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Getting Started with Flex 3 by Emily Kim, Jack D. Herrington

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Chapter 4. Flex Controls and Layout

The first step in building a Flex application is to create the user interface. Flex not only has a rich set of controls. It also has a complete set of layout mechanisms that make it easy to build user interfaces that look good and can scale appropriately as the display area of the Flash application is resized.

This chapter covers both layout mechanisms and controls. We will start by covering the layout mechanisms, and then we will discuss the available controls.

The Application Container

At the root of a Flex application is a single container, called the Application container, which holds all other containers and components. The Application container lays out all its children vertically by default (when the layout property is not specifically defined). There are three possible values for the Application component’s layout property:

vertical

Lays out each child component vertically from the top of the application to the bottom in the specified order

horizontal

Lays out each child component horizontally from the left of the application to the right in the specified order

absolute

Does no automatic layout, and requires you to explicitly define the location of each child component

If the Application component’s layout property is absolute, each child component must have an x and y coordinate defined; otherwise, the component will be displayed in the (0,0) position.

The Application container can also be formatted using any of the several style parameters that are available, including backgroundGradientColors and verticalGap. In Example 4-1, the Application tag is used to lay out the child controls.

Example 4-1. The Application MXML
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 backgroundGradientColors="[#FFFFFF, #FFDE00]" verticalGap="15"
 layout="horizontal">
 <mx:Image source="assets/animals03.jpg" />
 <mx:Label text="Photographed by Elsie Weil" fontSize="15" 
   fontWeight="bold" />
</mx:Application>

Figure 4-1 shows the result of this code.

Controls using the Application container
Figure 4-1. Controls using the Application container

The Box Class

The Box class is the base class for the VBox and HBox classes:

  • The VBox container renders all child display objects vertically.

  • The HBox container renders all child display objects horizontally.

The Application object behaves like a VBox by default (vertical layout), but you can also set it to use absolute or horizontal layout.

VBox and HBox flow like HTML, only in one direction.

Example 4-2 shows the default layout method used by the VBox container (vertical).

Example 4-2. Using the VBox container
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 backgroundColor="#FFFFFF" backgroundAlpha="0">
 <mx:VBox>
  <mx:Button label="&lt; prev" left="10" top="120" />
  <mx:Image source="assets/animals03.jpg" horizontalCenter="0" 
    top="30"/>
  <mx:Label text="Photographed by Elsie Weil" 
    horizontalCenter="0" top="250"/>
  <mx:Button label="next &gt;" right="10" top="120"/>
 </mx:VBox>
</mx:Application>

Figure 4-2 shows the result of this code.

A VBox layout
Figure 4-2. A VBox layout

Example 4-3 shows the default layout method used by the HBox container (horizontal).

Example 4-3. Using the HBox container
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 backgroundColor="#FFFFFF" backgroundAlpha="0">
 <mx:HBox>
  <mx:Button label="&lt; prev" left="10" top="120" />
  <mx:Image source="assets/animals03.jpg" horizontalCenter="0" 
    top="30"/>
  <mx:Label text="Photographed by Elsie Weil" 
    horizontalCenter="0" top="250"/>
  <mx:Button label="next &gt;" right="10" top="120"/>
 </mx:HBox>
</mx:Application>

Figure 4-3 shows the result.

An HBox layout
Figure 4-3. An HBox layout

You can also use both VBox and HBox to achieve a desired layout. For instance, Example 4-4 nests an HBox inside a VBox, demonstrating that container controls can have other containers as children.

Example 4-4. Using both the VBox and the HBox containers
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 backgroundColor="#FFFFFF" backgroundAlpha="0">
 <mx:VBox horizontalAlign="center" verticalGap="15">
   <mx:HBox verticalAlign="middle" horizontalGap="15">
     <mx:Button label="&lt; prev" left="10" top="120" />
     <mx:Image source="assets/animals03.jpg" 
       horizontalCenter="0" top="30"/>
     <mx:Button label="next &gt;" right="10" top="120"/>
   </mx:HBox>
   <mx:Label text="Photographed by Elsie Weil" 
     horizontalCenter="0" top="250"/>
 </mx:VBox>
</mx:Application>

Figure 4-4 shows the result of Example 4-4.

A combination VBox and HBox layout
Figure 4-4. A combination VBox and HBox layout

The Canvas Container (Absolute Positioning)

Canvas is the only container that lets you explicitly specify the location of its children within the container. The Canvas object has only one layout value: absolute. You can use the x and y properties of child components for pixel-perfect layouts. If the display window is resized, the child components stay fixed in place and may appear cut off. Using absolute positioning you can make child controls overlap if desired.

