Calling a Flex/AS3 Callback from Javascript - javascript

I have a Javascript API, which should be usable with GWT and Flex. Using the FABridge it is really easy to call Javascript methods from AS3 and vice versa. But when I try to register a callback to an AS3 method in my Javascript API I get stuck. Here is a short code sample:
public function initApp():void {
if (ExternalInterface.available) {
ExternalInterface.addCallback("foobar", foobar);
}
}
public function foobar():void {
//the callback function
Alert.show("Callback from API works!");
}
private function btnCallbackClicked():void {
ExternalInterface.call("testAPICallbackFromJS", Application.application.foobar);
}
And the simple JS method:
function testAPICallbackFromGWT(callback){
$clinit_26(); //added by the GWT compiler
alert('callback to be launched 3 2 1');
callback();
}
But this version does not work, because I always receive an empty function in my JS code. It seems that the FABridge is cutting the rest.
Then I tried a different approach. I wrote a little JS method, which takes the name of the function and creates the callback from the JS side.
registerFlexCallback = function(registerMethod, callback, id) {
/*
workaround to create a callback for an AS method, which can be called by Javascript
* registerMethod - Javascript method which shall be called for registration with the created callback as parameter
* callback - AS method that shall be called by Javascript (available over the FABridge interface)
* id - ID of the flash object (use Application.application.id in AS)
*/
var swf = document.getElementById(id);
eval(registerMethod + "(swf." + callback + ");");
};
This one works well with the Internet Explorer, but with no other browser. For example in Firefox I get the following error message:
NPMethod called on non-NPObject wrapped JSObject!
Can somebody tell me, what this error is about (maybe some kind of security issue)? Or does anyone have a better idea how to create callbacks for my AS3 methods which can be called by JS?

This is because functions don't serialize across the FABridge. Meaning in your
ExternalInterface.call("testAPICallbackFromJS", Application.application.foobar);
the second parameter will always be null. What I do is add a wrapper method on the HTML page via eval that points at my embed and therefore the added callback. So you have to add an extra, while annoying step:
ExternalInterface.addCallback("foobar", foobar);
var callBack:String = "";
var functionName:String = UIDUtil.createUUID;
callBack = "function " + functionName + "( ){ " +
"document.getElementById('applicationName').foobar(arguments);"+
"}";
ExternalInterface.call("eval", callback);
ExternalInterface.call("testAPICallbackFromJS", functionName);
The NPObject error you're seeing I'm pretty sure is a security error ( based on where it comes from in the FF code ) probably preventing you from dynamically injecting methods that can be eval'ed without the JS interpreter getting in the way.
I haven't even tried to compile the above so, hopefully you get the gist.

I notice two things right away
firstly it appears your ExternalInterface will die if the ExternalInterface is not ready.
public function initApp():void {
if (ExternalInterface.available) {
ExternalInterface.addCallback("foobar", foobar);
}
}
I would add a timout and then try again so that it tries again until Externalinterface is ready.
Also I don't see the function "foobar" in your javascript code. I see callback passed in as a variable but without varifying that it is in fact 'foobar' this is hte kind of thing that can make testing a misserable event.
function testAPICallbackFromGWT(callback){
$clinit_26(); //added by the GWT compiler
alert('callback to be launched 3 2 1');
callback();
}
I would simplify your testing example so that there are less moving parts.
// e.g. run just flash to javascript only
ExternalInterface.call("alert", "hello out there");
if that works
// establish the call from flash
ExternalInterface.addCallback("hello_out_there", foobar);
// and in javascript
alert(typeof('hello_out_there')); // will be 'function' if exists or undefined if ExternalInterface did not work
This way you can get a handle bit for bit what is working and where it breaks down.
Pay atention to the timing, if you can tigger your flash from button actions and your javascript from links you can illiminate a number of loading issues as well. of course you'll need to solve an autoload version for your launch but for testing manually triggered events can simplify things significantly.
also because it's javascript the browser is relevant.
I've seen consistent results in Firefox and Internet explorer that break down in safari and sometimes IE is the odd browser out.
Sometimes Firefox is the only one that breaks.
you just have to test them all.

