view渐变色，透明度渐变

1 功能描述

　　开发中经常遇到这样的需求：view2显示在view1上面，透过view2可以渐渐的看到view1.效果如图1所示：view1是一个imageView，view2是一个普通view。view1与view2完全重叠，view2从下到上由灰色到白色，由不透明到透明。图2为只有view2的效果，图3为只有view1的效果。

2 代码如下

　　如果只想颜色渐变，而不需要透明度的渐变，可以将alpha设为1.0。

 

- (void)viewDidLoad {
    [super viewDidLoad];
    UIImageView *imgView = [[UIImageView alloc] initWithFrame:self.view.bounds];
    imgView.image = [UIImage imageNamed:@"sun"];
    [self.view addSubview:imgView];
    
    UIView *gradView = [[UIView alloc] initWithFrame:self.view.bounds];
    CAGradientLayer *layer = [self getGradientWithFrame:gradView.bounds];
    [gradView.layer insertSublayer:layer atIndex:0];
    [self.view addSubview:gradView];
}

- (CAGradientLayer *)getGradientWithFrame:(CGRect) frame {
    int colorOneV = 33;
    int colorTwoV = 33;
    int colorThreeV = 33;
    // 设置颜色与不透明度alpha，如果只想渐变而不想透明，可以将alpha设为1.0。
    UIColor *colorOne = [UIColor colorWithRed:(colorOneV/255.0)  green:(colorOneV/255.0)  blue:(colorOneV/255.0)  alpha:0.0];
    UIColor *colorTwo = [UIColor colorWithRed:(colorTwoV/255.0)  green:(colorTwoV/255.0)  blue:(colorTwoV/255.0)  alpha:0.5];
    UIColor *colorThree = [UIColor colorWithRed:(colorThreeV/255.0)  green:(colorThreeV/255.0)  blue:(colorThreeV/255.0)  alpha:1.0];
    NSArray *colors = [NSArray arrayWithObjects:(id)colorOne.CGColor, colorTwo.CGColor, colorThree.CGColor , nil];
    NSNumber *stopOne = [NSNumber numberWithFloat:0.0];
    NSNumber *stopTwo = [NSNumber numberWithFloat:0.5];
    NSNumber *stopThree = [NSNumber numberWithFloat:1.0];
    NSArray *locations = [NSArray arrayWithObjects:stopOne, stopTwo, stopThree, nil];
    // 渐变图层
    CAGradientLayer *layer = [CAGradientLayer layer];
    layer.colors = colors;
    layer.locations = locations;
    layer.frame = frame;
    return layer;
}