iOS CAGradientLayer 颜色渐变

　　CAGradientLayer *layer = [CAGradientLayer layer];
layer.startPoint = (CGPoint){0.5f, 0.0f};
layer.endPoint = (CGPoint){0.5f, 1.0f};
layer.colors = @[(id)[UIColor purpleColor].CGColor,
(id)[UIColor cyanColor].CGColor,
(id)[UIColor magentaColor].CGColor,
(id)[UIColor redColor].CGColor];
layer.locations = @[@0.0f, @0.6f, @1.0f];
layer.frame = self.view.layer.bounds;
[self.view.layer insertSublayer:layer atIndex:0];