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Re: Getting NSScroller to respond to mouseUp: method

Hello...

The reason that mouseUp: isn't called in most NSControl type classes is that the mouse up event is eaten by the implementation of the mouseDown: method. Most controls start a seperate event tracking loop to handle dragging of the mouse in the mouseDown: method and remain in that event loop until a mouse up event occurs. Since the events that occur during the seperate event tracking loop are handled directly by the code in the NSControl, they are not dispatched using the standard AppKit techniques and don't trigger the normal methods that are inherited from NSResponder.

For more info on the mouse event tracking done by controls, take a look at the conceptual topic on "Basic Event Handling", specifically the section "Handling Mouse Events in Views" which has an example of how this sort of thing might be implemented near the end.
<http://developer.apple.com/documentation/Cocoa/Conceptual/BasicEventHandling/Tasks/HandlingMouseEvents.html#//apple_ref/doc/uid/20000906>


So if you wanted to force your subclass of NSScroller to use mouseUp: or add code which runs when the mouse button is released, all you have to do is override mouseDown: and put the code after the call to the superclass implementation.

- (void)mouseDown:(NSEvent *)theEvent
{
	[super mouseDown:theEvent];

	/* ... whatever you wanted to do at mouse up here ... */
}


If the code needs to run any time that the NSScrollView is scrolled, you could avoid subclassing NSScroller by listening for the NSViewBoundsDidChangeNotification notifications that occur when the bounds origin of the scroll view's NSClipView is changed (which is what creates the scroll effect).

You can also take advantage of the fact that the run loop is NOT being run in NSDefaultRunLoopMode while the scroller is being manipulated to delay the method invocation until right after the mouse button is released (most controls and other things like menus that do their own event handling loops use NSWindow's nextEventMatchingMask: method which runs the event loop using NSEventTrackingRunLoopMode).

- (void)awakeFromNib
{

	/* ... any other stuff  ... */

/* register for bounds changed notification for relevant clip view */
NSNotificationCenter *noteboard = [NSNotificationCenter defaultCenter];
[noteboard addObserver:self selector:@selector(clipViewDidScroll:) name:NSViewBoundsDidChangeNotification object:[theScrollView contentView]];

}

- (void)clipViewDidScroll:(NSNotification *)note
{
/* set up delayed action (and coalesce since should only happen once) */
[NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(someImportantButSlowMethod:) object:[note object]];
[self performSelector:@selector(someImportantButSlowMethod:) withObject:[note object] afterDelay:0.0];
}

- (void)someImportantButSlowMethod:(id)sender
{
/* ... do whatever expensive stuff you wanted to do after scrolling ... */
NSLog(@"doing someImportantButSlowMethod: only once");
}

If the same code needs to run any time the size of the visible area within the scrollview is changed due to the window being resized, then you could also register for the NSViewFrameDidChangeNotification for the clipview and send it through the same clipViewDidScroll: selector...


Hope that helps,

	Louis


Hi all, I am having some troubles and I wanted to shoot a question off and
see if anyone could offer some wisdom.

I am attempting to get a subclassed NSScroller to respond to mouseUp:
method.  When overriding the default behaviour for mouseDown: (by tossing an
NSLog entry in to let me know it's been called then and calling [super
mouseDown:theEvent]) it works just fine, but the instance does not seem to
respond to mouseUp: at all... (which according to my logic, it should as it
inherits from NSResponder...right?)

I did notice that NSControl overrides mouseDown: but does not include any
reference to mouseUp: would this level of inheritence remove the
functionality/presence of NSResponder's inhereted mouseUp: method?

Just to provide some scenery... The app I am building requires thumbnail
previews of images and currently bogs down quite a bit when dealing with a
large number of images (1000+). The reason that I am trying to get
NSScroller to respond to mouseUp: is to defer some of the repetitive and
expensive drawing operations performed on a subclassed NSMatrix of
NSImageCells in a subclassed NSScrollView, until after the user releases the
scroll knob.

Does anybody know what I am overlooking in regards to mouseUp: ?

Thanks in advance,
Joe Striedl


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