ActionScript™ 3.0 Bible

Chapter 19. Understanding Errors

Introduction

In this chapter

• Understanding failure states

• Using exceptions

• Handling asynchronous errors

Despite our best intentions and wishes, programs sometimes fail. In Chapter 5, you learned about ways that the compiler can catch your typos and invalid requests: syntax errors and type mismatches. These are kinds of compile‐time errors. But there are countless kinds of errors that can happen while your program is running, which may be the result of a logical error in your code, or just the result of unexpected circumstances out of your control. Whichever is the case, ActionScript 3.0 gives you the opportunity to handle these errors at runtime. This chapter gives you the tools to understand runtime errors and keep them from affecting the end user.

Comparing Ways to Fail

If a certain function fails, there are a few ways for it to signal this failure. Some functions return a value that indicates it did not complete successfully. Code that uses this function must then check to make sure all went as planned before continuing. There are a few problems with this approach.

First, returning a Boolean signifying if the function call was successful is not always possible. What if the function is already supposed to return a value? If you are returning a complex object, setting the value of this returned object to null is one option. If you are returning an int, you must decide on a reserved number to signify failure, such as –1. But these values might actually ...