YIInnerShadowView* innerShadowView = [[YIInnerShadowView alloc] initWithFrame:frame];
innerShadowView.shadowRadius = 5;
innerShadowView.shadowMask = YIInnerShadowMaskVertical;
[self.view addSubview:innerShadowView];

typedef enum {
    YIInnerShadowMaskNone       = 0,
    YIInnerShadowMaskTop        = 1 << 1,
    YIInnerShadowMaskBottom     = 1 << 2,
    YIInnerShadowMaskLeft       = 1 << 3,
    YIInnerShadowMaskRight      = 1 << 4,
    YIInnerShadowMaskVertical   = YIInnerShadowMaskTop | YIInnerShadowMaskBottom,
    YIInnerShadowMaskHorizontal = YIInnerShadowMaskLeft | YIInnerShadowMaskRight,
    YIInnerShadowMaskAll        = YIInnerShadowMaskVertical | YIInnerShadowMaskHorizontal
} YIInnerShadowMask;