Related

Understanding callback function purpose

I realise this is more of a general question, but I've read through similar answers on here but I can't find more of an overview. I'm new to callbacks and I'm trying to understand when they should be used.
The MDN web docs has this example;
function greeting(name) {
alert('Hello ' + name);
}
function processUserInput(callback) {
var name = prompt('Please enter your name.');
callback(name);
}
processUserInput(greeting);
However I'm struggling to see how this is more beneficial than the following, where I'm not passing the greeting function as a parameter?
function greeting(name) {
alert('Hello ' + name);
}
function processUserInput() {
var name = prompt('Please enter your name.');
greeting(name);
}
processUserInput();
As Javascript is async, sometimes it is difficult to handle response from non-blocking functions, for ex. if you are making an ajax call then it'll be executed asynchronously and results will be returned sometime later, by that time the main execution flow will pass the ajax code and starts executing following statements, in that case, its very difficult to catch the response to process further.
To handle those cases, callbacks comes into picture where you pass a function as the parameter to the ajax function and once the response is returned then call the callback by passing response data as a parameter to process further.
more info here http://callbackhell.com/
In simple terms you can say a callback is a way of asking a question (or requesting a task) in advance, i.e. when you're done with this do this (usually with the result). The whole point is to set aside functions that are to be done later, usually because you don't have the required inputs to do them now.
The 2 main differences between your implementation and the MDN one is that yours is harder to maintain and harder to reason about hence test.
1. Maintanance / Reusability
Imagine you're a few thousand lines of code into a code base then you're required to change what processUserInput() does. Its much easier to change or write a new callback function instead of changing the function processUserInput(). This would be evident if processUserInput was a bit more complicated. This also means the MDN one is much more useful in various scenarios unlike your implementation. You can reuse it in different situations like saying good bye, capitalizing names etc simply by writing different callbacks to plug into processUserInput().
2. Testing / Easier to reason about
The MDN implementation is much more easier to understand. Its easier to assume that the function processUserInput(greeting) will probably return a greeting than it is to assume what processUserInput() does. This makes it easier to test because you can always be sure the MDN implementation will always return the same output given an input.
Callbacks can be extremely useful depending on the circumstances; for example, when working with JavaScript for Google Chrome browser extension development, a callback can be used for intercepring web requests once it has been setup.
The purpose of a callback in general is to have the callback routine executed upon a trigger - the trigger being an event of some kind. Usually, functionality follows an interface of chained APIs. By implementing callback support, you can redirect execution flow during a stage of an operation. Callbacks are especially useful to third-party developers when dealing with someone elses library depending on what they are trying to do. Think of them like a notification system.
Functions in general taking in parameters is useful for flexibility and maintenance. If you use different functions for different things, the functions can be simply re-used over and over again to provide different functionality - whilst still preventing bloating the source code with more-or-less the same code over and over again. At the same time, if you use functions to your own library and a bug shows up, you can simply patch it for the one function and then it will be solved.
In your example, your passing a callback routine to the function you're calling - the function you're calling will call the callback function and pass the correct parameters. This is flexible because it allows you to have a callback routine called for printing the contents of the variable, and another for calculating the length of the string passed in, or another for logging it somewhere, etc. It allows you to re-use the function you setup, and have a different function called with the correct parameters without re-making the original function.
This example is not appropriate for understanding callbacks
In simple Language callbacks functions are used when we have to do some stuff after or in response of some other event or function or expression.
i.e when the parent function completes its execution then callback gets executed.
simple Example
function hungerStatus(status,cb){
return cb(status)
}
function whatToDo(status){
return status ? "order Pizza" : "lets play"
}
hungerStatus(false,whatToDo)
Another example
// global variable​
​var allUserData = [];
​
​// generic logStuff function that prints to console​
​function logStuff (userData) {
if ( typeof userData === "string")
{
console.log(userData);
}
else if ( typeof userData === "object")
{
for (var item in userData) {
console.log(item + ": " + userData[item]);
}
​
}
​
}
​
​// A function that takes two parameters, the last one a callback function​
​function getInput (options, callback) {
allUserData.push (options);
callback (options);
​
}
​
​// When we call the getInput function, we pass logStuff as a parameter.​
​// So logStuff will be the function that will called back (or executed) inside the getInput function​
getInput ({name:"Rich", speciality:"JavaScript"}, logStuff);
refer callback exaplanation

