如何在Core Animation中平滑更新动画的toValue /终点？

Question

I'm trying to emulate the effect Apple has as a progress indicator when downloading an app in iOS 7, somewhat of a pie chart:

My code's coming along pretty well. I can give it a value to go to and it will update to that.

- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect];

    CGFloat radius = CGRectGetWidth(self.frame) / 2;
    CGFloat inset  = 1;
    CAShapeLayer *ring = [CAShapeLayer layer];
    ring.path = [UIBezierPath bezierPathWithRoundedRect:CGRectInset(self.bounds, inset, inset)cornerRadius:radius-inset].CGPath;

    ring.fillColor = [UIColor clearColor].CGColor;
    ring.strokeColor = [UIColor whiteColor].CGColor;
    ring.lineWidth = 2;

    self.innerPie = [CAShapeLayer layer];
    inset = radius/2; 
    self.innerPie.path = [UIBezierPath bezierPathWithRoundedRect:CGRectInset(self.bounds, inset, inset)
                                               cornerRadius:radius-inset].CGPath;
    self.innerPie.fillColor   = [UIColor clearColor].CGColor;
    self.innerPie.strokeColor = [UIColor whiteColor].CGColor;
    self.innerPie.lineWidth   = (radius-inset)*2;

    [self.layer addSublayer:ring];
    [self.layer addSublayer:self.innerPie];

    self.progress = 0.0;
    self.innerPie.hidden = YES;
}

- (void)setProgress:(CGFloat)progress animated:(BOOL)animated {
    if (animated) {
        self.innerPie.hidden = NO;
        self.innerPie.strokeEnd = progress;

        CABasicAnimation *pathAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
        pathAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear];
        pathAnimation.duration = 0.5;
        pathAnimation.fromValue = [NSNumber numberWithFloat:self.progress];
        pathAnimation.toValue = [NSNumber numberWithFloat:progress];

        [self.innerPie addAnimation:pathAnimation forKey:@"strokeEnd"];
    }
    else {
        [CATransaction setDisableActions:YES];
        [CATransaction begin];
        self.innerPie.hidden = NO;
        self.innerPie.strokeEnd = progress;
        [CATransaction commit];
    }

    self.progress = progress;
}

My issue lies in updating the progress while in the middle of an animation . For example, if I'm downloading a file and the progress jumps from 0.1 to 0.2 (10% to 20%), I want to animate that. Say I give it a 0.5 second animation. What if, 0.2 seconds into the 0.5 second animation it jumps to 0.4 (40%)?

With my current code, it jumps to where it should have ended (when it should have animated) in the first animation (20%) and then starts animating again toward 40%.

What I'd like is for it to almost update the toValue to 0.4. At least that's what I think I want. Conceptually I'd like it to continue the animation toward the new value without interrupting the previous value. Smoothness, to put it simply.

How would I accomplish this?

I know there's libraries to do this. I want to do it myself for learning purposes.

Answer 1

You should replace

pathAnimation.fromValue = [NSNumber numberWithFloat:self.progress];

with :

pathAnimation.fromValue = [NSNumber numberWithFloat:[self.innerPie.presentationLayer strokeEnd]];

Initially, [self.innerPie.presentationLayer strokeEnd]] will be 1, so you will need to set the initial value to 0. Add these 2 lines to place where you create your 'innerPie' :

self.innerPie.strokeStart = 0;
self.innerPie.strokeEnd = 0;

I've tested and it works.