How to wrap a callback using JavaScript overlay types (GWT)?

Question

In the Display Object class, I have all the wrapped exception from events. I can not understand the template and really need an example.

In JavaScript, you create a callback for an object as follows:

displayObject.onPress = function(event) { $wnd.alert("object pressed"); } 

I wrapped the Mouse Event parameter:

 public class MouseEventImpl extends JavaScriptObject { protected MouseEventImpl() {} public static native MouseEventImpl create(String type, int stageX, int stageY, DisplayObjectImpl target, JavaScriptObject nativeEvent) /*-{ return new $wnd.MouseEvent(type, stageX, stageY, target, nativeEvent); }-*/; ...other methods excluded... } public class MouseEvent { private MouseEventImpl impl; public MouseEvent(String type, int stageX, int stageY, DisplayObject target, JavaScriptObject nativeEvent) { this.impl = MouseEventImpl.create(type, stageX, stageY, target.getOverlay(), nativeEvent); } ...other methods excluded... } 

The display object uses the same overlay pattern. How can I write a callback in java and pass it to JSO? Give an example if you can. :)

Answer 1

Pre-editing: I wrote this answer, not trying to understand why you tried to do this, assuming you were using some kind of non-browser event that is already pretty well wrapped, and if you want to get more data from the NativeEvent instance, you can write JSNI methods in your own classes to access it, or another subclass of NativeEvent to add additional methods and .cast() to your class. Add a handler to the widget using the Widget.addDomHandler method and the corresponding MouseEvent subclass to get an instance of the type.

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In JavaScript, callbacks are simply functions that will be called when something happens. Unless specifically indicated where they are being transferred, they are usually called in a global context, and not on a specific instance of the object.

 var callback = function() { alert("callback called!"); }; // later, in something that consumes that callback: callback(); 

To call a function in an instance (i.e. make it a method call), you can wrap this call in a function that does not need an instance:

 var obj = {}; obj.name = "Me"; obj.whenSomethingHappens = function() { alert("my name is " + this.name); }; // wont work, so commented out: //var callback = obj.whenSomethingHappens; // instead we wrap in a function // this is making a closure (by closing over obj) var callback = function() { obj.whenSomethingHappens(); }; // later, in something that consumes that callback: callback(); 

In Java, you cannot refer specifically to a method (without reflection), but only to instances of objects. The easiest way to create a simple callback is to implement an interface, and the code that receives the callback takes an instance of the interface and calls a specific method.

GWT declares a Command interface for functions with a null argument and a common Callback<T,F> interface for cases that may or may not be executed, with one common argument for each parameter. Most event handlers in GWT simply define a single method with the specific data passed to that method.

We must use all this knowledge to transfer Java instances using the call function in JavaScript and make sure that they are called on the right instance. This example is a function that accepts a Callback instance, and using JSNI terminates the JS call.

 // a callback that has a string for either success or failure public native void addCallback(Callback<String, String> callback) /*-{ var callbackFunc = function() { // obviously the params could come from the function params [e-mail] ::onSuccess(Ljava/lang/String;)("success!"); }; doSomethingWith(callbackFunc);//something that takes (and presumably calls) the callback }-*/; 

One last part - for correct error handling and GWT planning to work correctly, it is important to transfer the call back to java in $ entry - the last line should be

  doSomething($entry(callbackFunc));