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