使用CoreGraphics 绘制渐变效果。

-(void)drawRect:(CGRect)rect{    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();    UIColor * startColor = [UIColor blueColor];    CGFloat * startColorComponents = (CGFloat *)CGColorGetComponents([startColor CGColor]);    UIColor * endColor = [UIColor greenColor];    CGFloat * endColorComponents = (CGFloat *)CGColorGetComponents([endColor CGColor]);        CGFloat colorComponents[8] = {        startColorComponents[0],        startColorComponents[1],        startColorComponents[2],        startColorComponents[3],                endColorComponents[0],        endColorComponents[1],        endColorComponents[2],        endColorComponents[3]    };        CGFloat colorIndices[2] = {0.0f,1.0f};    CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, (const CGFloat *)&colorComponents, (const CGFloat *)&colorIndices, 2);     CGColorSpaceRelease(colorSpace);        CGRect screenBounds = [[UIScreen mainScreen] bounds];    CGPoint startPoint, endPoint;    startPoint = CGPointMake(0.0f, screenBounds.size.height/2.0f);    endPoint = CGPointMake(screenBounds.size.width, startPoint.y);        CGContextRef currentContext = UIGraphicsGetCurrentContext();    CGContextDrawLinearGradient(currentContext,gradient , startPoint, endPoint, 0);        CGGradientRelease(gradient);     }