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From "Josh Tynjala (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLEX-34897) Scope is wrong when calling member function from a variable reference
Date Sun, 28 Jun 2015 21:32:04 GMT
Josh Tynjala created FLEX-34897:
-----------------------------------

             Summary: Scope is wrong when calling member function from a variable reference
                 Key: FLEX-34897
                 URL: https://issues.apache.org/jira/browse/FLEX-34897
             Project: Apache Flex
          Issue Type: Bug
          Components: FalconJX
    Affects Versions: Apache FalconJX 0.0.3
            Reporter: Josh Tynjala


In Flash Player, when you save a reference to a member function in a variable, you can call
the function reference, and "this" will still be bound to the instance where it came from.

public class Test
{
    public function Test()
    {
        this.func();
        var func:Function = this.func;
        func();
    }

    private function func():void
    {
        trace(this); //in Flash, "this" will always be an instance of Test
    }
}

Basically, in the code above, the two calls to func() will behave the same in Flash Player.
However, in the current implementation of the transpiler, that behavior is lost. When the
variable reference to the function is called, "this" ends up referring to the global window
object instead.

JavaScript function objects have a bind() function that let's you set what "this" will refer
to when the function is called:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind

After modifying my code to use bind(), the two calls to func() will behave the same:

public function Test()
{
    this["func"] = this.func.bind(this);
    this.func();
    var func:Function = this.func;
    func();
}

Would it be possible for the transpiler to automatically bind all member functions to the
correct scope to preserve the behavior that AS3 developers expect?

Event listeners are the typical use case where you'd pass a reference to a member function
somewhere else where a reference needs to be saved in a variable. AS3 in Flash Player makes
this easy by automatically binding all member functions to the instance. JavaScript usually
requires some manual intervention to get event listeners to be called with the right scope.



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