Re: [platform-swt-dev] Efficient eventHandlers in a subclass

[Steve Northover <Steve_Northover@xxxxxxxxxx>]

Due to the dynamic fragile subclass
problem, you almost never subclass in SWT to get behavior.  It's a
design decision.

Mauk van der Laan <mauk@xxxxxxxxxxxx>
Sent by: platform-swt-dev-bounces@xxxxxxxxxxx

09/28/2006 03:49 AM

Please respond to
"Eclipse Platform SWT component developers list." <platform-swt-dev@xxxxxxxxxxx>

To	platform-swt-dev@xxxxxxxxxxx
cc
Subject	[platform-swt-dev] Efficient eventHandlers in a subclass

Hi,

I create a lot of Composite or Canvas subclasses. To be able to handle
their events,
the following code is used:

class X extends Canvas implements Listener {
    X() {
        ...
        addListener(SWT.Paint, this);
        addListener(SWT.MouseMove,this);
        ... lots more
    }
    public void handleEvent(Event e) {
        switch(e.type) {
        case SWT.Paint:   
            onPaint();
            break;
        ... etc


This is too 'heavy' in my opinion: each component has a big EventTable
and must do a lookup
for every event, only to send it to the same handleEvent() method where
it is tested again
by the switch.

It would be better to override some public method, the eventTable would
be empty that way
(unless another component registers). The problem is that 
Widget.sendEvent() isn't public.

To make this possible, i propose one of the following changes:
- make Widget.sendEvent() public,
- let it call a new public method 'handleAllEvents()' before it does the
EventTable lookup
- create a special 'catch-all' event constant like SWT.AllEvents which
is checked in 
EventTable.sendEvent() to that it sends all events to a handler that is
registered with
that constant.

Regards,
Mauk

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