Example 4-5 is some sample code for an absolutely positioned layout.

Example 4-5. An absolutely positioned layout
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 layout="absolute" backgroundColor="#FFFFFF" 
   backgroundAlpha="0">
 <mx:Canvas x="23" y="34">
  <mx:Button label="&lt; prev" x="4" y="97" />
  <mx:Image source="assets/animals03.jpg" x="85" y="4" />
  <mx:Label text="Photographed by Elsie Weil" x="85" 
    y="230" />
  <mx:Button label="next &gt;" x="381" y="97" />
 </mx:Canvas>
</mx:Application>

Figure 4-5 shows the result.

An absolutely positioned image
Figure 4-5. An absolutely positioned image

The Canvas Container (Relative Positioning)

With relative positioning, also called constraint-based layout, you can anchor the sides or center of a component to positions which are relative to the component’s container. The size and position of the components change when the user resizes the application window. The container’s layout property must be set to absolute. All constraints are set relative to the edges of the container, not to other controls in the container. The left, right, top, bottom, horizontalCenter, and verticalCenter properties are anchors in constraint-based layouts.

Example 4-6 shows the code for positioning children in a constraint-based layout using the top, bottom, left, right, horizontalCenter, and verticalCenter styles.

Example 4-6. Photo.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 layout="absolute backgroundColor="#FFFFFF" 
   backgroundAlpha="0">
 <mx:HDividedBox width="100%" height="300">
  <mx:Canvas backgroundColor="#FFFFCC" width="150" 
    height="100%">
   <mx:Label text="Adjust this section" left="15" />
  </mx:Canvas>
  <mx:Canvas>
   <mx:Button label="&lt; prev" left="10" top="120"/>
   <mx:Image source="animals03.jpg" horizontalCenter="0" 
     top="30"/>
   <mx:Label text="Photographed by Elsie Weil" 
     horizontalCenter="0" top="250"/>
   <mx:Button label="next &gt;" right="10" top="120"/>
  </mx:Canvas>
 </mx:HDividedBox>
</mx:Application>

When you launch this application you should see something similar to Figure 4-6.

A constraint-based layout
Figure 4-6. A constraint-based layout

You can adjust the size of the panel on the right by grabbing the control and moving the mouse to the left or right. This will move the image to match the size of the panel.

The Form Container

The Form container lets you control the layout of a form, mark form fields as required or, optionally, handle error messages, and bind your form data to the Flex data model to perform data checking and validation.

The Form container, like all containers, encapsulates and lays out its children. The Form container controls the size and layout of the contents of the form. The FormHeader defines a heading for your Form. Multiple FormHeading controls are allowed. A FormItem container specifies a Form element consisting of the following parts:

  • A single label

  • One or more child controls or containers, such as input controls

You can also insert other types of components into a Form container.

The code in Example 4-7 demonstrates use of a Form container control.

Example 4-7. CommentForm.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 layout="absolute" backgroundColor="#FFFFFF" 
   backgroundAlpha="0">
 <mx:Form x="50" y="50" verticalGap="15">
  <mx:FormHeading label="Send us comments" />
  <mx:FormItem label="Full Name:">
   <mx:TextInput id="fullName" />
  </mx:FormItem>
  <mx:FormItem label="Email:">
   <mx:TextInput id="email" />
  </mx:FormItem>
  <mx:FormItem label="Comments:">
   <mx:TextArea id="comments" />
  </mx:FormItem>
  <mx:FormItem>
   <mx:Button id="submit" label="submit" />
  </mx:FormItem>
  </mx:Form>
</mx:Application>

Figure 4-7 shows the result of this code.

A form-based layout
Figure 4-7. A form-based layout

Combined Layouts

Containers can hold other containers. You can nest them to create sophisticated layouts, and you can create custom components that are made up of existing components. Example 4-8 shows an example of a complex nested layout. You should take care to use these container classes wisely and not to overuse them. Using too many nested containers can be the cause of performance problems in your application.

