Using RSLs with Flex Builder

You can start using Flex 3 RSLs at any point in the development cycle of your application. Either develop the application using RSLs from the beginning or implement them at the end after the application is done.

Creating a sample program

Let's create a Flex project with a simple "Hello World" program (see Figure 1). Here is the source:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
   <mx:Label x="165" y="10" text="Hello World"/>
</mx:Application> 

Figure 1. Flex "Hello World" program

In order to reduce the download time, it is helpful to compare the size of your application before and after using RSLs. By default, Flex Builder adds debugging information to SWF files, so in order to see the true size of the application, turn off the debugging information. To do this, add -debug=false to the Additional Compiler Arguments section of the Flex Compiler Properties dialog box (see Figure 2).

Figure 2. Turning off debugging information

Paste the source code for the "Hello World" app into a Flex application file and save it. Check the size of the application by viewing the properties of the application's SWF file created in the bin directory. You can view the properties of a file by right-clicking the file (Option-clicking on Mac OS) and choosing the Properties option in Windows (Get Info on Mac). The size of the application SWF file for the "Hello World" project is 152,269 bytes or about 149K (your result may vary).

Reducing the application SWF file size

Now let's see how you can reduce the size of this application SWF file by pulling the Flex framework code out of the SWF and loading it at runtime as an RSL.

To change how the Flex framework is linked, modify the build properties of the project. First, access the project's properties by selecting the project name in the Navigator View and choosing File > Properties. Next, choose the Flex Build Path properties and click the Library Path tab (see Figure 3).

Figure 3. Changing how the Flex framework is linked in the Library Path dialog box

In the Framework Linkage pop-up menu, change the link type from "Merged into code" to "Runtime shared library (RSL)." Click OK to save the changes. If the option "Project - Build Automatically" is set, your project will be recompiled. If not, you'll need to manually rebuild the project. After recompiling, the size of the application SWF file is reduced to 47,466 bytes (46K), a savings of 104,803 bytes (103K) compared to the SWF that statically links the Flex framework.

You can easily switch between a statically linked application and a dynamically linked application just by toggling the option in the Framework Linkage pop-up menu from "Merged into code" to "Runtime shared library (RSL)." You may want to switch back to "Merged into code" periodically in order to debug code in one of the framework RSLs, because when you choose the option to use RSLs, all of the debug information is removed.

The framework RSL is the only RSL that is configured by default in the flex-config.xml file because every Flex application uses over 100K of framework classes. If you do add RSLs, the compiler will always load any RSLs you specify whether or not they are used by your application.

Adding and changing the linkage of a SWC file

For this example I'll change the linkage of the SDK library rpc.swc. Normally you would do this only if your application uses classes from rpc.swc.

To change the linkage, access the project properties (File > Properties), choose the Flex Build Path, and select the Library Path tab. Make sure the Framework Linkage pop-up menu is set to "Runtime shared library (RSL)." Click the SDK folder to open it up and then open the rpc.swc entry. Next, click the "RSL URL" entry (see Figure 4).

Figure 4. After setting the framework linkage to RSL, drilling down inside rpc.swc to edit the RSL URL options

While RSL URL is selected, click the Edit button to add RSL information for rpc.swc (see Figure 5).

Figure 5. Library Path Item Options dialog box

Select the option to "Use same linkage as framework." This means rpc.swc will use whatever link type is set in the Framework Linkage pop-up menu.

Now click the "Digests" radio button to use Flex 3 RSLs. (If you want to use the RSLs as supported by Flex 2.0.1, choose None.) After selecting Digests, a table with an Add button appears below the radio buttons. Click the Add button to add an RSL URL (see Figure 6).

Figure 6. Specifying the URL of the RSL in the dialog box

A signed RSL is displayed by default. Click OK. After making these changes, the Library Path Item Options dialog box is updated (see Figure 7).

Figure 7. Final settings after making changes to the Library Path Item Options dialog box

Notice the message that appears at the bottom of the dialog box. Because we've specified only a signed RSL, a version of Flash Player that knows how to load RSLs signed by Adobe is required to run the application. The informational message in Figure 7 reflects a known bug in the release. The message says, "This projects settings will be changed to require Flash Player {0}." The message should say, "This project's settings will be changed to require Flash Player 9,0,60." Flash Player 9,0,60 is the earliest version of Flash Player that can load signed RSLs. To allow the application to run in previous versions of Flash Player, add an unsigned RSL as a failover. At runtime, if the version of Flash Player running the application is earlier than version 9,0,60, then the signed RSL will be skipped and the unsigned RSL will be loaded. Since Flash Player 9,0,60 is a beta version, if you need to require a Flash Player version that knows how to load signed RSLs, you should require Flash Player 9,0,115—the latest release as of this writing.

Let's do this now. Click the Add button to add an unsigned RSL. When the dialog box opens, the default URL will be a signed RSL again. All you need to do is change the extension from ".swz" to ".swf" (see Figure 8).

Figure 8. Specifying an unsigned RSL by changing the file extension to .swf

Click OK. After adding the unsigned RSL, the Library Path Item Options dialog box is updated (see Figure 9).

Figure 9. Library Path Item Options dialog box updated after adding the unsigned RSL

Notice that after supplying the project with an unsigned RSL as a failover, Flash Player version 9,0,115,0 is no longer required. The RSL for rpc.swc is now properly configured.

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