Loading and Returning Text Data with ActionScript 3 (URLLoader)

Question

I'm trying to do something exceedingly simple: write a function that reads text from a text file and returns the text in a string using AS3.

The Function

    public function readData(path:String):String
 {
  var dataSet:String;

  var urlRequest:URLRequest = new URLRequest(path);
  var urlLoader:URLLoader = new URLLoader();
  urlLoader.dataFormat = URLLoaderDataFormat.TEXT;
  urlLoader.addEventListener(Event.COMPLETE, urlLoader_complete);
  urlLoader.load(urlRequest);

  function urlLoader_complete(evt:Event):void {
   dataSet = urlLoader.data;
   trace(dataSet)
  }
  trace(dataSet);
  return dataSet;
 }

Calling the Function

    var dataString:String = aq.readData("http://example.com/data.txt");
    trace(dataString);

This code returns a null string when I run it. Why?

EDIT: Ok, I now see that this doesn't work because urlLoader is acting asynchronously. I'm writing a program that reads in a data file and acts on it. Does this mean that I need to write the rest of my program inside function urlLoader_complete? Or should I pause the program until urlLoader is finished?

Answer 1

In Flash and Flex, all network I/O is asynchronous. It has to be this way in order to avoid blocking your browser.

As a result, it is not possible to write a readData() function that directly returns the result of a network read operation. You will have to pass a callback function to the readData() function. When readData() has finished reading the data, it can call the callback function.

For example:

/**
 * Asynchronous function to read data as a string. When the data has been read,
 * the callback function is called with the result.
 * 
 * @param path     the URL to read
 * @param callback the function that is called with the result; should take
 *                 one string argument.
 */
public function readData(path:String, callback:Function):void
{
    var dataSet:String;

    var urlRequest:URLRequest = new URLRequest(path);
    var urlLoader:URLLoader = new URLLoader();
    urlLoader.dataFormat = URLLoaderDataFormat.TEXT;
    urlLoader.addEventListener(Event.COMPLETE, urlLoader_complete);
    urlLoader.load(urlRequest);

    function urlLoader_complete(evt:Event):void {
        dataSet = urlLoader.data;
        trace(dataSet);
        callback(dataSet);
    }
}

Here is how you might call that function from Flex:

<mx:Label id="mylabel" />
<mx:Button click="readData('http://www.google.com/',
    function(s:String):void {mylabel.text = s})" />