ios - Forcing a UIView to redraw its contents inside of an animation block -

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i have superview , subview b. in a's drawrect(_:) line drawing depends on position , size of subview b. in class (but not in drawrect(_:)) have animation block moves , scales b.

the problem drawing in drawrect(_:) happens before animation takes place, either using final position , size of subview or using initial position , size, never using intermediate values.

case 1: drawing uses final state

the image below screen shot of simulator while animation scales subview in progress. note how lines drawn in final state.

the code line drawing in a's drawrect(_:). code animation (it's in class self refers superview):

uiview.animatewithduration(3.0, delay: 0.5,     options: uiviewanimationoptions.curvelinear,     animations: {          [unowned self] in         self.subviewwidthconstraint.constant = 0.75 * cgrectgetwidth(self.bounds)          self.layoutifneeded()         self.setneedsdisplay() // has no effect since self's bounds don't change      }) { (completed) -> void in }

enter image description here

case 2: drawing uses initial state

looking around answers found suggestion use self.layer.displayifneeded() in place of self.setneedsdisplay() in animation block, tried well

func animateframe() {     uiview.animatewithduration(3.0, delay: 0.5,         options: uiviewanimationoptions.curvelinear,         animations: {              [unowned self] in             self.subviewwidthconstraint.constant = 0.75 * cgrectgetwidth(self.bounds)              self.layoutifneeded()             self.layer.displayifneeded() // difference here          }) { (completed) -> void in } }

but results in following. again, screen shot while animation scales subview in progress. now, lines again drawn before animation starts using subview's initial position , size.

enter image description here

i think understand case 1. entire animation block queued execution end result computed time animation starts it's no surprise drawinrect(_:) using final state.

i don't understand case 2 that's because have little experience dealing directly view layer.

either way, effect i'm looking achieve lines drawn vertices of subview while it's being moved and/or scaled. suggestions or pointers documentation appreciated. thanks!


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