As part of the Flex SDK, Adobe includes FDB, a free command-line debugger. This debugger is fully featured, although usually you will opt to use the Flex Builder debugger if available. With that said, it is great to have access to a free debugging tool included as part of the SDK. This allows developers who do not want to purchase Flex Builder access to a fully featured debugger. We won’t be covering FDB in depth, but we will discuss the basics and some of the possible benefits it has to offer.

You launch FDB from the command line as you would any other command-line application. Once it is started, you will be prompted with the FDB prompt (fdb). You can type help at the prompt for a list of available commands.

The starting point for a debug session with FDB is to launch an .swf compiled with debugging enabled in the Debug Player and establish a connection with FDB. You do this by first executing the run command at the FDB prompt. Once the command is executed, FDB will confirm that it is waiting for the player to connect. To connect Flash Player to FDB, open a debug-enabled .swf with the Debug Player. When a debug-enabled .swf is opened, the player will attempt to auto-connect to the local debugger, if available. If you open an application without the debugger listening for a connection from the player, the Flash Debug Player will prompt you to select the debugger you want to use. Although typically a user will be running the application and the debugger on the same machine, which means you may never receive the prompt requesting you to select the debugger, it is possible to initiate a remote debugging session. Remote debugging allows you to execute an application on a machine that is separate from the debugger, allowing you to debug problems that are reproducible on only certain machines, or even debug across platforms wherein a Mac OS X machine executes an application with the debugger running on a Windows machine. For this purpose, all debugging communication occurs through TCP on port 7935. We cover remote debugging later in this chapter.

Once a connection is established, you can set breakpoints or instruct the debugger to continue execution of the application. To continue execution you can issue the continue command. Application trace messages are shown in the debugger as they are encountered.

Breakpoints can be set using several methods. The most typical method of setting a breakpoint is to specify the class and line number for the breakpoint, which you can achieve by issuing the break command. For example, if you wanted to insert a breakpoint in line 56 of the class MainApp, you would input the command break MainApp.mxml:56. If you want a breakpoint when a button is pressed but you are not sure of the method that will be called when the event occurs, you could enter break button. This will return a list of methods that begin with the word button. From the list, you should see buttonPressed and the location of the method. With that information, you could set the breakpoint by calling break buttonPressed. Once you do that, you will need to call the command continue, which will tell the debugger to allow the player to continue executing the application. There are other methods of setting breakpoints with FDB, which you can find by issuing the command help breakpoint.

When an application is executing, the debugger will inform you when it encounters a breakpoint. Once a breakpoint is encountered, you have several options. You can issue the continue command, which will continue execution of the application, step through the application using the step command, and set the value of a variable using the set command. When done debugging, you can exit FDB to end the debugging session, which will automatically end the active connection with the Debug Player, or you can execute the kill command.

As discussed, FDB makes it easy to search for methods on which you want to set breakpoints. Some of the other nice features of FDB that Flex Builder’s debugger doesn’t support are the ability to set conditional breakpoints using the condition command, and the ability to review a list of all the loaded types and functions with the info functions command.

FDB can be a powerful tool and is worth exploring, but as you will see in the next section, Flex Builder provides a more practical method of application debugging, which you will likely opt for over FDB.