UIImageView *testImgView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 200, 100)];
    
    [testImgView setBackgroundColor:[UIColor yellowColor]];
    
    // 阴影颜色
    testImgView.layer.shadowColor = [UIColor blackColor].CGColor;
    // 阴影偏移，默认(0, -3)
    testImgView.layer.shadowOffset = CGSizeMake(0,0);
    // 阴影透明度，默认0
    testImgView.layer.shadowOpacity = 0.5;
    // 阴影半径，默认3
    testImgView.layer.shadowRadius = 5;
    
    [self.view addSubview:testImgView];
    

//单边阴影
    
    UILabel *testLabel = [[UILabel alloc]initWithFrame:CGRectMake(100, 300, 200, 100)];
    
    [testLabel setBackgroundColor:[UIColor yellowColor]];
    
    // 阴影颜色
    testLabel.layer.shadowColor = [UIColor blackColor].CGColor;
    
    // 阴影偏移，默认(0, -3)
    testLabel.layer.shadowOffset = CGSizeMake(0,0);
    
    // 阴影透明度，默认0
    testLabel.layer.shadowOpacity = 0.5;
    // 阴影半径，默认3
    testLabel.layer.shadowRadius = 5;
    
    // 单边阴影 顶边
    float shadowPathWidth = testLabel.layer.shadowRadius;
    CGRect shadowRect = CGRectMake(-shadowPathWidth/2.0, 0-shadowPathWidth/2.0, testLabel.bounds.size.width+shadowPathWidth, shadowPathWidth);
    UIBezierPath *path = [UIBezierPath bezierPathWithRect:shadowRect];
    testLabel.layer.shadowPath = path.CGPath;
    
    [self.view addSubview:testLabel];

/** 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;