How to avoid asynchronous nature of scrollToVisibleRect causing intermittent timing issues - cocoa

Question

I am writing a Cocoa UI and am calling NSView's scrollRectToVisible repeatedly in a short space of time as a result of the user holding down a certain key (and hence repeatedly firing events on the main queue).
Through logging I can see that successive keyDown() events are firing prior to the scrollRectToVisible having completed its prior changing of the visibleRect. This is resulting in the subsequent scrollRectToVisible being called with incorrect inputs (namely the wrong starting visibleRect) and is leading to non-sensical UI behaviour. This happens about 50% of the time which I guess is to be expected dealing with an asynchronous problem.
How can I address this?
One way I can think of is by somehow turning scrollRectToVisible into a synchronous call. The problem is that it is an AppKit API and the method returns immediately with a boolean indicating whether it is going to scroll or not.

Answer 1

Try putting the code in the view that responds to the -keyDown:.
This example controls the scrolling with a timer instead of the repeated keyDowns from holding the key.
I couldn't get the UI unwanted behavior, so I don't know if this will help.
- (BOOL)acceptsFirstResponder {
return YES;
}
- (BOOL)canBecomeKeyView {
return YES;
}
- (void)keyDown:(NSEvent *)theEvent {
if (timer == nil) {
timer = [NSTimer scheduledTimerWithTimeInterval:0.01
target:self
selector:#selector(doScroll:)
userInfo:nil
repeats:YES];
}
NSLog(#"event %# with Timer%#", theEvent, timer);
}
- (void)keyUp:(NSEvent *)theEvent {
[timer invalidate];
timer = nil;
NSLog(#"keyUp timer:%#", timer);
NSLog(#"event %#", theEvent);
}
- (void)doScroll:theTimer {
NSEvent * current = [NSApp currentEvent]; //use this to parse
NSLog(#"current press %#", current);
[self scrollRectToVisible:NSMakeRect(self.visibleRect.origin.x + 10,
self.visibleRect.origin.y + 10,
self.bounds.size.width,
self.bounds.size.height)];
}