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WWDC2014_Session235_Advanced_Scrollviews_and_Touch_Handling_Techniques.md

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WWDC2014 Session 235 Advanced Scrollviews and Touch Handling Techniques

This is one of the very technique session in WWDC 2014. I have made a duplicate demo from this session and published it here.

Here is a list of problems and solutions from the talk.

  • When UIScrollView with DrawerView (UIVisuallEffectView) added to controller's view(AView), AView's user interaction is blocked. How to keep AView unblocked from user?

Set UIScrollView's userInteractionEnabled to be NO.

  • When UIScrollView's userInteractionEnabled is false, how can we keep UIScrollView's panGestureRecognizer, so the UIScrollView could be pulled up or down.

Easy solution would be pass the UIScrollView's property panGestureRecognizer to its superview, so the superview could recognize the pan gesture, while not being blocked.

  • How to add subviews to UIVisualEffectView?

Since UIVisualEffectView would do tons of work for the blurring effect on the view, it's best to add any subview to its contentView, which is a UIView on top of it. Then these subviews would not interfere with the computation of visual effect.

  • How does - hitTest:withEvent: work?

This function is defined in UIView, the following is the pseudo code.

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event { 

	if (/* point is in our bounds */) {
        for (/* each subview, in reverse order */) {
            UIView *hitView = /* recursive call on subview */
            if (hitView != nil) {
                return hitView;
			}
		}
		return self; 
	}
	return nil;

}

  • How to reenable user interaction on UIScrollView without blocking its parent view?
  1. Set UIScrollView's userInteractionEnabled to YES.
  2. Subclass UIScrollView, override - hitTest:withEvent: function, so this function either returns UIScrollView's subview, or its superview, rather itself.
  • (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event { UIView *hitView = [super hitTest:point withEvent:event]; if (hitView == self) { return nil; } return hitView; }
  • How to not cancel other touch event when UILongPressGestureRecognizer is triggered?

Set cancelsTouchesInView of UILongPressGestureRecognizer to NO.

  • How to make several UIGestureRecognizer recognize simultaneously?

Implement UIGestureRecognizer delegate method: - gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer:, and return YES when certain both of two UIGestureRecognizer could be triggered.

This way, UIGestureRecognizer could work simultaneously, between siblings and self, also between superview and self.

Thank system for supporting resting touches.

  • Good UX to implement:
  1. When grab / drop a dot: Apply a simple animation could be very good choice. Just scale the transform and change the alpha of view.
  2. When user does not click on the center of view, there might be a 'Jump' at first: This could be fixed simply move it to the user touch location when view is grabbed.
  • Fastest Fix on conflict between UIView's touch event and UIScrollView's panGestureRecognizer.

Just disable, then enable UIScrollView's panGestureRecognizer when UIView's UILongPressGestureRecognizer triggered. This will disable panGestureRecognizer for current user touch, but reenable it will allow panGestureRecognizer to recognize new touches from user.

  • How to apply the feature "Touch Delay Gesture Recognizer" from UIScrollView to UIView.
  1. Subclass UIGestureRecognizer.
  2. #import <UIKit/UIGestureRecognizerSubclass.h>
  3. Introduce a NSTimer for the purpose of delay.
  4. Set delaysTouchesBegan to YES in initializer.
  5. Set UIGestureRecognizer's State to Failed when timer expired, touch cancelled or touch ended.
  6. Override - reset function to reset timer.
  7. Add this customized UIGestureRecognizer to the view, which is a subclass of UIView.
  • One More Thing: how to more easily recognize user's touch when subclass of UIView is pretty small for user to click on?

Override method - pointInside:withEvent: of UIView, in the method, check bigger area instead, rather than the bounds of the view.

44 is a very common number used in iOS system, so use it frequently and wisely.