How to expose events from child objects on the containing composite object? - gwt

Question

I have a composite widget that contains many buttons (eg. Button1, Button2, etc). I can't figure how to expose button click events on the composite widget. I'm trying to avoid creating custom events such as Button1ClickEvent and Button2ClickEvent and instead reuse the existing GWT ClickEvent for both of them. The following code snippet can give an idea what I'm trying to do:
public class WidgetWithTwoButtons extends Composite {
...
#UiField Button button1;
#UiField Button button2;
#UiHandler("button1")
void onButton1Click(ClickEvent event) {
// TODO fire click event on Button1ClickHandler
}
public HandlerRegistration addButton1ClickHandler(ClickHandler handler) {
return addDomHandler(handler, ClickEvent.getType());
}
#UiHandler("button2")
void onButton1Click(ClickEvent event) {
// TODO fire click event on Button2ClickHandler
}
public HandlerRegistration addButton2ClickHandler(ClickHandler handler) {
return addDomHandler(handler, ClickEvent.getType());
}
}
I think this is not the best practice. I'd appreciate if you give pointers to recommended solutions/examples to this problem in your answers. Thanks!

Answer 1

I started thinking about your problem and first noticed instead of using addDomHandler you can change your current addButton1ClickHandler(ClickHandler handler) simply to :
public HandlerRegistration addButton1ClickHandler(ClickHandler handler) {
return button1.addClickHandler(handler);
}
If you had only one button, you could expose it by making your Composite implement HasClickHandlers and implement the below method to pass it through to your button:
#Override
public HandlerRegistration addClickHandler(ClickHandler handler) {
return button1.addClickHandler(handler);
}
But since you have many buttons, you need a method that takes the button you want to add your clickHandler to as a parameter such as :
public HandlerRegistration addClickHandlerToButton(ClickHandler handler, Button target){
return target.addClickHandler(handler);
}
The problem with this approach is, to use this you need to have a reference to your buttons which means you need to define getter methods for your buttons in your composite such as public Button getButton1() . When you expose your buttons like this, the problem is one does not need a passthrough method on the composite anymore since he/she can directly access buttons anyway making the above approach that takes a target button as parameter obsolete. Worst part is he/she can even change the styles and even detach those buttons.
To solve this you can expose your buttons over the HasClickHandlers interface.
So IMHO this is how i beleive it should be done:
public class ComplexComposite extends Composite {
private Button button1 = new Button("btn1");
private Button button2 = new Button("btn2");
public ComplexComposite(){
HorizontalPanel panel = new HorizontalPanel();
panel.add(button1);
panel.add(button2);
initWidget(panel);
}
public HasClickHandlers getButton1(){
return button1;
}
public HasClickHandlers getButton2(){
return button2;
}
}
With this approach you expose your buttons only over the desired interface and one can add
click handlers such as :
ComplexComposite composite = new ComplexComposite();
composite.getButton1().addClickHandler(new ClickHandler() {
#Override
public void onClick(ClickEvent event) {
...
}
});