About MXML
You use two languages to write Flex applications: MXML and ActionScript. MXML is an XML markup language that you use to lay out user interface components. You also use MXML to declaratively define nonvisual aspects of an application, such as access to data sources on the server and data bindings between user interface components and data sources on the server.
Like HTML, MXML provides tags that define user interfaces. MXML will seem very familiar if you have worked with HTML. However, MXML is more structured than HTML, and it provides a much richer tag set. For example, MXML includes tags for visual components such as data grids, trees, tab navigators, accordions, and menus, as well as nonvisual components that provide web service connections, data binding, and animation effects. You can also extend MXML with custom components that you reference as MXML tags.
One of the biggest differences between MXML and HTML is that MXML-defined applications are compiled into SWF files and rendered by Adobe® Flash® Player or Adobe® AIR™, which provides a richer and more dynamic user interface than page-based HTML applications.
You can write an MXML application in a single file or in multiple files. MXML also supports custom components written in MXML and ActionScript files.
Writing a simple application
Because MXML files are ordinary XML files, you have a wide choice of development environments. You can write MXML code in a simple text editor, a dedicated XML editor, or an integrated development environment (IDE) that supports text editing. Flex supplies a dedicated IDE, called Adobe® Flex® Builder™, that you can use to develop your applications.
The following example shows a simple "Hello World" application that contains just an <mx:Application> tag and two child tags, the <mx:Panel> tag and the <mx:Label> tag. The <mx:Application> tag defines the Application container that is always the root tag of a Flex application. The <mx:Panel> tag defines a Panel container that includes a title bar, a title, a status message, a border, and a content area for its children. The <mx:Label> tag represents a Label control, a very simple user interface component that displays text.
<?xml version="1.0"?>
<!-- mxml\HellowWorld.mxml -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Panel title="My Application"
paddingTop="10"
paddingBottom="10"
paddingLeft="10"
paddingRight="10"
>
<mx:Label text="Hello World!" fontWeight="bold" fontSize="24"/>
</mx:Panel>
</mx:Application>
The executing SWF file for the previous example is shown below:
Save this code to a file named hello.mxml. MXML filenames must end in a lowercase .mxml file extension.
The following image shows the "Hello World" application rendered in a web browser window:
The first line of the document specifies an optional declaration of the XML version. It is good practice to include encoding information that specifies how the MXML file is encoded. Many editors let you select from a range of file encoding options. On North American operating systems, ISO-8859-1 is the dominant encoding format, and most programs use that format by default. You can use the UTF-8 encoding format to ensure maximum platform compatibility. UTF-8 provides a unique number for every character in a file, and it is platform-, program-, and language-independent.
If you specify an encoding format, it must match the file encoding you use. The following example shows an XML declaration tag that specifies the UTF-8 encoding format:
<?xml version="1.0" encoding="utf-8"?>
About the <mx:Application> tag
In addition to being the root tag of a Flex application, the <mx:Application> tag represents an Application container. A container is a user-interface component that contains other components and has built-in layout rules for positioning its child components. By default, an Application container lays out its children vertically from top to bottom. You can nest other types of containers inside an Application container, such as the Panel container shown above, to position user interface components according to other rules. For more information, see Using Flex Visual Components.
The properties of an MXML tag, such as the text, fontWeight, and fontSize properties of the <mx:Label> tag, let you declaratively configure the initial state of the component. You can use ActionScript code in an <mx:Script> tag to change the state of a component at run time. For more information, see Using ActionScript.
Compiling MXML to SWF Files
You can deploy your application as a compiled SWF file or as a SWF fuke included in an AIR application, or if you have Adobe LiveCycle Data Services ES, you can deploy your application as a set of MXML and AS files.
If you are using Flex Builder, you compile and run the compiled SWF file from within Flex Builder. After your application executes correctly, you deploy it by copying it to a directory on your web server or application server. Users then access the deployed SWF file by making an HTTP request in the form:
http://hostname/path/filename.html
Flex also provides a command-line MXML compiler, mxmlc, that lets you compile MXML files. You can use mxmlc to compile hello.mxml from a command line, as the following example shows:
cd flexInstallDir/bin mxmlc --show-actionscript-warnings=true --strict=true c:/appDir/hello.mxml
In this example, flexInstallDir is the Flex installation directory, and appDir is the directory containing hello.mxml. The resultant SWF file, hello.swf, is written to the same directory as hello.mxml.
For more information about mxmlc, see Using the Flex Compilers. For more information about the debugger version of Flash Player, see Logging.
The relationship of MXML tags to ActionScript classes
Adobe implemented Flex as an ActionScript class library. That class library contains components (containers and controls), manager classes, data-service classes, and classes for all other features. You develop applications by using the MXML and ActionScript languages with the class library.
MXML tags correspond to ActionScript classes or properties of classes. Flex parses MXML tags and compiles a SWF file that contains the corresponding ActionScript objects. For example, Flex provides the ActionScript Button class that defines the Flex Button control. In MXML, you create a Button control by using the following MXML statement:
<mx:Button label="Submit"/>
When you declare a control using an MXML tag, you create an instance object of that class. This MXML statement creates a Button object, and initializes the label property of the Button object to the string "Submit".
An MXML tag that corresponds to an ActionScript class uses the same naming conventions as the ActionScript class. Class names begin with an uppercase letter, and uppercase letters separate the words in class names. Every MXML tag attribute corresponds to a property of the ActionScript object, a style applied to the object, or an event listener for the object. For a complete description of the Flex class library and MXML tag syntax, see the Adobe Flex Language Reference.
Understanding a Flex application structure
You can write an MXML application in a single file or in multiple files. You typically define a main file that contains the <mx:Application> tag. From within your main file, you can then reference additional files written in MXML, ActionScript, or a combination of the two languages.
A common coding practice is to divide your Flex application into functional units, or modules, where each module performs a discrete task. In Flex, you can divide your application into separate MXML files and ActionScript files, where each file corresponds to a different module. By dividing your application into modules, you provide many benefits, including the following:
Ease of development
Different developers or development groups can develop and debug modules independently of each other.
Reusability
You can reuse modules in different applications so that you do not have to duplicate your work.
Maintainability
You can isolate and debug errors faster than if your application is developed in a single file.
In Flex, a module corresponds to a custom component implemented either in MXML or in ActionScript. These custom components can reference other custom components. There is no restriction on the level of nesting of component references in Flex. You define your components as required by your application.