shadow属性 from Xcode

这是从xcode里面摘取的内容，苹果的官方文档
/** Shadow properties. **/

/* The color of the shadow. Defaults to opaque black. Colors created
* from patterns are currently NOT supported. Animatable. */

@property(nullable) CGColorRef shadowColor;
/* The opacity of the shadow. Defaults to 0. Specifying a value outside the
* [0,1] range will give undefined results. Animatable. */
@property float shadowOpacity;

/* The shadow offset. Defaults to (0, -3). Animatable. */
@property CGSize shadowOffset;
/* The blur radius used to create the shadow. Defaults to 3. Animatable. */
@property CGFloat shadowRadius;
/* When non-null this path defines the outline used to construct the

layer's shadow instead of using the layer's composited alpha
channel. The path is rendered using the non-zero winding rule.
Specifying the path explicitly using this property will usually
improve rendering performance, as will sharing the same path
reference across multiple layers. Upon assignment the path is copied.
Defaults to null. Animatable. */

@property(nullable) CGPathRef shadowPath;`

其实就是三个属性

//加阴影
_imageView.layer.shadowColor = [UIColor blackColor].CGColor;//shadowColor阴影颜色
_imageView.layer.shadowOffset = CGSizeMake(4,4);//shadowOffset阴影偏移,x向右偏移4，y向下偏移4，默认(0, -3),这个跟shadowRadius配合使用
_imageView.layer.shadowOpacity = 0.8;//阴影透明度，默认0

_imageView.layer.shadowRadius = 4;//阴影半径，默认3

后面这个是做的效果图
form http://blog.csdn.net/rhljiayou/article/details/10178723

_imageView1.layer.shadowColor = [UIColor yellowColor].CGColor;//shadowColor阴影颜色
_imageView1.layer.shadowOffset = CGSizeMake(0,0);//shadowOffset阴影偏移，默认(0, -3),这个跟shadowRadius配合使用
_imageView1.layer.shadowOpacity = 1;//阴影透明度，默认0
_imageView1.layer.shadowRadius = 3;//阴影半径，默认3

//路径阴影
UIBezierPath *path = [UIBezierPath bezierPath];

float width = _imageView1.bounds.size.width;
float height = _imageView1.bounds.size.height;
float x = _imageView1.bounds.origin.x;
float y = _imageView1.bounds.origin.y;
float addWH = 10;

CGPoint topLeft      = _imageView1.bounds.origin;
CGPoint topMiddle = CGPointMake(x+(width/2),y-addWH);
CGPoint topRight     = CGPointMake(x+width,y);

CGPoint rightMiddle = CGPointMake(x+width+addWH,y+(height/2));

CGPoint bottomRight  = CGPointMake(x+width,y+height);
CGPoint bottomMiddle = CGPointMake(x+(width/2),y+height+addWH);
CGPoint bottomLeft   = CGPointMake(x,y+height);


CGPoint leftMiddle = CGPointMake(x-addWH,y+(height/2));

[path moveToPoint:topLeft];
//添加四个二元曲线
[path addQuadCurveToPoint:topRight
             controlPoint:topMiddle];
[path addQuadCurveToPoint:bottomRight
             controlPoint:rightMiddle];
[path addQuadCurveToPoint:bottomLeft
             controlPoint:bottomMiddle];
[path addQuadCurveToPoint:topLeft
             controlPoint:leftMiddle];
//设置阴影路径
 _imageView1.layer.shadowPath = path.CGPath;