If true, specifies to clip the children to the boundaries of the viewport. If false, the container children extend past the container boundaries, regardless of the size specification of the component.

The default value is false.

The DisplayObject that this layout uses for the drop indicator during a drag-and-drop operation. Typically you do not set this property directly, but instead define a dropIndicator skin part in the skin class of the drop target.

The List control sets this property in response to a DragEvent.DRAG_ENTER event. The List initializes this property with an instance of its dropIndicator skin part. The List clears this property in response to a DragEvent.DRAG_EXIT event.

The x coordinate of the origin of the viewport in the component's coordinate system, where the default value is (0,0) corresponding to the upper-left corner of the component. If clipAndEnableScrolling is true, setting this property typically causes the viewport to be set to:

      new Rectangle(horizontalScrollPosition, verticalScrollPosition, width, height)
      

The default value is 0.

This property can be used as the source for data binding.

The GroupBase container whose elements are measured, sized and positioned by this layout.

Subclasses may override the setter to perform target specific actions. For example a 3D layout may set the target's maintainProjectionCenter property here.

The default value is null.

See also

Used by layouts when fixed row/column sizes are requested but a specific size isn't specified. Used by virtual layouts to estimate the size of layout elements that have not been scrolled into view.

This property references a component that Flex uses to define the height of all container children, as the following example shows:

      <s:Group>
        <s:layout>
          <s:VerticalLayout variableRowHeight="false"
              typicalLayoutElement="{b3}"/> 
        </s:layout>
        <s:Button id="b1" label="Button 1"/>
        <s:Button id="b2" label="Button 2"/>
        <s:Button id="b3" label="Button 3" fontSize="36"/>
        <s:Button id="b4" label="Button 4" fontSize="24"/>
      </s:Group>

If this property has not been set and the target is non-null then the target's first layout element is cached and returned.

The default value is the target's first layout element.

The default value is null.

See also

A container can hold any number of children. However, each child requires an instance of an item renderer. If the container has many children, you might notice performance degradation as you add more children to the container.

Instead of creating an item renderer for each child, you can configure the container to use a virtual layout. With virtual layout, the container reuses item renderers so that it only creates item renderers for the currently visible children of the container. As a child is moved off the screen, possible by scrolling the container, a new child being scrolled onto the screen can reuse its item renderer.

To configure a container to use virtual layout, set the useVirtualLayout property to true for the layout associated with the container. Only DataGroup or SkinnableDataContainer with layout set to VerticalLayout, HorizontalLayout, or TileLayout supports virtual layout. Layout subclasses that do not support virtualization must prevent changing this property.

Note: The BasicLayout class throws a run-time error if you set useVirtualLayout to true.

When true, layouts that support virtualization must use the target.getVirtualElementAt() method, rather than getElementAt(), and must only get the elements they anticipate will be visible given the value of getScrollRect().

When true, the layout class must be able to compute the indices of the layout elements that overlap the scrollRect in its updateDisplayList() method based exclusively on cached information, not by getting layout elements and examining their bounds.

Typically virtual layouts update their cached information in the updateDisplayList() method, based on the sizes and locations computed for the elements in view.

Similarly, in the measure() method, virtual layouts should update the target's measured size properties based on the typicalLayoutElement and other cached layout information, not by measuring elements.

Containers cooperate with layouts that have useVirtualLayout = true by recycling item renderers that were previously constructed, but are no longer in use. An item is considered to be no longer in use if its index is not within the range of getVirtualElementAt() indices requested during the container's most recent updateDisplayList() invocation.

The default value is false.

See also

The y coordinate of the origin of the viewport in the component's coordinate system, where the default value is (0,0) corresponding to the upper-left corner of the component. If clipAndEnableScrolling is true, setting this property typically causes the viewport to be set to:

      new Rectangle(horizontalScrollPosition, verticalScrollPosition, width, height)
      

The default value is 0.

This property can be used as the source for data binding.

Constructor.

Calculates how much to scroll for the specified dropLocation during a drag and drop gesture. Called by the showDropIndicator() method to calculate the scroll during drag-scrolling.

