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A trick for capturing all touch input for the duration of a touch

By Michael Tyson | Published: August 14, 2010

If you’ve ever tried to implement an interactive control that makes use of gestures within a UITableView, or tried to implement a view that you can drag around, like pins on a MKMapView, you’ll know that you’re either generally thwarted by the scroll view (and the table view will just scroll, instead of your control getting the vertical drag gesture), or as soon as the touch moves outside the bounds of your view, you’ll get no more touchesMoved events, making for a very frustrating dragging interaction.

Some platforms give you a way to capture all pointer input for a time, but this doesn’t appear to be available out of the box on the iPhone — at least, I haven’t found it.

Here’s a way to make it work:

  • Subclass UIWindow and replace sendEvent: with your own method
  • Provide a way for objects to be registered with your UIWindow subclass to gain ‘touch priority’ – either add a property that taken a UIView/UIResponder, or add a mutable array to be able to register multiple views.
  • When you receive a touch began event, check to see if the touch was within the bounds of any ‘priority’ views – if they are, the view subsequently gets all events until the touch ended event.

Here’s what the UIWindow subclass could look like:

 
@interface CTTouchInterceptingWindow : UIWindow {
    NSMutableArray *views;
 
    @private
    UIView *touchView;
}
 
- (void)addViewForTouchPriority:(UIView*)view;
- (void)removeViewForTouchPriority:(UIView*)view;
 
@end
 
 
@implementation CTTouchInterceptingWindow
 
- (void)dealloc {
    if ( views ) [views release];
    [super dealloc];
}
 
- (void)addViewForTouchPriority:(UIView*)view {
    if ( !views ) views = [[NSMutableArray alloc] init];
    [views addObject:view];
}
- (void)removeViewForTouchPriority:(UIView*)view {
    if ( !views ) return;
    [views removeObject:view];
}
 
- (void)sendEvent:(UIEvent *)event {
    if ( !views || [views count] == 0 ) {
        [super sendEvent:event];
        return;
    }
 
    UITouch *touch = [[event allTouches] anyObject];
 
    switch ( touch.phase ) {
        case UITouchPhaseBegan:
            for ( UIView *view in views ) {
                if ( CGRectContainsPoint([view frame], [touch locationInView:[view superview]]) ) {
                    touchView = view;
                    [touchView touchesBegan:[event allTouches] withEvent:event];
                    return;
                }
            }
            break;
        case UITouchPhaseMoved:
            if ( touchView ) {
                [touchView touchesMoved:[event allTouches] withEvent:event];
                return;
            }
            break;
        case UITouchPhaseCancelled:
            if ( touchView ) {
                [touchView touchesCancelled:[event allTouches] withEvent:event];
                return;
            }
            touchView = nil;
            break;
        case UITouchPhaseEnded:
            if ( touchView ) {
                [touchView touchesEnded:[event allTouches] withEvent:event];
                return;
            }
            touchView = nil;
            break;
    }
 
    [super sendEvent:event];
}
 
@end

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