iOS 视图颜色渐变详解

    // 初始化一个试图 (方法一)
    UIView *view = [[UIView alloc] initWithFrame:CGRectMake(10, 100, 200, 30)];
    view.backgroundColor = [UIColor greenColor];
    [self.view addSubview:view];
    CAGradientLayer *layer = [CAGradientLayer layer];
    layer.frame = view.bounds;
    layer.colors = [NSArray arrayWithObjects:(id)[UIColor redColor].CGColor,
                    (id)[UIColor yellowColor].CGColor,
                    (id)[UIColor greenColor].CGColor,nil]; // 可以只填一中颜色
    layer.startPoint = CGPointMake(0, 1);
    layer.endPoint = CGPointMake(1, 1); // 这里面数字范围改动在(0-1)
    [view.layer insertSublayer:layer atIndex:0]; // 添加到试图layer上

  // 方法二
    CGContextRef context = UIGraphicsGetCurrentContext();
    UIGraphicsPushContext(context);
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGFloat locations[] = {0,1};
    NSArray *colors = @[(id)[UIColor redColor].CGColor,
                        (id)[UIColor yellowColor].CGColor,
                        (id)[UIColor greenColor].CGColor];
    CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (CFArrayRef) colors, locations);
    CGColorSpaceRelease(colorSpace);
    
    CGPoint startPoint = (CGPoint){view.frame.size.width * 0, view.frame.size.height * 1};
    CGPoint endPoint = (CGPoint){view.frame.size.width * 1, view.frame.size.height * 1};
    CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);
    
    CGGradientRelease(gradient);
    UIGraphicsPopContext();

 // 方法三
    CIFilter *ciFilter = [CIFilter filterWithName:@"CILinearGradient"];
    CIVector *vector0 = [CIVector vectorWithX:view.frame.size.width * _Point0.x Y:view.frame.size.height * (1 - _Point0.y)];
    CIVector *vector1 = [CIVector vectorWithX:view.frame.size.width * Point1.x Y:view.frame.size.height * (1 - Point1.y)];
    [ciFilter setValue:vector0 forKey:@"Point0"];
    [ciFilter setValue:vector1 forKey:@"Point1"];
    [ciFilter setValue:[CIColor colorWithCGColor:_inputColor0.CGColor] forKey:@"inputColor0"];
    [ciFilter setValue:[CIColor colorWithCGColor:_inputColor1.CGColor] forKey:@"inputColor1"];
    
    CIImage *ciImage = ciFilter.outputImage;
    CIContext *con = [CIContext contextWithOptions:nil];
    CGImageRef resultCGImage = [con createCGImage:ciImage
                                         fromRect:rect];
    UIImage *resultUIImage = [UIImage imageWithCGImage:resultCGImage];
    CGImageRelease(resultCGImage);
    
    [resultUIImage drawInRect:rect];