myfile</lookup>
</closure>
```

https://docs.gtk.org/gtk4/class.Expression.html

  • Constructor Details

  • Method Details

    • getClassHandler

      public static ClassHandler getClassHandler()

    • bind

      public ExpressionWatch bind(@Nonnull Pointer target, @Nonnull Str property, @Nullable Pointer this_)
      Bind `target`'s property named `property` to `self`.

      The value that `self` evaluates to is set via `g_object_set()` on
      `target`. This is repeated whenever `self` changes to ensure that
      the object's property stays synchronized with `self`.

      If `self`'s evaluation fails, `target`'s `property` is not updated.
      You can ensure that this doesn't happen by using a fallback
      expression.

      Note that this function takes ownership of `self`. If you want
      to keep it around, you should [method@Gtk.Expression.ref] it beforehand.
      Parameters:
      target - the target object to bind to
      property - name of the property on `target` to bind to
      this_ - the this argument for the evaluation of `self`
      Returns:
      a `GtkExpressionWatch`

    • bind

      public ExpressionWatch bind(@Nonnull Pointer target, String property, @Nullable Pointer this_)
      Bind `target`'s property named `property` to `self`.

      The value that `self` evaluates to is set via `g_object_set()` on
      `target`. This is repeated whenever `self` changes to ensure that
      the object's property stays synchronized with `self`.

      If `self`'s evaluation fails, `target`'s `property` is not updated.
      You can ensure that this doesn't happen by using a fallback
      expression.

      Note that this function takes ownership of `self`. If you want
      to keep it around, you should [method@Gtk.Expression.ref] it beforehand.
      Parameters:
      target - the target object to bind to
      property - name of the property on `target` to bind to
      this_ - the this argument for the evaluation of `self`
      Returns:
      a `GtkExpressionWatch`

    • evaluate

      public boolean evaluate(@Nullable Pointer this_, @Nonnull Value value)
      Evaluates the given expression and on success stores the result
      in @value.

      The `GType` of `value` will be the type given by
      [method@Gtk.Expression.get_value_type].

      It is possible that expressions cannot be evaluated - for example
      when the expression references objects that have been destroyed or
      set to `NULL`. In that case `value` will remain empty and `FALSE`
      will be returned.
      Parameters:
      this_ - the this argument for the evaluation
      value - an empty `GValue`
      Returns:
      `TRUE` if the expression could be evaluated

    • getValueType

      public long getValueType()
      Gets the `GType` that this expression evaluates to.

      This type is constant and will not change over the lifetime
      of this expression.
      Returns:
      The type returned from [method@Gtk.Expression.evaluate]

    • isStatic

      public boolean isStatic()
      Checks if the expression is static.

      A static expression will never change its result when
      [method@Gtk.Expression.evaluate] is called on it with the same arguments.

      That means a call to [method@Gtk.Expression.watch] is not necessary because
      it will never trigger a notify.
      Returns:
      `TRUE` if the expression is static

    • ref

      public Expression ref()
      Acquires a reference on the given `GtkExpression`.
      Returns:
      the `GtkExpression` with an additional reference

    • unref

      public void unref()
      Releases a reference on the given `GtkExpression`.

      If the reference was the last, the resources associated to the `self` are
      freed.

    • watch

      public ExpressionWatch watch(@Nullable Pointer this_, Expression.OnExpressionNotify notify, @Nullable Pointer user_data, Expression.OnDestroyNotify user_destroy)
      Watch the given `expression` for changes.

      The @notify function will be called whenever the evaluation of `self`
      may have changed.

      GTK cannot guarantee that the evaluation did indeed change when the @notify
      gets invoked, but it guarantees the opposite: When it did in fact change,
      the @notify will be invoked.
      Parameters:
      this_ - the `this` argument to watch
      notify - callback to invoke when the expression changes
      user_data - user data to pass to the `notify` callback
      user_destroy - destroy notify for `user_data`
      Returns:
      The newly installed watch. Note that the only reference held to the watch will be released when the watch is unwatched which can happen automatically, and not just via [method@Gtk.ExpressionWatch.unwatch]. You should call [method@Gtk.ExpressionWatch.ref] if you want to keep the watch around.

    • getTypeID

      public static long getTypeID()

    • getParentTypeID

      public static long getParentTypeID()

    • getTypeSize

      public static TypeSystem.TypeSize getTypeSize()

    • getParentTypeSize

      public static TypeSystem.TypeSize getParentTypeSize()

    • getInstanceSize

      public static int getInstanceSize()