User Guide

Error handling in ActionScript 3.0 259
ActionScript 3.0 error handling elements
ActionScript 3.0 includes many tools for error handling, including:
■ Error classes. In compliance with the ECMAScript (ECMA-262) edition 4 draft language
specification, ActionScript 3.0 includes a broad range of Error classes to expand the scope
of situations that may produce error objects. Each Error class helps applications handle
and respond to specific error conditions, whether they are related to system errors (like a
MemoryError condition), coding errors (like an ArgumentError condition), networking
and communication errors (like a URIError condition), or other situations. For more
information on each class, see “Comparing the Error classes” on page 271.
■ Fewer silent failures. In earlier versions of Flash Player, errors were generated and reported
only if you explicitly used the
throw statement. For Flash Player 9, native ActionScript
methods and properties throw run-time errors that allow you to handle these exceptions
more effectively when they occur, and then individually react to each exception.
■ Clear error messages displayed during debugging. When you are using the debugger
version of Flash Player, problematic code or situations will generate robust error messages,
which help you easily identify reasons why a particular block of code fails. This makes
fixing errors more efficient. For more information, see “Working with the debugger
version of Flash Player” on page 260.
■ Precise errors allow for clear error messages displayed to users at run time. In previous
versions of Flash Player, the
FileReference.upload() method returned a Boolean value
of
false if the upload() call was unsuccessful, indicating one of five possible errors. If an
error occurs when you call the
upload() method in ActionScript 3.0, you can throw one
of four specific errors, which helps you display more accurate error messages to end users.
■ Refined error handling. Distinct errors are thrown for many common situations. For
example, in ActionScript 2.0, before a FileReference object has been populated, the
name
property has the value null (so, before you can use or display the name property, you need
to ensure that the value is set and not
null). In ActionScript 3.0, if you attempt to access
the
name property before it has been populated, Flash Player throws an
IllegalOperationError, which informs you that the value has not been set, and you can use
try..catch..finally blocks to handle the error. For more information see “Using
try..catch..finally statements” on page 261.
■ No significant performance drawbacks. Using try..catch..finally blocks to handle
errors takes little or no additional resources compared to previous versions of
ActionScript.
■ An ErrorEvent class that allows you to build listeners for specific asynchronous error
events. For more information see “Responding to error events and status” on page 267.