用到的類及其簡單的解釋：

1、UIBezierPath：使用此類可以定義簡單的形狀介时，如橢圓或者矩形隙畜，或者有多個直線和曲線段組成的形狀

2、CAShapeLayer：繼承自CALayer鹰服，因此，可使用CALayer的所有屬性杜恰。但是获诈，CAShapeLayer需要和貝塞爾曲線配合使用才有意義仍源。

3、CABasicAnimation：使用方式就是基本的關(guān)鍵幀動畫舔涎。所謂關(guān)鍵幀動畫笼踩，就是將Layer的屬性作為KeyPath來注冊，指定動畫的起始幀和結(jié)束幀亡嫌，然后自動計(jì)算和實(shí)現(xiàn)中間的過渡動畫的一種動畫方式嚎于。

4、CATransaction：CATransaction的作用是保證多個“Animatable”的變化同時進(jìn)行挟冠。也就是說CALayer的屬性修改需要依賴CATransaction于购。

扇形代碼：

- (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;

}

如果你根據(jù)下載的進(jìn)度來控制動畫，方法如下：

- (void)startAnimationWithProgress:(CGFloat)progress {

  [CATransaction begin];
  [CATransaction setAnimationDuration:0.1];
  [CATransaction setCompletionBlock:^{
  }];
  [self.shapeLayer setStrokeEnd:progress];
  [CATransaction commit];

}

如果想直接是一個扇形的動畫：

- (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"];

}

效果圖如下：

效果.gif