QlikView runs BodyOnLoadFunctionNames callbacks too early - why?

I have a problem with QlikView in the browser: I have a listbox and try to access it using an initialize script.
The script is registered by using the InitWorkbench function, using its BodyOnLoadFunctionNames parameter. So far, this works, and the initializer is run at startup.
Inside the initializer I try to do the following:
var doc = Qv.GetCurrentDocument();
var listbox = doc.GetObject('LB01');
Afterwards, when I have a look at listbox.Type, unfortunately it is undefined. If I delay execution of this query, it correctly says LB, hence apparently the query works - but only when it is executed delayed.
So, obviuosly there's a timing problem, and it seems as if the initializer runs too early (or I am doing something wrong).
Can anybody point out what the solution is (or give me a hint on what I am doing wrong)?
Okay, I've found the solution: The internal update function did not run yet, and all the values are only available once this function ran, so you need to provide a callback to the call to GetObject (that gets called after the update function):
var doc = Qv.GetCurrentDocument();
var listbox = doc.GetObject('LB01', function () {
console.log(listbox.Type); // => 'LB'
});

JS to monitor the CSS property change like "display:none"=>"display:block"?

I want to run some JS code when an image's CSS property "display" has been changed by any other JS script/functions. Is there any method to monitor that change and setup a callback function?
$(this).bind.('propertychange', function(){})
cannot do this, and setInterval is also a bad idea.
What else could be done?
This is what you are looking for:
document.documentElement.addEventListener('DOMAttrModified', function(e){
if (e.attrName === 'style') {
console.log('prevValue: ' + e.prevValue, 'newValue: ' + e.newValue);
}
}, false);
This is inside the legacy JavaScript files that you do not want to modify:
// this is your original, unmodified function
function originalFunction(sel) {
alert(sel);
$(sel).css("display","none");
}
This is in your code:
// here is a sample callback function you pass into the extended function below
function myCallback(s) {
alert("The image with src = '" + $(s).attr("src") + "' has been modified!");
}
// here is how you can extend the function to do what you want
// without needing to modify the actual code above
originalFunction = (function(legacyFn, callback) {
// 1 arg function to be returned and reassigned to originalFunction
return function(sel) {
// call "original" originalFunction, with alert and image hide.
legacyFn(sel);
if(callback) callback(sel); // invoke your callback
}
})(originalFunction, myCallback);
The variable originalFunction is assigned a function that takes one argument. The function that takes one argument is returned by an anonymous, self-executing function that takes 2 arguments, the reference to the originalFunction before it is modified, and the reference to the callback function. These two function references become "locked" inside the closure so that when the originalFunction is then assigned a new value by the self-executing function, the legacyFn parameter still contains a reference to the originalFunction prior to it being modified.
In summary, at a higher level, originalFunction and myCallback are passed in as parameters to the self-executing anonymous function and are passed into the variables legacyFn and callback, and a new function is then assigned to originalFunction.
Now, when you call originalFunction('.someClassOnAnImage'), the legacyFn will fire, which will alert the selector and set the display property to none. Afterwards, the callback function, if it exists, will fire, and you'll then see:
The image with src = '.someClassOnAnImage' has been modified!
While this isn't as nice as a hypothetical or platform-specific addEventListener, it does allow you to modify the behavior of the functions in the legacy code without having to physically crack open those files and modify them. This simply extends the functions to perform additional behaviors but without needing to modify the original functions or even the original files for that matter.
You could neatly include all of your extensions in a separate JavaScript file (or whatever JavaScript file you're working in) and if you ever want to go back to the original behavior, you simply remove your extended functions.
The Answer: See this other post >> is there an alternative to DOMAttrModified that will work in webkit
The Rant:
The DOM Mutation events hold the key to your problem. However, in the new wave of browser wars, Wekit and Gecko can't agree on stuff. While Gecko has DOMAttrModified, webkit has something called mutation observer (which breaks the pattern of event handlers being attached to events but hey who cares for consistency when we want to lock users/coders in right? ;)
P.S: Just adding this here for future seekers of the same wisdom.
Building upon Jeff's suggestion, I would recommend writing a single function that modifies the image style property and then using that function as the bottleneck that all other functions must go through to modify that image style property.
function showImage(selector, callback) {
$(selector).css("display","block");
if(callback)
callback();
}
function hideImage(selector, callback) {
$(selector).css("display","none");
if(callback)
callback();
}
Something like the above two functions can be invoked from anywhere in your JavaScript when you must change the image CSS property. The functions also take a function as a parameter, which would be executed afterwards assuming the function was passed in as the 2nd parameter.
You could further simplify this into a single function, but I'll leave that to you as I don't know exactly what your goals are in doing this.