Example 4-8. A complex nested layout
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 backgroundColor="#000000" layout="horizontal" 
   horizontalGap="25">
 <mx:Style> Panel { backgroundAlpha: 1; borderAlpha: 1;
   headerColors: #c7c7c7, #ffffff;
   footerColors: #ffffff, #c7c7c7;
   paddingTop: 15; paddingRight: 15;
   paddingLeft: 15; paddingBottom: 15;
   shadowDirection: "right"; }
  .header { color: #ffffff; fontSize: 15;
   fontWeight: "bold"; }</mx:Style>
 <mx:VBox verticalGap="10">
  <mx:Panel title="Featured Photograph">
   <mx:Image source="assets/animals03.jpg" horizontalCenter="0" 
     top="30" />
   <mx:Label text="Photographed by Elsie Weil" 
     horizontalCenter="0" top="250" />
  </mx:Panel>
  <mx:Panel title="Provide feedback">
   <mx:Form x="50" y="50" verticalGap="15">
   <mx:FormHeading label="Send us comments" />
   <mx:FormItem label="Full Name:"><mx:TextInput 
     id="fullName" />
   </mx:FormItem>
   <mx:FormItem label="Email:"><mx:TextInput id="email" />
   </mx:FormItem>
   <mx:FormItem label="Comments:"><mx:TextArea 
     id="comments" />
   </mx:FormItem>
   <mx:FormItem><mx:Button id="submit" label="submit" />
   </mx:FormItem>
   </mx:Form>
  </mx:Panel>
 </mx:VBox>
 <mx:VBox verticalGap="25">
  <mx:Canvas>
   <mx:Label text="Category: Animals" styleName="header"  />
   <mx:Image source="assets/animals03_sm.jpg" y="30" />
   <mx:Image source="assets/animals08_sm.jpg" y="120" />
   <mx:Image source="assets/animals09_sm.jpg" y="120" 
     x="120" />
   <mx:Image source="assets/animals10_sm.jpg" y="120" 
     x="240" />
   <mx:Image source="assets/animals11_sm.jpg" y="211" />
   <mx:Image source="assets/animals12_sm.jpg" y="211" 
     x="120" />
   <mx:Image source="assets/animals06_sm.jpg" y="30" 
     x="120" />
   <mx:Image source="assets/animals07_sm.jpg" y="30" 
     x="240" />
  </mx:Canvas>
  <mx:Canvas>
   <mx:Label text="Category: Cities" styleName="header" />
   <mx:Image source="assets/city01_sm.jpg" y="30" />
   <mx:Image source="assets/city02_sm.jpg" y="30"  x="120"/>
   <mx:Image source="assets/city03_sm.jpg" y="30" x="240" />
   <mx:Image source="assets/city04_sm.jpg" y="120" x="0" />
  </mx:Canvas>
 </mx:VBox>
</mx:Application>

Figure 4-8 shows the result of Example 4-8.

A complex layout using various types of layout mechanisms
Figure 4-8. A complex layout using various types of layout mechanisms

The Panel Container

The Panel container consists of a title bar, a caption, a status message, a border, and a content area for its children. You can use Panel containers to wrap self-contained application modules. You can control the display layout by using the layout property set to vertical (the default), horizontal, or absolute. Each child must have its x and y positions set when using an absolute layout, or they must use anchors for a constraint-based layout.

Example 4-9 shows a sample Panel layout.

Example 4-9. Photo2.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
 backgroundGradientColors="[#FFFFFF, #000000]">
 <mx:Panel title="Featured Photograph"
  backgroundAlpha=".25" borderAlpha="1"
  headerColors="[#c7c7c7, #ffffff]"
  footerColors="[#ffffff, #c7c7c7]"
  paddingTop="15" paddingRight="15" paddingLeft="15" 
    paddingBottom="15"
  shadowDirection="right">
   <mx:Image source="assets/animals03.jpg" 
     horizontalCenter="0" top="30" />
   <mx:Label text="Photographed by Elsie Weil" 
     horizontalCenter="0" top="250" />
 </mx:Panel>
</mx:Application>

Figure 4-9 shows this Panel-based layout.

A layout using the Panel container
Figure 4-9. A layout using the Panel container

In addition to panels, you also can use a TitleWindow class to provide windowing-style functionality. This can come in handy when you want to bring up an alert, or a modal dialog.

Controls

So many controls are available for you to use with Flex that it’s almost hard to know where to begin. I suppose the best place to start is with the basic controls, such as labels, buttons, checkboxes, and so on. Example 4-10 shows an MXML application that provides a heaping helping of the basic control types.

