AS3 -TypeError #1009 - any easy way to find out *w

问题:

We all get "TypeError #1009 Cannot access a property or method of a null object reference" now and then - no big deal, but sometimes frustrating to debug.

Flash gives you the call stack (which is a start), but leaves it up to you to figure out where the null object is - is it possible to find out exactly which reference is throwing the error?

Given the following (error prone) function:

function nullObjectReferenceError():void
    {
        var obj:Object;
        var prop:* = obj.nonExistentProperty;
    }

Rather than just the call stack from the TypeError, I'd like to trace something like: "Cannot access a property or method of a null object reference at obj.nonExistentProperty" - Is this even possible?

回答1:

If you check Permit Debugging under Publish Settings in the Flash IDE, it gives you the line number in your code causing the error.

回答2:

The obvious solution is to stop using such generic error-prone code in the first place. You should never use '*' type, and almost never should use 'Object' type.

To catch it at runtime, you could always say:

if(obj == null)
  throw new Error("null obj passed in!!");

if(obj.nonExistentProperty == null)
  throw new Error("obj doesn't have the prop!! the obj was: "+obj);

回答3:

TypeError will not give you any more information if you catch it.

As far as I know, there is no known way to achieve this (i.e which object threw the Error).

Your best bet will be to set a breakpoint in the beginning of the function and investigate the variables manually. That's what I do, and that works fairly well for me.