iOS：给View添加渐变色

给View添加渐变色的方法有多种，这里采用CAGradientLayer来实现渐变效果。CAGradientLayer的好处在于绘制使用了硬件加速。

CAGradientLayer在背景色上绘制颜色渐变，填充图层的形状（包括圆角）。

一个简单的例子：

UIView *colorView = [[UIView alloc] init];
[colorView setFrame:CGRectMake(20, 160, 
        self.view.frame.size.width - 40, self.view.frame.size.height - 320)];
[self.view addSubview:colorView];
    
CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = colorView.bounds;
gradient.colors = [NSArray arrayWithObjects:
                   (id)[UIColor colorWithRed:0 green:143/255.0 blue:234/255.0 alpha:1.0].CGColor, 
                   (id)[UIColor colorWithRed:0 green:173/255.0 blue:234/255.0 alpha:1.0].CGColor, 
                   (id)[UIColor whiteColor].CGColor, nil];
[colorView.layer addSublayer:gradient];

效果如下：

CAGradientLayer属于QuartzCore，QuartzCore提供动画和动画显示层次功能的应用程序。
CAGradientLayer有5个属性：

• colors: An array of CGColorRef
  objects defining the color of each gradient stop. Animatable.
• locations: An optional array of NSNumber objects defining the location of each gradient stop. Animatable.
• endPoint: The end point of the gradient when drawn in the layer’s coordinate space. Animatable.
• startPoint: The start point of the gradient when drawn in the layer’s coordinate space. Animatable.
• type: Style of gradient drawn by the layer.

colors中设置渐变的颜色；locations中设置颜色的范围，大小在[0, 1]内，默认为平均；startPoint和endPoint设置渐变的起始位置，范围在[0, 1]内；type设置渐变样式，目前仅支持线性渐变(kCAGradientLayerAxial)。

对角线渐变，渐变范围0.0，0.2，0.5，一定要确保locations
数组和colors数组大小相同，否则会得到一个空白的渐变。

CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = colorView.bounds;
gradient.colors = [NSArray arrayWithObjects:
                            (id)[UIColor magentaColor].CGColor, 
                            (id)[UIColor cyanColor].CGColor, 
                            (id)[UIColor greenColor].CGColor, nil];
gradient.startPoint = CGPointMake(0, 0);
gradient.endPoint = CGPointMake(1, 1);
gradient.locations = @[@0.0, @0.2, @0.5];
[colorView.layer addSublayer:gradient];

效果如下：

参考：
CAGradientLayer