Example 4-10. Buttons2.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="horizontal">
<mx:VBox horizontalAlign="left">
 <mx:Label text="Text label" />
 <mx:Label htmlText="&lt;b&gt;HTML&lt;/b&gt; text" />
 <mx:Button label="Button" />
 <mx:CheckBox label="Check box" />
 <mx:RadioButtonGroup id="cardType"/>
 <mx:RadioButton label="Visa" groupName="cardType" />
 <mx:RadioButton label="MasterCard" groupName="cardType"/>
 <mx:ComboBox dataProvider="{['a','b','c']}" />
 <mx:HSlider />
 <mx:TextInput />
</mx:VBox>
<mx:VBox horizontalAlign="left">
 <mx:List dataProvider="{['a','b','c']}" width="200" 
   height="100" />
 <mx:ButtonBar dataProvider="{['a','b','c']}" />
 <mx:NumericStepper />
 <mx:Image source="@Embed('megan.jpg')" />
</mx:VBox>
</mx:Application>

When I run this in Flex Builder I see Figure 4-10.

A collection of the basic control types
Figure 4-10. A collection of the basic control types

As you would expect, in addition to these controls, you also have available labels with flat text and HTML, push buttons, checkboxes and radio boxes, combos, text inputs, and lists, as well as some cool new controls such as sliders, numeric steppers, button bars, and images, among others.

Data Grids

We regularly have to build tables of structured information. This is easy in Flex, thanks to two controls: the DataGrid and the AdvancedDataGrid. I’ll start by showing the DataGrid control (see Example 4-11).

Example 4-11. Datagrid.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="vertical">
 <mx:XMLList id="employees">
  <employee>
   <name>Christina Coenraets</name>
   <phone>555-219-2270</phone>
   <email>ccoenraets@fictitious.com</email>
   <active>true</active>
  </employee>
  ...
 </mx:XMLList>
 <mx:DataGrid width="100%" height="100%" dataProvider=
   "{employees}">
  <mx:columns>
   <mx:DataGridColumn dataField="name" headerText="Name"/>
   <mx:DataGridColumn dataField="phone" headerText="Phone"/>
   <mx:DataGridColumn dataField="email" headerText="Email"/>
  </mx:columns>
 </mx:DataGrid>
</mx:Application>

When I run this in Flex Builder I see Figure 4-11.

A simple data grid
Figure 4-11. A simple data grid

You don’t even have to define the columns in the DataGrid unless you want to. The DataGrid control is smart enough to detect the columns from the data and set itself up if you haven’t defined the columns.

The AdvancedDataGrid is just like the DataGrid but with a more powerful set of features. For example, it has the ability to roll up sections of the data and provide users with spinners so that they can drill down into the data.

Example 4-12 shows AdvancedDataGrid in action.

Example 4-12. Advgrid.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="vertical">
<mx:Script>
<![CDATA[
import mx.collections.ArrayCollection;
[Bindable]
private var dpHierarchy:ArrayCollection = new ArrayCollection([
  {Region:"Southwest", children: [ ... ]}
]);
]]>
</mx:Script>

<mx:AdvancedDataGrid width="100%" height="100%">
 <mx:dataProvider>
  <mx:HierarchicalData source="{dpHierarchy}"/>
 </mx:dataProvider>
 <mx:columns>
  <mx:AdvancedDataGridColumn dataField="Region"/>
  <mx:AdvancedDataGridColumn dataField="Territory_Rep"
   headerText="Territory Rep"/>
  <mx:AdvancedDataGridColumn dataField="Actual"/>
  <mx:AdvancedDataGridColumn dataField="Estimate"/>
 </mx:columns>
</mx:AdvancedDataGrid>
</mx:Application>

When I run this in the browser and click around a little bit I get something similar to Figure 4-12.

The advanced data grid
Figure 4-12. The advanced data grid

As with any control, you can use the itemRenderer functionality in Flex Builder to format each cell however you choose.

In-Place Editing

The DataGrid control also allows for editing cell contents in place. Example 4-13 shows just how easy this is.

Example 4-13. Edit_table.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="vertical">
  <mx:XMLList id="customers" xmlns="">
    <customer><first>Jack</first>
    <last>Herrington</last></customer>
    <customer><first>Lori</first>
    <last>Herrington</last></customer>
    <customer><first>Megan</first>
    <last>Herrington</last></customer>
  </mx:XMLList>
  <mx:DataGrid dataProvider="{customers}" editable="true">
  <mx:columns>
    <mx:DataGridColumn dataField="first" />
    <mx:DataGridColumn dataField="last" />
  </mx:columns>
  </mx:DataGrid>
</mx:Application>

All I needed to do was add the editable attribute to the DataGrid and set it to true.

