Why am I unable to detect when my UIView I push off screen using UIKit Dynamics is no longer visible?

Question

I'm using UIKit Dynamics to push a UIView off screen, similar to how Tweetbot performs it in their image overlay.

I use a UIPanGestureRecognizer, and when they end the gesture, if they exceed the velocity threshold it goes offscreen.

    [self.animator removeBehavior:self.panAttachmentBehavior];

    CGPoint velocity = [panGestureRecognizer velocityInView:self.view];

    if (fabs(velocity.y) > 100) {
        self.pushBehavior = [[UIPushBehavior alloc] initWithItems:@[self.scrollView] mode:UIPushBehaviorModeInstantaneous];
        [self.pushBehavior setTargetOffsetFromCenter:centerOffset forItem:self.scrollView];
        self.pushBehavior.active = YES;

        self.pushBehavior.action = ^{
            CGPoint lowestPoint = CGPointMake(CGRectGetMinX(self.imageView.bounds), CGRectGetMaxY(self.imageView.bounds));
            CGPoint convertedPoint = [self.imageView convertPoint:lowestPoint toView:self.view];

            if (!CGRectIntersectsRect(self.view.bounds, self.imageView.frame)) {
                NSLog(@"outside");
            }
        };

        CGFloat area = CGRectGetWidth(self.scrollView.bounds) * CGRectGetHeight(self.scrollView.bounds);
        CGFloat UIKitNewtonScaling = 5000000.0;
        CGFloat scaling = area / UIKitNewtonScaling;
        CGVector pushDirection = CGVectorMake(velocity.x * scaling, velocity.y * scaling);

        self.pushBehavior.pushDirection = pushDirection;

        [self.animator addBehavior:self.pushBehavior];
    }

I'm having an immense amount of trouble detecting when my view actually completely disappears from the screen.

My view is setup rather simply. It's a UIScrollView with a UIImageView within it. Both are just within a UIViewController. I move the UIScrollView with the pan gesture, but want to detect when the image view is off screen.

In the action block I can monitor the view as it moves, and I've tried two methods:

1. Each time the action block is called, find the lowest point in y for the image view. Convert that to the view controller's reference point, and I was just trying to see when the y value of the converted point was less than 0 (negative) for when I "threw" the view upward. (This means the lowest point in the view has crossed into negative y values for the view controller's reference point, which is above the visible area of the view controller.)

This worked okay, except the x value I gave to lowestPoint really messes everything up. If I choose the minimum X, that is the furthest to the left, it will only tell me when the bottom left corner of the UIView has gone off screen. Often times as the view can be rotating depending on where the user pushes from, the bottom right may go off screen after the left, making it detect it too early. If I choose the middle X, it will only tell me when the middle bottom has gone off, etc. I can't seem to figure out how to tell it "just get me the absolute lowest y value.

2. I tried CGRectIntersectsRect as shown in the code above, and it never says it's outside, even seconds after it went shooting outside of any visible area.

What am I doing wrong? How should I be detecting it no longer being visible?

score 0 · Answer 1 · answered Jul 25 '14 at 14:53

If you take a look on UIDynamicItem protocol properties, you can see they are center, bounds and transform. So UIDynamicAnimator actually modifies only these three properties. I'm not really sure what happens with the frame during the Dynamics animations, but from my experience I can tell it's value inside the action block is not always reliable. Maybe it's because the frame is actually being calculated by CALayer based on center, transform and bounds, as described in this excellent blog post.

But you for sure can make use of center and bounds in the action block. The following code worked for me in a case similar to yours:

CGPoint parentCenter = CGPointMake(CGRectGetMidX(self.view.bounds), CGRectGetMidY(self.view.bounds));
self.pushBehavior.action = ^{
    CGFloat dx = self.imageView.center.x - parentCenter.x;
    CGFloat dy = self.imageView.center.y - parentCenter.y;
    CGFloat distance = sqrtf(dx * dx + dy * dy);
    if(distance > MIN(parentCenter.y + CGRectGetHeight(self.imageView.bounds), parentCenter.x + CGRectGetWidth(self.imageView.bounds))) {
       NSLog(@"Off screen!");
    }
};

Why am I unable to detect when my UIView I push off screen using UIKit Dynamics is no longer visible?

1 Answers1