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:];
     [self.view addSubview:gradView];
 }

 - (CAGradientLayer *)getGradientWithFrame:(CGRect) frame {
     int colorOneV = ;
     int colorTwoV = ;
     int colorThreeV = ;
     // 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;
 }