When I bring this up in the browser and double-click on a cell, I see something similar to Figure 4-13.

The editable grid
Figure 4-13. The editable grid

Of course, to make the example functional I would need to listen to the editing events and update the backend data store to match.

By default, the DataGrid uses a text editor to edit the cell contents, but you can provide your own editor renderer to use whatever controls you like to edit the value in the cell.

Tabs and Accordions

Sometimes you have more content than you can reasonably fit on the screen, so you need some way to let the user navigate around groupings of content. Flex provides several solutions, two of which, tabs and accordions, I’ll demonstrate here.

Tabs are very easy to create, as you can see in Example 4-14.

Example 4-14. Tabs.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="vertical">
 <mx:TabNavigator borderStyle="solid" width="100%" 
   height="100%">
   <mx:VBox label="Tab One">
     <mx:Label text="Tab one's content" />
   </mx:VBox>
   <mx:VBox label="Tab Two">
     <mx:Label text="Tab two's content" />
   </mx:VBox>
   <mx:VBox label="Tab Three">
     <mx:Label text="Tab three's content" />
   </mx:VBox>
 </mx:TabNavigator>
</mx:Application>

When I run this example from Flex Builder I see Figure 4-14.

The tab control
Figure 4-14. The tab control

Yep, it’s really that easy. And you can reskin the tabs to be in whatever form you please with CSS and skinning (more on that shortly).

An accordion works exactly the same way, as you can see in Example 4-15.

Example 4-15. Accord.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="vertical">
 <mx:Accordion borderStyle="solid” width="100%” height="100%">
   <mx:VBox label="Tab One">
     <mx:Label text="Tab one's content" />
   </mx:VBox>
   <mx:VBox label="Tab Two">
     <mx:Label text="Tab two's content" />
   </mx:VBox>
   <mx:VBox label="Tab Three">
     <mx:Label text="Tab three's content" />
   </mx:VBox>
 </mx:Accordion>
</mx:Application>

All I did was change the tag name from TabNavigator to Accordion and the example works, as you can see in Figure 4-15. These are just two of the controls that you can use to manage the presentation of large sets of interface elements in a way that doesn’t overwhelm the user.

The Accordion control
Figure 4-15. The Accordion control

Flex also has support for menus, including those that appear at the top of the window as well as pop-up menus. Example 4-16 shows how to create a menu bar along the top of the window.

Example 4-16. Menu.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="absolute">
 <mx:MenuBar labelField="@label">
  <mx:XMLList>
   <menuitem label="File">
    <menuitem label="New" />
    <menuitem label="Open"/>
   </menuitem>
   <menuitem label="Edit"/>
   <menuitem label="Source"/>
  </mx:XMLList>
 </mx:MenuBar>
</mx:Application>

When I run this in Flex Builder I see something similar to Figure 4-16.

An example menu
Figure 4-16. An example menu

There is also a handy control called ApplicationControlBar that gives a nice-looking control set along the top of the window. Example 4-17 is the code for a sample ApplicationControlBar.

Example 4-17. Appbar.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="vertical">
<mx:ApplicationControlBar dock="true">
    <mx:ButtonBar dataProvider="{['People','Places',
      'Things']}" />
</mx:ApplicationControlBar>
</mx:Application>

When I launch this example in Flex Builder I see the nice presentation shown in Figure 4-17.

An application control bar
Figure 4-17. An application control bar

One thing I love about Flex is that even by default, it looks really good. I’m not a graphic designer by any stretch, so I like the fact that Figure 4-17 looks very slick but required absolutely no effort on my part.

Divider Boxes

Flex provides an easy way for your users to customize their own layout with divider boxes. The code in Example 4-18 shows just how easy it is to use a divider box.

Example 4-18. Divbox.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="horizontal">
<mx:HDividedBox width="100%" height="100%">
<mx:HBox backgroundColor="#ff9999" width="50%" height="100%" 
  borderStyle="solid">
    <mx:Label text="Left part" />
</mx:HBox>
<mx:HBox width="50%" height="100%" borderStyle="solid">
    <mx:Label text="Right part" />
</mx:HBox>
</mx:HDividedBox>
</mx:Application>

When I run this in Flex Builder I see something similar to Figure 4-18.

Two sections divided by an adjustable divider
Figure 4-18. Two sections divided by an adjustable divider

I can the drag the divider control to adjust the size of the left and right parts to match my needs.

