Module javafx.graphics

Package javafx.scene.layout

Provides classes to support user interface layout. Each layout pane class supports a different layout strategy for its children and applications may nest these layout panes to achieve the needed layout structure in the user interface. Once a node is added to one of the layout panes, the pane will automatically manage the layout for the node, so the application should not position or resize the node directly; see "Node Resizability" for more details.

Scene Graph Layout Mechanism

The scene graph layout mechanism is driven automatically by the system once the application creates and displays a Scene. The scene graph detects dynamic node changes which affect layout (such as a change in size or content) and calls requestLayout(), which marks that branch as needing layout so that on the next pulse, a top-down layout pass is executed on that branch by invoking layout() on that branch's root. During that layout pass, the layoutChildren() callback method will be called on each parent to layout its children. This mechanism is designed to maximize layout efficiency by ensuring multiple layout requests are coalesced and processed in a single pass rather than executing re-layout on on each minute change. Therefore, applications should not invoke layout directly on nodes.

Node Resizability

The scene graph supports both resizable and non-resizable node classes. The isResizable() method on Node returns whether a given node is resizable or not. A resizable node class is one which supports a range of acceptable sizes (minimum <= preferred <= maximum), allowing its parent to resize it within that range during layout, given the parent's own layout policy and the layout needs of sibling nodes. Node supports the following methods for layout code to determine a node's resizable range: 


    public Orientation getContentBias()
    public double minWidth(double height)
    public double minHeight(double width)
    public double prefWidth(double height)
    public double prefHeight(double width)
    public double maxWidth(double height)
    public double maxHeight(double width)

Non-resizable node classes, on the other hand, do not have a consistent resizing API and so are not resized by their parents during layout. Applications must establish the size of non-resizable nodes by setting appropriate properties on each instance. These classes return their current layout bounds for min, pref, and max, and the resize() method becomes a no-op.


Resizable classes: Region, Control, WebView
Non-Resizable classes: Group, Shape, Text

For example, a Button control (resizable) computes its min, pref, and max sizes which its parent will use to resize it during layout, so the application only needs to configure its content and properties: 

    Button button = new Button("Apply");
However, a Circle (non-resizable) cannot be resized by its parent, so the application needs to set appropriate geometric properties which determine its size: 
    Circle circle = new Circle();
    circle.setRadius(50);

Resizable Range

Each resizable node class computes an appropriate min, pref, and max size based on its own content and property settings (it's 'intrinsic' size range). Some resizable classes have an unbounded max size (all layout panes) while others have a max size that is clamped by default to their preferred size (buttons) (See individual class documentation for the default range of each class). While these defaults are geared towards common usage, applications often need to explicitly alter or set a node's resizable range to achieve certain layouts. The resizable classes provide properties for overriding the min, pref and max sizes for this purpose.

For example, to override the preferred size of a ListView:

    listview.setPrefSize(200,300);

Or, to change the max width of a button so it will resize wider to fill a space: 

    button.setMaxWidth(Double.MAX_VALUE);

For the inverse case, where the application needs to clamp the node's min or max size to its preferred: 

    listview.setMaxSize(Region.USE_PREF_SIZE, Region.USE_PREF_SIZE);
And finally, if the application needs to restore the intrinsically computed values: 
    listview.setPrefSize(Region.USE_COMPUTED_SIZE, Region.USE_COMPUTED_SIZE);

CSS Styling and Node Sizing

Applications cannot reliably query the bounds of a resizable node until it has been added to a scene because the size of that node may be dependent on CSS. This is because CSS is used to style many aspects of a node which affect it's preferred size (font, padding, borders, etc) and so the node cannot be layed out (resized) until CSS has been applied and the parent can access valid size range metrics. This is always true for Controls (and any panes that contain them), because they rely on CSS for their default style, even if no user-level style sheets have been set. Stylesheets are set at the Scene level, which means that styles cannot even be determined until a node's enclosing scene has been initialized. Once a Scene is initialized, CSS is applied to nodes on each pulse (when needed) just before the layout pass.

Visual Bounds vs. Layout Bounds

A graphically rich user interface often has the need to make a distinction between a node's visual bounds and the bounds used for layout. For example, the tight visual bounds of a Text node's character glyphs would not work for layout, as the text would not be aligned and leading/trailing whitespace would be discounted. Also, sometimes applications wish to apply affects and transforms to nodes without disturbing the surrounding layout (bouncing, jiggling, drop shadows, glows, etc). To support this distinction in the scene graph, Node provides the layoutBounds property to define the 'logical' bounds of the node for layout and boundsInParent to define the visual bounds once all effects, clipping, and transforms have been applied.

These two bounds properties will often differ for a given node and layoutBounds is computed differently depending on the node class:

Bounds Computation Table
Node Type Layout Bounds
Shape,ImageView Includes geometric bounds (geometry plus stroke). Does NOT include effect, clip, or any transforms.
Text logical bounds based on the font height and content width, including white space. can be configured to be tight bounds around chars glyphs by setting boundsType. Does NOT include effect, clip, or any transforms.
Region, Control, WebView always [0,0 width x height] regardless of visual bounds, which might be larger or smaller than layout bounds.
Group Union of all visible childrens' visual bounds (boundsInParent) Does NOT include effect, clip, or transforms set directly on group, however DOES include effect, clip, transforms set on individual children since those are included in the child's boundsInParent.

So for example, if a DropShadow is added to a shape, that shadow will not be factored into layout by default. Or, if a ScaleTransition is used to pulse the size of a button, that pulse animation will not disturb layout around that button. If an application wishes to have the effect, clip, or transform factored into the layout of a node, it should wrap that node in a Group.