Now VGroup and VerticalLayout have a “verticalAlign” property with the three supported values of “top“, “middle“, “bottom“. Accordingly HGroup and HorizontalLayout get a “horizontalAlign” property with the supported values of “left“, “center” and “right“. The alignment modes work for the layouts with and without virtualization enabled.
An important difference to note is that when “clipAndEnableScrolling” is set to “true” and the containers are scrolling (the content is bigger than the container), the Spark layouts ensure that the entire content can be scrolled in view by internally reverting to the “top“/”left” alignment respectively.

When playing with the attached sample, you’ll notice difference between the clipping and the non-clipping containers when scrolling is in effect (drag the slider left to force smaller height for the containers).

The sample was compiled with SDK version 4.0.0.13553, source: MajorAxisAlignment.mxml

Let me show off the end result first:

And the source code is available here.

Update: The source has been updated to compile with Flash Builder / Flex SDK beta 2. I also added a horizontalAlign property, just because I can =)

Adding a Layout Property

I want to implement two properties – a horizontalGap for between the elements within a row, and verticalAlign which will control how the elements within the row are aligned. For the latter, I’ll have “top”, “middle” and “bottom” as options, and of course I’ll have to beef up the FlowLayout to support elements with various heights.

I’ll add the properties directly to the FlowLayout class. When the value changes, I need to invalidate the container which will cause the layout to run its updateDisplayList() and at that point I’ll take into account the new values:

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    //---------------------------------------------------------------
    //  horizontalGap
    //---------------------------------------------------------------
 
    private var _horizontalGap:Number = 10;
 
    public function set horizontalGap(value:Number):void
    {
        _horizontalGap = value;
 
        // We must invalidate the layout
        var layoutTarget:GroupBase = target;
        if (layoutTarget)
            layoutTarget.invalidateDisplayList();
    }
 
    //---------------------------------------------------------------
    //  verticalAlign
    //---------------------------------------------------------------
 
    private var _verticalAlign:String = "bottom";
 
    public function set verticalAlign(value:String):void
    {
        _verticalAlign = value;
 
        // We must invalidate the layout
        var layoutTarget:GroupBase = target;
        if (layoutTarget)
            layoutTarget.invalidateDisplayList();
    }

Then I will add a drop-down and a slider in the flowTest.mxml to configure the the properties like this:

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    <!-- The slider to control the horizontal gap -->
    <s:HSlider id="hGapSlider" minimum="0" maximum="50"
                         value="10" liveDragging="true"/>
 
    <!-- The drop-down to select vertical alignment -->
    <s:DropDownList id="vAlign" requiresSelection="true">
        <s:ArrayCollection>
            <fx:String>bottom</fx:String>
            <fx:String>middle</fx:String>
            <fx:String>top</fx:String>
        </s:ArrayCollection>
    </s:DropDownList>                         
 
    <!-- A Spark List -->
    <s:List id="list1" width="{widthSlider.value}"
            selectedIndex="7"
            dataProvider="{new ArrayCollection(
            'The quick fox jumped over the lazy sleepy\n\dog'.split(' '))}">
 
        <!-- Configure the layout to be the FlowLayout -->
        <s:layout>
            <my:FlowLayout1 verticalAlign="{vAlign.selectedItem}"
                            horizontalGap="{hGapSlider.value}"/>
        </s:layout>
    </s:List>

Then the harder part – actually implementing the support for these properties in the FlowLayout’s updateDisplayList() method.  Since I’ll need to know the row height before I can decide how to position the elements, I’ll have to replace my original loop with a little more complicated code, something along the lines of:

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        // loop through the elements
        // while we can start a new row
        var rowStart:int = 0;
        while (rowStart < count)
        {
            // The row always contains the start element
            element = layoutTarget.getElementAt(rowStart);
            var rowWidth:Number = element.getPreferredBoundsWidth();
            var rowHeight:Number =  element.getPreferredBoundsHeight();
 
            // Find the end of the current row
            var rowEnd:int = rowStart;
            while (rowEnd + 1 < count)
            {
                element = layoutTarget.getElementAt(rowEnd + 1);
 
                // Since we haven't resized the element just yet, get its preferred size
                elementWidth = element.getPreferredBoundsWidth();
                elementHeight = element.getPreferredBoundsHeight();
 
