CODEWITHSUNDEEP
javagwt
Daniel
Daniel

Reputation: 1

Avoiding create new ClickHandler Instances on every Click

im sitting on this for 4 hours now, and once again I end up on Stackoverflow because I just cant solve this (simple) problem.

I want to fire a method when I click a button, Google gives an Example like this:

// Listen for mouse events on the Add button.
addStockButton.addClickHandler(new ClickHandler() {
  public void onClick(ClickEvent event) {
    addStock();
  }
});

But this creates a new Instance(?..How can they even create an instance of Clickhandler, since its an Interface) everytime the button is clicked. How can I solve this that all buttons share a Clickhandler and the Handler askes the Button which button he is, so he can fire the method attached to that button.

Any Ideas? If you this is to vage information and you require more code please let me know.

Thanks in advance, Daniel

Upvotes: 0

Views: 2121

Answers (3)

janhink
janhink

Reputation: 5023

Eelke has already answered your question. I just add that if you would use GWT's UiBinder feature, you could achieve what you want like this:

@UiField
Button addStockButton;

@UiField
Button removeStockButton;

@UiHandler({ "addStockButton", "removeStockButton" })
void handleClickEvents(ClickEvent event)
{
    if (event.getSource() == addStockButton)
    {
        addStock();
    }
    else if (event.getSource() == removeStockButton)
    {
        removeStock();
    }
}

Upvotes: 2

Eelke
Eelke

Reputation: 22013

Java creates a new instance of an anonymous class that implements ClickHandler. Which it can do because you provide an implementation for the onClick function specified by the interface.

This class is however not created when you click on the button but at the moment you call addClickhandler. If you need the handler for multiple events do something like: 

ClickHandler handler = new ClickHandler() {
  public void onClick(ClickEvent event) {
    addStock();
  }
};
addStockButton.addClickHandler(handler);
someOtherButton.addClickHandler(handler);

Within the handler you can identify from where the event is coming using event.getSource().

If you have access to your button variables you could simply check the pointer

if (addStockButton == event.getSource()) ...

Or you can cast the result of getSource to the appropriate type and access the properties/methods of the object.

Upvotes: 2

mark-cs
mark-cs

Reputation: 4707

Its an anonymous instance of the interface, this is like declaring a new class that implements that interface.

I would have to ask why you would want to do this, you would need to make the ClickHandler contain a reference to its parent. You would also need to make the buttons identifiable so you can select the right one in the body of the ClickHandler. Is your need to only have a single instance really that bad that you can't have multiple anonymous instances ?

Upvotes: 0

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