mouseDragged events get stolen by another view

Question

I have several views of class MyView (subclass of NSView) inside another NSView. MyView implements -mouseEntered:, -mouseExited:, -mouseDown:, -mouseDragged:, and -mouseUp:.

Almost always, when a MyView receives a mouse-down event, all subsequent mouse-dragged events are received by the same MyView until the next mouse-up event. Even if the cursor goes outside of the MyView. That is the expected behavior.

Occasionally, a MyView will receive a mouse-down event, but will only receive mouse-dragged and mouse-up events while the cursor remains inside the MyView. If the cursor moves onto a different MyView, then that MyView starts receiving mouse-dragged events (without first receiving a mouse-down event) and can receive the subsequent mouse-up event.

In case it matters, the mouse-down event creates a FooView (subclass of NSView) on top of the MyView, and the mouse-dragged events resize the frame of the FooView. This might be related, as I've only been able to reproduce the problem after one of these FooViews has been created. FooView does not implement any of the mouse event methods.

I've been messing with this for a while now and haven't been able to either purposely reproduce the problem or recreate the problem in a simple example. I'd be happy to answer any questions about my code, I'm just not sure what would be the relevant part to post.

score 2 · Accepted Answer · answered Sep 19 '11 at 04:00

2

Not sure what the root issue is (this Cocoa behavior seems inconsistent to me)... but here's one possible workaround:

In the superview, create an instance variable tracking the MyView instance in which the -mouseDown: occurred.
When you receive a -mouseDragged: in MyView, instead of operating on self, operate on the MyView instance reference stored in the superview.

...then you'll be able to consistently track which object is being dragged, without having to run your own event loop.

answered Sep 19 '11 at 04:00

smokris

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The cause of the problem turned out to be NSView not handling overlapping sibling views in a predictable way. Your workaround fixed the problem -- thanks. – jlstrecker Sep 21 '11 at 16:21
Thank you, this answer just saved me hours of headaches! – DrRocket Nov 08 '15 at 16:51
if a child of MyView dissapears (by getting removed or resized) from under the cursor while dragging, things start to go crazy, fast. – Radu Simionescu Jan 19 '16 at 15:08

Davyd Geyl · Answer 2 · 2011-09-17T09:54:26.077

0

You need to run you own mouse tracking loop in the view until mouse is up. You can extend it to process more types of events by passing them into nextEventMatchingMask:.

- (void)mouseDown:(NSEvent*)event
{
    CGPoint hitPoint = [self pointInViewSpaceFromEvent:event];

    BOOL isDragging = NO;
    BOOL isTracking = YES;

    while (isTracking)
    {       
        switch ([event type])
        {
            case NSLeftMouseDown:
                [self singleMouseDownAtPoint:hitPoint withModifierFlags:[event modifierFlags]];
                break;

            case NSLeftMouseUp:
                isTracking = NO;
                if (isDragging)
                    [self mouseDraggingDidEndAtPoint:hitPoint];
                else
                    [self singleMouseUpAtPoint:hitPoint withEvent:event];
                break;

            case NSLeftMouseDragged:
                if (isDragging)
                    [self mouseDraggingAtPoint:hitPoint withModifierFlags:[event modifierFlags]];
                else
                    isDragging = YES;
                break;
            default:
                break;
        }

        if (isTracking)
        {
            event = [[self window] nextEventMatchingMask:NSLeftMouseDraggedMask | NSLeftMouseUpMask];
            hitPoint = [self pointInViewSpaceFromEvent:event];
        }
    }
}

edited Sep 17 '11 at 09:54

answered Sep 17 '11 at 08:54

Davyd Geyl

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Thanks. This doesn't exactly fix the problem but maybe it gives a clue. When the mouse-tracking loop is running, the drag and mouse-up events are *not* getting stolen -- presumably because there are no other `MyView`s to steal the event, because the other `MyView`s are blocked from receiving events. However, I don't think the mouse-tracking loop will work for me *because* it blocks all other events. Events to other `MyView`s, and other objects in the application, need to get through. I tried wrapping the loop in a `dispatch_async`, but once again the drag and mouse-up events got stolen. – jlstrecker Sep 19 '11 at 02:14
1

If you really need to process dragging events in your view this is the only correct way to do it. This is how all Cocoa controls work. Try to drag the knob of a NSSlider. If it did not track the mouse in its own loop it would stop responding to the drag events as soon as cursor goes out of its bounds. And yes all the events of those types you specify in nextEventMatchingMask: will go to this particular view, other view will not receive them. However, this may be not what you need. If you want to process drags between the views, you need to do it one level up: int the superview of those views. – Davyd Geyl Sep 19 '11 at 03:22
What do you mean by "this is the only correct way"? Not that Apple docs are flawless, but the [Cocoa Event-Handling Guide](http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/EventOverview/HandlingMouseEvents/HandlingMouseEvents.html#//apple_ref/doc/uid/10000060i-CH6) seems to imply that `mouseDragged` and `mouseUp` should work. Is my problem a bug in Apple's code or my own misunderstanding of how to use it? Note that `mouseDragged` and `mouseUp` *usually* work as I had expected. – jlstrecker Sep 19 '11 at 16:45
2

Well, I am sorry, my answer was wrong. You do not need to implement the code I have provided, unless you need more flexibility with events; the window does this job for you calling the methods like mouseDragged:, etc. I have tried to reproduce the case you had described. My guess was that the first responder may be changed during dragging. This might explain why other views start receiving messages. However, I have not managed to reproduce any of those scenarios. I would try to watch becomeFirstResponder and resignFirstResponder in your views. I'm sorry I cannot help without seeing the code. – Davyd Geyl Sep 20 '11 at 02:58
so mouseDown gets triggered when there's a NSLeftMouseDragged event or NSLeftMouseUp? I don't get it – Radu Simionescu Jan 19 '16 at 15:00

mouseDragged events get stolen by another view

2 Answers2