CSS

The best way to control the look of your Flex application is through CSS. If you are familiar with CSS for HTML you will find the CSS that’s supported by Flex to be very familiar.

To demonstrate I’ll take a very simple data entry form, make the font size huge, and change the colors of the text inputs based on CSS classes (see Example 4-19).

Example 4-19. CSS.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="absolute">
<mx:Style>
Application { font-size: 30; }
TextInput { color: #0000ff; }
.important { color: #ff0000; }
</mx:Style>
  <mx:Form>
    <mx:FormItem label="First Name">
      <mx:TextInput id="first" width="300" />
    </mx:FormItem>
    <mx:FormItem label="Last Name">
      <mx:TextInput id="last" width="300" />
    </mx:FormItem>
    <mx:FormItem label="Email">
      <mx:TextInput id="email" styleName="important" 
        width="300" />
    </mx:FormItem>
    <mx:FormItem>
      <mx:Button label="Subscribe" />
    </mx:FormItem>
  </mx:Form>
</mx:Application>

The CSS styles are defined in the mx:Styles tag. I’ve defined three classes. The Application class, which controls all of the content within the Application tag, increases the font size. For the TextInput colors I specify that the text should be blue. For any control of the class important, the color should be red.

When I launch this in Flex Builder I see Figure 4-19 in my browser.

A simple CSS example
Figure 4-19. A simple CSS example

In this example, I’ve defined the CSS inline, but you can reference an external CSS file if you want to maintain styles across several applications. In addition, Flex Builder can help you manage your classes in Design mode.

Skinning

Flex also allows you to change the look of your whole application in a process called skinning. You can use CSS to apply new skins to your Flex controls. Skins are available for free as well as for purchase on the Web. A good repository for Flex skins is Scale Nine (http://www.scalenine.com/).

To demonstrate this I went to the Scale Nine website and found a pretty skin called “blue plastic.” I downloaded the ZIP file and copied the contents into my Flex Builder 3 project folder via drag-and-drop.

I then modified my form by adding a Panel and replacing my own styles with a reference to the “blue plastic” skin (see Example 4-20).

Example 4-20. Styleform.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="vertical">
<mx:Style source="/blueplastic/blue_plastic.css" />
<mx:Panel title="Subscription form" paddingTop="20">
  <mx:Form>
    <mx:FormItem label="First Name">
      <mx:TextInput id="first" width="300" />
    </mx:FormItem>
    <mx:FormItem label="Last Name">
      <mx:TextInput id="last" width="300" />
    </mx:FormItem>
    <mx:FormItem label="Email">
      <mx:TextInput id="email" styleName="important" 
        width="300" />
    </mx:FormItem>
    <mx:FormItem>
      <mx:Button label="Subscribe" />
    </mx:FormItem>
  </mx:Form>
</mx:Panel>
</mx:Application>

Figure 4-20 shows the result.

The skinned subscription form
Figure 4-20. The skinned subscription form

As you can see, the panel has gotten a bit glossy. The font of the title of the panel has changed, and the background color for the entire design has also changed.

Filters and Effects

Flex supports a wide variety of filters and effects that you can apply to any user interface object. Take, for example, how easy it is to add a drop shadow to an image. The code for two images, one with a shadow and one without, appears in Example 4-21.

Example 4-21. Dropfilter.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="horizontal">
 <mx:Image source="@Embed('megan.jpg')" />
 <mx:Image source="@Embed('megan.jpg')">
   <mx:filters>
     <mx:DropShadowFilter />
   </mx:filters>
 </mx:Image>
</mx:Application>

It’s almost as easy to apply filters and effects to text-based controls, but in many cases you’ll have to embed the font. The result, when I look at it in the browser, looks like Figure 4-21.

The drop shadow filter applied to an image
Figure 4-21. The drop shadow filter applied to an image

You can also apply filters based on certain events, such as rollovers, to provide interactive effects. Example 4-22 shows a button that glows when you roll your mouse pointer over it.

Example 4-22. Effect.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  layout="horizontal">
 <mx:Button label="Push me!">
   <mx:rollOverEffect>
     <mx:Glow blurXTo="5" blurYTo="5" color="#ff0000" />
   </mx:rollOverEffect>
 </mx:Button>
</mx:Application>

As I roll my mouse pointer over the button, the effect looks similar to Figure 4-22.

A button that glows
Figure 4-22. A button that glows

These kinds of effects can bring an interface to life for your customers. It’s worth taking the time to learn how to use them effectively so that you have a complete set of tools at your disposal to make your applications grab people’s attention.

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