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33 lines
1.6 KiB
Markdown
33 lines
1.6 KiB
Markdown
# JustUI: Widget hierarchy
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## The tree hierarchy
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Every widget (either of type `jwidget` or of a structure type that begins with
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a `jwidget`) has a parent. This relationship forms a tree. The widgets that
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have a common parent *w* are called the children of *w*.
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The motivation for the widget hierarchy is to have groups of widgets behave as
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one. A complex UI element can have many widgets to implement its complex
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interface, but group them together as children of a parent and expose only the
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parent. This is an essential tool to build complex interfaces out of smaller
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components.
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When a widget is created, for instance with `jwidget_create()` or
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`jlabel_create()`, its parent is specified as the last parameter.
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In the tree one of the widgets is the ancestor of all the others, and is called
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the *root* of the scene. In JustUI the root of the tree is normally a `jscene`,
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because that is how event handling and keyboard input are managed.
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## Managing ownership
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Every widget *owns* its children with regards to memory allocation. This means
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that when a widget is destroyed, its children are destroyed along with it.
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Thus, even though the user program performs a function call for each widget to
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allocate in a scene, a single call freeing the root will destroy all of them.
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Whenever children are moved around, they change ownership. This is important
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because we always want the parent to outlive the children. Specifically, if a
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source widget holds a pointer of reference to a target widget, then the source
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should make sure that either the target is one of its parents, or it is
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informed when the target is destroyed.
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