用到的类及其简单的解释：
1、UIBezierPath：使用此类可以定义简单的形状，如椭圆或者矩形，或者有多个直线和曲线段组成的形状
2、CAShapeLayer：继承自CALayer，因此，可使用CALayer的所有属性。但是，CAShapeLayer需要和贝塞尔曲线配合使用才有意义。
3、CABasicAnimation：使用方式就是基本的关键帧动画。所谓关键帧动画，就是将Layer的属性作为KeyPath来注册，指定动画的起始帧和结束帧，然后自动计算和实现中间的过渡动画的一种动画方式。
4、CATransaction：CATransaction的作用是保证多个“Animatable”的变化同时进行。也就是说CALayer的属性修改需要依赖CATransaction。
扇形代码：
- (void)createSectorView{
UIView *bgView = [[UIView alloc]initWithFrame:self.bounds];
bgView.backgroundColor = [UIColor colorWithRed:255 green:255 blue:255 alpha:0.4];
[self addSubview:bgView];
CGFloat width = self.frame.size.width;
CGFloat height = self.frame.size.height;
CGFloat centerX = width / 2;
CGFloat centerY = height / 2;
UIBezierPath *bezierPath = [UIBezierPath bezierPathWithArcCenter:CGPointMake(centerX, centerY) radius:width / 4 startAngle:(CGFloat)(-M_PI / 2) endAngle:(CGFloat)(3 * M_PI / 2) clockwise:YES];
_shapeLayer = [CAShapeLayer layer];
[bgView.layer addSublayer:_shapeLayer];
_shapeLayer.fillColor = [UIColor clearColor].CGColor;
_shapeLayer.strokeColor = [UIColor colorWithRed:255 green:255 blue:255 alpha:0.6].CGColor;
_shapeLayer.lineWidth = width / 2.0;
_shapeLayer.path = bezierPath.CGPath;
bgView.layer.mask = _shapeLayer;
}
如果你根据下载的进度来控制动画，方法如下：
- (void)startAnimationWithProgress:(CGFloat)progress {
[CATransaction begin];
[CATransaction setAnimationDuration:0.1];
[CATransaction setCompletionBlock:^{
}];
[self.shapeLayer setStrokeEnd:progress];
[CATransaction commit];
}
如果想直接是一个扇形的动画：
- (void)startAnimation{
CABasicAnimation *animation = [CABasicAnimation animation];
animation.keyPath = @"strokeEnd";
animation.duration = 2.0;
animation.fromValue = @0.0;
animation.toValue = @1.0;
animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
animation.removedOnCompletion = NO;
//animation.fillMode = kCAFillModeRemoved;
//[_shapeLayer setStrokeEnd:toEnd];
[animation setDelegate:self];
[_shapeLayer addAnimation:animation forKey:@"strokeEndAnimation"];
}
效果图如下：
效果.gif