                // Can we add one more element to this row?
                if (rowWidth + hGap + elementWidth > containerWidth)
                    break;
 
                rowWidth += hGap + elementWidth;
                rowHeight = Math.max(rowHeight, elementHeight);
                rowEnd++;
            }
 
            // Layout all the elements within the row
            for (var i:int = rowStart; i <= rowEnd; i++)
            {
                element = layoutTarget.getElementAt(i);
                ...
                // Position the element
                element.setLayoutBoundsPosition(x, y + elementY);
            }
 
            // Next row will start with the first element after the current row's end
            rowStart = rowEnd + 1;
 
            // Update the position to the beginning of the row
            ...
        }

Enabling Scrolling

Now I’m almost done, but I want the FlowLayout to be scrollable. In theory this would mean that I need do the following steps:

1. Add scroll-bars.
2. Hook up the scroll-bars with the layout’s horizontalScrollPosition and verticalScrollPosition properties.
3. Calculate the ranges for the scroll-bars and keep them in sync whenever the values change.

In practice, I can save a lot of work by using a ready stock Spark component – the Scroller. Without going into too much details, the Scroller can wrap a container and automatically put up scroll-bars, keeping everything in sync with the container’s layout.  And since in my flowTest.mxml I’m using a List, the Scroller is already in there!!! It’s a part of the List default skin   Now all that’s left to do for me is actually calculate the ranges for the scroll bars. This is basically the total width and height of the scrollable area of the container a.k.a. the “content size”.

Calculating the content size is easy enough – I’ll just find the maximum extents of all the children while I’m resizing and arranging them in the FlowLayout’s updateDisplayList(). At the very end of that method I’ll set the updated content size like this:

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        // Set the content size which determines the scrolling limits
        // and is used by the Scroller to calculate whether to show up
        // the scrollbars when the the scroll policy is set to "auto"
        layoutTarget.setContentSize(maxRowWidth, y);

That’s it!

I want to show how easy it is to create a custom Spark layout.  I’ve chosen a flow layout example where all the elements are arranged horizontally, wrapping to the next row when the width limit of the container is reached.  For simplicity I’m going to assume that all elements are of equal height.

Update: Check out part 2 of the article here.

Step 1 – Simple List Application with a Stock TileLayout

For starter, let’s get a simple application going with a Spark List with a few items. I’m going to add a slider that controls the width of the List. By default the List’s layout is the VerticalLayout, but I’m going to switch it to the stock TileLayout just to contrast it with the FlowLayout that I’m going to put together further below.

And the source code:

FlowTest.mxml

Step 2 – A Custom Layout Skeleton Class

Next step is creating the skeleton for the custom layout. I’ll create a new AS class FlowLayout.as, and since it is a custom layout, it must subclass LayoutBase.  The LayoutBase provides a lot of default functionality, but the most important methods I need to override are updateDisplayList() and measure().

• updateDisplayList() is called by the container to size and arrange the container’s elements.
• measure() is called to calculate the default size of the container based on the container’s elements. I’m not going to override measure(), as the List skin already has default size.

FlowLayout.as

Step 3 – Make Sure It Does Something

Now I’m going to iterate through all of the container’s elements.  I’ll access the container through the layout’s “target” property, which is of type GroupBase.  To work with the elements I’ll be using the ILayoutElement interface, specifically the following APIs:

• setLayoutBoundsSize(width, height) – where I can also pass in NaN leaving the element to size itself to its preferred size.
• getLayoutBoundsWidth(), getLayoutBoundsHeight() – to get the current bounds of the element.
• setLayoutBoundsPosition(x, y) – to position the element.  Coordinates are relative to the container’s origin.

Before delving into too much coding, let’s just size every element to it’s preferred size, stagger them vertically by 10 pixels each and setup the List to use the the new FlowLayout.

And the source code:

FlowLayout.as

The update to the flowText.mxml

Final Step – Finish the FlowLayout

And now let’s finish the FlowLayout – I want to arrange elements left to right, space them by 10 pixels, and move the elements down when there’s not enough space to fit in the containerWidth.  With a couple of lines I get the final result:

And the final code looks like this:

Final source code of the FlowLayout.as

Hope you enjoyed it!

Download the source files here.