I have two views: A and B. A is positioned at the top of the screen, B is positioned at the bottom of the screen.
When the user presses a button, view B animates upwards with a EaseInEaseOut bezier curve until it reaches y = 0. While B is on its way to its destination, it should push A up when it hits A. In other words, when B has passed a certain y coordinate (A's y origin + height) during its transition from bottom to top, A should stick to B so it seems B pushes A upwards.
What I have tried so far:
- Register a target + selector to a CADisplayLink immediately after the user pressed the button. Inside this selector, request view B's y coordinate by accessing its presentationLayer and adjust A's y coordinate accordingly. However, this method turns out to be not accurate enough: the presentationLayer's frame is behind on B's current position on the screen (this is probably because -presentationLayer recalculates the position of the animating view on the current time, which takes longer than 1 frame). When I increase B's animation duration, this method works fine.
- Register a target + selector to a CADisplayLink immediately after the user pressed the button. Inside this selector, calculate B's current y coordinate by solving the bezier equation for x = elapsed time / animation duration (which should return the quotient distance traveled / total distance). I used Apple's open source UnitBezier.h for this (http://opensource.apple.com/source/WebCore/WebCore-955.66/platform/graphics/UnitBezier.h). However, the results are not correct.
Any suggestions on what I can try next?
You said that you tried the "animate B and use display link to update A" technique, and that it resulted in "A" lagging behind "B". You could theoretically animate a new view, "C", and then adjust B and A's frames accordingly in the display link, which should eliminate any lag (relative to each other).