CGPathQuery is used for calculating points along a CGPath. This is intended for composing points along the path as part of a more complex graphic (such as a ShapeLayer).
It works by using core animation itself to pre-calculate values along a given range, then can be 'queried' later. Specifically, it makes use of immediately calculating a collection of points by making the animation.speed zero then reading the presentationLayer's position upon completion. If a value in-between two precalculated points is queried, averaging is applied to determine the value.
The classes should be considered experimental 'proof-of-concept' code at this time.
####What CGPathQuery isn't for
- Animating layers along a path. Core animation already does a great job of doing this by setting an animation.path property and an appropriate speed. This said, CGPathQuery could help if you're trying to animate the layer along a small partial segment of a path.
####References CGPathQuery uses the following techniques/libraries:
- Nick Lockwood's "iOS Core Animation: Advanced Techniques" - Ch. 9 - Manual Animation, which mentions the technique of setting speed to zero to get values immediately.
- CAAnimationBlocks, which is used in this library.
- This advice on creating an opengl context - required in order to get a non-nil presentationLayer: https://stackoverflow.com/questions/3429925/carenderer-never-produces-output/3430544#3430544
####Alternatives CGPathQuery was made to avoid replicating math at the expense of being a slightly less 'functional' solution. There's some other alternatives worth looking at:
- Creating a dashed line along a path and iterating along path elements (see https://stackoverflow.com/questions/841111/how-can-i-get-all-points-in-cgpath-curve-or-quad-curve )
- Using an external library like Wykobi (GPL/commercial) or Claw (LGPL) or other bezier math to calculate/query a similarly-created path.
- Possibly the bezierPointAtT method found in performanceBezier: https://github.com/adamwulf/PerformanceBezier/blob/master/PerformanceBezier/UIBezierPath%2BPerformance.m#L494-L522
- Using SpriteKit's SKAction "followPath:asOffset:orientToPath:duration:" method.
####Warnings
- CGPathQuery, at this time of writing, appears to work best for very simple paths - paths where the segments are equal in overall length. Be careful when using more complex paths - (e.g. from personal observation CGPathQuery can yield unexpected results when drawing long paths with ended with smaller rounded corners - where querying indicates 'bunching up' around the corners)
###TODO
- Add usage documentation.
- Make the pre-calculation step optional. So far I haven't yet found a way to retrieve animation values with core animation without causing a deadlock on the main thread.
- Add rotation support. I've been able to grab the rotation angle in other demos from the presentationValue while layers are visible, but haven't been able to see it update in the current code.