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Returns the index where a new item should be inserted if the user releases the mouse at the specified coordinates while completing a drag and drop gesture. Called by the calculatedDropLocation() method.

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Calculates the bounds for the drop indicator that provides visual feedback to the user of where the items will be inserted at the end of a drag and drop gesture. Called by the showDropIndicator() method.

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Calculates the drop location in the data provider of the drop target for the specified dragEvent.

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When useVirtualLayout is true, this method can be used by the layout target to clear cached layout information when the target changes.

For example, when a DataGroup's dataProvider or itemRenderer property changes, cached elements sizes become invalid.

When the useVirtualLayout property changes to false, this method is called automatically.

Subclasses that support useVirtualLayout = true must override this method.

Called by the target after a layout element has been added and before the target's size and display list are validated. Layouts that cache per element state, like virtual layouts, can override this method to update their cache.

If the target calls this method, it's only guaranteeing that a a layout element will exist at the specified index at updateDisplayList() time, for example a DataGroup with a virtual layout will call this method when an item is added to the targets dataProvider.

By default, this method does nothing.

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This method must is called by the target after a layout element has been removed and before the target's size and display list are validated. Layouts that cache per element state, like virtual layouts, can override this method to update their cache.

If the target calls this method, it's only guaranteeing that a a layout element will no longer exist at the specified index at updateDisplayList() time. For example, a DataGroup with a virtual layout calls this method when an item is added to the dataProvider property.

By default, this method does nothing.

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Returns the specified element's layout bounds as a Rectangle or null if the index is invalid, the corresponding element is null, includeInLayout=false, or if this layout's target property is null.

Layout subclasses that support useVirtualLayout=true must override this method to compute a potentially approximate value for elements that are not in view.

Parameters

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Returns the bounds of the first layout element that either spans or is above the scrollRect's top edge.

This is a convenience method that is used by the default implementation of the getVerticalScrollPositionDelta() method. Subclasses that rely on the default implementation of getVerticalScrollPositionDelta() should override this method to provide an accurate bounding rectangle that has valid top and bottom properties.

By default this method returns a Rectangle with width=0, height=1, whose top edge is one less than the top edge of the scrollRect, and left=0.

Subclasses should override this method to provide an accurate bounding rectangle that has valid top and bottom properties.

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Returns the bounds of the first layout element that either spans or is below the scrollRect's bottom edge.

This is a convenience method that is used by the default implementation of the getVerticalScrollPositionDelta() method. Subclasses that rely on the default implementation of getVerticalScrollPositionDelta() should override this method to provide an accurate bounding rectangle that has valid top and bottom properties.

By default this method returns a Rectangle with width=0, height=1, whose bottom edge is one more than the bottom edge of the scrollRect, and left=0.

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Returns the bounds of the first layout element that either spans or is to the left of the scrollRect's left edge.

This is a convenience method that is used by the default implementation of the getHorizontalScrollPositionDelta() method. Subclasses that rely on the default implementation of getHorizontalScrollPositionDelta() should override this method to provide an accurate bounding rectangle that has valid left and right properties.

By default this method returns a Rectangle with width=1, height=0, whose left edge is one less than the left edge of the scrollRect, and top=0.

Parameters

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Returns the bounds of the first layout element that either spans or is to the right of the scrollRect's right edge.

This is a convenience method that is used by the default implementation of the getHorizontalScrollPositionDelta() method. Subclasses that rely on the default implementation of getHorizontalScrollPositionDelta() should override this method to provide an accurate bounding rectangle that has valid left and right properties.

By default this method returns a Rectangle with width=1, height=0, whose right edge is one more than the right edge of the scrollRect, and top=0.

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Returns the change to the horizontal scroll position to handle different scrolling options. These options are defined by the NavigationUnit class: END, HOME, LEFT, PAGE_LEFT, PAGE_RIGHT, and RIGHT.

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Delegation method that determines which item to navigate to based on the current item in focus and user input in terms of NavigationUnit. This method is used by subclasses of ListBase to handle keyboard navigation. ListBase maps user input to NavigationUnit constants.

