画勾：
CAShapLayer给定一个路径它就能跟着你的路径画出一个你想要的图形
CAAnimation——速度控制函数
速度控制函数(CAMediaTimingFunction)
kCAMediaTimingFunctionLinear（线性）：匀速，给你一个相对静态的感觉
kCAMediaTimingFunctionEaseIn（渐进）：动画缓慢进入，然后加速离开
kCAMediaTimingFunctionEaseOut（渐出）：动画全速进入，然后减速的到达目的地
kCAMediaTimingFunctionEaseInEaseOut（渐进渐出）：动画缓慢的进入，中间加速，然后减速的到达目的地。这个是默认的动画行为。
设置动画的执行节奏
anim.timingFunction= [CAMediaTimingFunction
functionWithName:kCAMediaTimingFunctionLinear];
- (void)drawPic:(CGRect)rect context:(CGContextRef)context {
_shap = [CAShapeLayer layer];
CGPoint point[3] = {{150,200},{200,250},{300,150}};
CGMutablePathRef mu = CGPathCreateMutable();
CGPathAddLines(mu, nil, point, 3);
_shap.path = mu;
_shap.strokeStart = 0;
_shap.strokeEnd = 1;
_shap.fillColor = [UIColor clearColor].CGColor;
_shap.strokeColor = [UIColor redColor].CGColor;
_shap.lineWidth = 13;
[self.layer addSublayer:_shap];
//动画
CABasicAnimation *base = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
base.fromValue = @(0);
base.toValue = @(1);
base.duration = 2;
base.repeatCount = CGFLOAT_MAX;
// base.fillMode = @"forwards";
// base.removedOnCompletion = NO;
[_shap addAnimation:base forKey:@"d"];
CGPathRelease(mu);
}