Unattached anonymous functions and doubly named methods in javascript?

I'm debugging an app that uses .NET's scriptmanager.
It may be a glitch in firebug, but when I read through the code there are a lot of lines like the following:
// anonymous functions not attached as handlers and not called immediately
function () {
//code
}
// named functions added as methods
myObj = {
myMethod: function myFunctionName() {
//code
}
}
Are these lines valid and, if so, what do they do and what possible reason would there be for coding like this (and I won't accept "It's microsoft - what d'you expect" as an answer)?
This might be worth a read: How does an anonymous function in JavaScript work?
They are there because some busy programmer was intending to do something and ran out of time, but left the stub as a reminder of work to be done. They do nothing as of yet.
or to watermark the code for checks that are done elsewhere in the logic
or simply put there to obfuscate...

Strange behavior on function calling

When calling a Javascript function, it seems like JS gives priority to functions without parameters first, even if I have the same function name with parameters.
The strange behavior only happens in the following scenario:
I have a an HTML page with embedded Javascript, like this:
//Javascript in the page
function testAbc(){
alert('testAbc no params');
}
//Javascript in common.js
function testAbc(x){
alert('testAbc with param:'+x);
}
function testAbcFunc(x){
testAbc(x);
}
Now from somewhere in the page, im calling testAbcFunc from the common.js expecting it to call testAbc with parameter which is the common function. But strangely, JS calls back the function in the original page without params!!
I have been debugging this bug fore few hours now, and i tried this short code to reproduce the bug, it does happen each time.
NOTE: if all functions are in the same page, the correct function (with params) will be called, but when ther are split between the page and the JS file. JS seems to give priority to the function in the page even though is doesn't have parameter
JavaScript does not support method overloading based on parameters. It simply uses the last-defined function if multiple functions have the same name. The version in the page will override the included version. When it worked for you, I assume that the include version (with the argument signature) was inlined after the original.
JavaScript doesn't have overloaded function. It doesn't care about signatures, it calls functions solely by names and nothing else. It is strange that later function does not completely hide the first one but well, there's no spec about that behaviour.
So just don't do that, check the number of params with arguments.length inside the function and don't try to use overloading which will never work.
function testAbc(){
if (arguments.length == 0) {
alert('testAbc no params');
} else {
var x = arguments[0];
alert('testAbc with param:'+x);
}
}
There is no function overloading in JavaScript. If you are defining a function with two times with diffrent number of parameters the last one to be defined will be called.
Also, you should be namespacing your JavaScript.
Like so:
var common = {
testABC: function () {
//Stuff
}
};
Then call testABC like this
common.testABC();

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