Subclasses can override this method to compute other values that are based on the current index and key stroke encountered.

Parameters

Computes the verticalScrollPosition and horizontalScrollPosition deltas needed to scroll the element at the specified index into view.

This method attempts to minimize the change to verticalScrollPosition and horizontalScrollPosition.

If clipAndEnableScrolling is true and the element at the specified index is not entirely visible relative to the target's scroll rectangle, then return the delta to be added to horizontalScrollPosition and verticalScrollPosition that scrolls the element completely within the scroll rectangle's bounds.

Parameters

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Returns the bounds of the target's scroll rectangle in layout coordinates. Layout methods should not get the target's scroll rectangle directly.

Returns the change to the vertical scroll position to handle different scrolling options. These options are defined by the NavigationUnit class: DOWN, END, HOME, PAGE_DOWN, PAGE_UP, and UP.

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Hides the previously shown drop indicator, created by the showDropIndicator() method, removes it from the display list and also stops the drag scrolling.

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Measures the target's default size based on its content, and optionally measures the target's default minimum size.

This is one of the methods that you must override when creating a subclass of LayoutBase. The other method is updateDisplayList(). You do not call these methods directly. Flex calls this method as part of a layout pass. A layout pass consists of three phases.

First, if the target's properties are invalid, the LayoutManager calls the target's commitProperties method.

Second, if the target's size is invalid, LayoutManager calls the target's validateSize() method. The target's validateSize() will in turn call the layout's measure() to calculate the target's default size unless it was explicitly specified by both target's explicitWidth and explicitHeight properties. If the default size changes, Flex will invalidate the target's display list.

Last, if the target's display list is invalid, LayoutManager calls the target's validateDisplayList. The target's validateDisplayList will in turn call the layout's updateDisplayList method to size and position the target's elements.

When implementing this method, you must set the target's measuredWidth and measuredHeight properties to define the target's default size. You may optionally set the measuredMinWidth and measuredMinHeight properties to define the default minimum size. A typical implementation iterates through the target's elements and uses the methods defined by the ILayoutElement to accumulate the preferred and/or minimum sizes of the elements and then sets the target's measuredWidth, measuredHeight, measuredMinWidth and measuredMinHeight.

See also

Called when the verticalScrollPosition or horizontalScrollPosition properties change.

The default implementation updates the target's scrollRect property by calling updateScrollRect(). Subclasses can override this method to compute other values that are based on the current scrollPosition or scrollRect.

See also

Sizes, positions and parents the drop indicator based on the specified drop location. Use the calculateDropLocation() method to obtain the DropLocation object.

Starts/stops drag-scrolling when necessary conditions are met.

Parameters

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Sizes and positions the target's elements.

This is one of the methods that you must override when creating a subclass of LayoutBase. The other method is measure(). You do not call these methods directly. Flex calls this method as part of a layout pass. A layout pass consists of three phases.

First, if the target's properties are invalid, the LayoutManager calls the target's commitProperties method.

Second, if the target's size is invalid, LayoutManager calls the target's validateSize() method. The target's validateSize() will in turn call the layout's measure() to calculate the target's default size unless it was explicitly specified by both target's explicitWidth and explicitHeight properties. If the default size changes, Flex will invalidate the target's display list.

Last, if the target's display list is invalid, LayoutManager calls the target's validateDisplayList. The target's validateDisplayList will in turn call the layout's updateDisplayList method to size and position the target's elements.

A typical implementation iterates through the target's elements and uses the methods defined by the ILayoutElement to position and resize the elements. Then the layout must also calculate and set the target's contentWidth and contentHeight properties to define the target's scrolling region.

Parameters

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Called by the target at the end of its updateDisplayList to have the layout update its scrollRect.

If clipAndEnableScrolling is true, the default implementation sets the origin of the target's scrollRect to verticalScrollPosition, horizontalScrollPosition. It sets its size to the width, height parameters (the target's unscaled width and height).

If clipAndEnableScrolling is false, the default implementation sets the scrollRect to null.

Parameters

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