UIView的投影设置主要通过UIView的layer的相关属性来设置
阴影的颜色：imgView.layer.shadowColor = [UIColor blackColor].CGColor;
阴影的透明度：imgView.layer.shadowOpacity = 0.4f;
阴影的圆角：imgView.layer.shadowRadius = 4.0f;
阴影偏移量：imgView.layer.shadowOffset = CGSizeMake(0,0);//Defaults to (0, -3). Animatable.
