What Is ESSSquareProgressIndicator?
It’s an indeterminate progress indicator originally developed for the iOS game ZEN.
It uses Core Animation (specifically, CAShapeLayers) to do its job and it’s pretty straight forward.
CAShapeLayer has two animatable properties – -strokeStart and -strokeEnd (with a minimum value of 0.0 – start of path – and 1.0 – end of path). Going beyond 1.0 or below 0.0 doesn’t work.
So when trying to animate those values along a rectangular path – which was the first thing I tried when creating this – you do get a nice animation, but it ends at the 0.0/1.0 (which is, basically, the same) mark. So you end up with something like this:
The goal, then, was to animate beyond 1.0. The solution I came to – I’m sure there are other ways – was to use a second CAShapeLayer that animated alongside the first for the last/first part of the animation.
For a square, the values of the CAShapeLayer’s -strokeStart and -strokeEnd are as follows (starting at the top left corner):
The CGPath begins and ends in the top left corner because I drew the path that way (you could make it start in the lower left corner or in the middle of a side, it all depends on where you start the CGPath with CGPathMoveToPoint).
The progress indicator starts at the middle of the left line and reaches to the middle of the top line. These are the two CAShapeLayers at work. The left part is one shape layer, the top line is another. I’ll do this in pictures I drew in code so it’s simpler to understand (the lines represent where the layer animated to, the dots are the invisible rest of the square).
We start at this position. The line to the left is layer2(strokeStart:0.875, strokeEnd:1.0), the line at the top is layer1(strokeStart:0.0, strokeEnd:0.125).
The transition to this is the reason we need two layers as we can not animate one layer beyond the 1.0 value.
So we animate layer2 to (1.0, 1.0) and at the same time animate layer1 to (0.0, 0.25) which makes it look like one line moving.
This next part is easy. We animate layer1 to (0.75, 1.0).
Lastly, we animate with two layers again to get to the initial position so we can repeat the whole thing from there on.
We animate layer2 from (0.75, 1.0) to (0.875, 1.0) and layer1 from (0.0, 0.0) to (0.0, 0.125) at the same time, again making it look like one line moving.
The Source Code
The repository (a sample iOS app, but the class works the same on OS X) is available on Github.
It was developed for iOS 7.1 but should work on earlier systems, and has been tested on OS X Yosemite, but should work on earlier systems as well.
You just drop in an ordinary UIView or NSView and set its class to ESSSquareProgressIndicator. Done. You can then set the color and stroke width right within Interface Builder thanks to the fairly new Xcode macros IB_Designable and IBInspectable.
I have some more source code available here (or directly on my github profile page) if you’re interested. If you have any questions or feedback regarding this progress indicator, please be sure to mail or tweet me ;)