蒙版渐变的实现

要求整个view渐渐alpha变透明，包括view上的子view，需要一起变透明。

-(void)testAlpha{     
    [self.view layoutIfNeeded];     
    UIColor *color1 = [UIColor colorWithRed:(0)  green:(0)  blue:(0)   alpha:1.0];     
    UIColor *color2 = [UIColor colorWithRed:(0)  green:(0)  blue:(0)  alpha:0.8];     
    UIColor *color3 = [UIColor colorWithRed:(0)  green:(0)  blue:(0)  alpha:0.2];     
    NSArray *colors = [NSArray arrayWithObjects:(id)color1.CGColor, color2.CGColor,color3.CGColor, nil];     
    NSArray *locations = [NSArray arrayWithObjects:@(0.0), @(0.7),@(1.0), nil];     
    CAGradientLayer *gradientLayer = [CAGradientLayer layer];     
    gradientLayer.colors = colors;     
    gradientLayer.locations = locations;     
    gradientLayer.frame = _inputView.bounds;     
    gradientLayer.startPoint = CGPointMake(0, 0);     
    gradientLayer.endPoint   = CGPointMake(1, 0);     
    _inputView.layer.mask = gradientLayer; 
}