先用下面的方法生成一张渐变色图片

具体的渐变方向改CGContextDrawLinearGradient参数，或者用其他样式的渐变，例如CGContextDrawRadialGradient

- (UIImage *)createImageWithSize:(CGSize)imageSize gradientColors:(NSArray *)colors percentage:(CGFloat)percents {
    NSMutableArray *ar = [NSMutableArray array];
    for(UIColor *c in colors) {
        [ar addObject:(id)c.CGColor];
    }
    UIGraphicsBeginImageContextWithOptions(imageSize, YES, 1);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSaveGState(context);
    CGColorSpaceRef colorSpace = CGColorGetColorSpace([[colors lastObject] CGColor]);
    CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (CFArrayRef)ar, &percents);
    CGPoint start = CGPointMake(0.0, 0.0);
    CGPoint end = CGPointMake(imageSize.width, imageSize.height);
    CGContextDrawLinearGradient(context, gradient, start, end, kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    CGGradientRelease(gradient);
    CGContextRestoreGState(context);
    CGColorSpaceRelease(colorSpace);
    UIGraphicsEndImageContext();
    return image;
}