漸變色
給View添加漸變色的方法有多種，這里采用CAGradientLayer來實現漸變效果。CAGradientLayer的好處在于繪制使用了硬件加速。
CAGradientLayer在背景色上繪制顏色漸變，填充圖層的形狀（包括圓角）。
一個簡單的例子：
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：
藍色漸變
CAGradientLayer屬于QuartzCore，QuartzCore提供動畫和動畫顯示層次功能的應用程序。
CAGradientLayer有5個屬性：
-
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中設置漸變的顏色；locations中設置顏色的范圍，大小在[0, 1]內，默認為平均；startPoint和endPoint設置漸變的起始位置，范圍在[0, 1]內；type設置漸變樣式，目前僅支持線性漸變(kCAGradientLayerAxial)。
對角線漸變，漸變范圍0.0，0.2，0.5，一定要確保locations
數組和colors數組大小相同，否則會得到一個空白的漸變。
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：
對角線漸變
參考：
CAGradientLayer
