渐变背景颜色Core Graphics

iOS Core Graphics中有两个方法用于绘制渐变颜色，CGContextDrawLinearGradient可以用于生成线性渐变，CGContextDrawRadialGradient用于生成圆半径方向颜色渐变。函数可以自定义path，无论是什么形状都可以，原理都是用来做Clip，所以需要在CGContextClip函数前调用CGContextAddPath函数把CGPathRef加入到Context中。

另外一个需要注意的地方是渐变的方向，方向是由两个点控制的，点的单位就是坐标。因此需要正确从CGPathRef中找到正确的点，方法当然有很多种看具体实现，本例中，我就是简单得通过调用CGPathGetBoundingBox函数，返回CGPathRef的矩形区域，然后根据这个矩形取两个点，读者可以根据自行需求修改具体代码。

- (void)viewDidLoad{

//创建view，并设置该view背景渐变

UIView *view = [[UIView alloc]initWithFrame:CGRectMake(0, 0, kScreenWidth - 100, kScreenHeight- 180 - 170)];

[self addImgView:view];

view.layer.masksToBounds = YES;

view.layer.cornerRadius = 8;

}

- (void)drawLinearGradient:(CGContextRef)context

path:(CGPathRef)path

startColor:(CGColorRef)startColor

endColor:(CGColorRef)endColor

{

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

CGFloat locations[] = { 0.0, 1.0 };

NSArray *colors = @[(__bridge id) startColor, (__bridge id) endColor];

CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef) colors, locations);

CGRect pathRect = CGPathGetBoundingBox(path);

//具体方向可根据需求修改

CGPoint startPoint = CGPointMake(CGRectGetMinX(pathRect), CGRectGetMinY(pathRect));

CGPoint endPoint = CGPointMake(CGRectGetMinX(pathRect), CGRectGetMaxY(pathRect));

CGContextSaveGState(context);

CGContextAddPath(context, path);

CGContextClip(context);

CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);

CGContextRestoreGState(context);

CGGradientRelease(gradient);

CGColorSpaceRelease(colorSpace);

}

- (UIView *)addImgView:(UIView *)view

{

//创建CGContextRef

UIGraphicsBeginImageContext(view.bounds.size);

CGContextRef gc = UIGraphicsGetCurrentContext();

//创建CGMutablePathRef

CGMutablePathRef path = CGPathCreateMutable();

//绘制Path

CGRect rect = CGRectMake(0, 0, view.width, view.height);

//矩形区域

CGPathAddRect(path, NULL, rect);

CGPathCloseSubpath(path);

//绘制渐变

[self drawLinearGradient:gc path:path startColor:[UIColor whiteColor].CGColor endColor:[UIColor colorWithRed:255.0/255.0 green:230.0/255.0 blue:230.0/255.0 alpha:1.0].CGColor];

//注意释放CGMutablePathRef

CGPathRelease(path);

//从Context中获取图像，并显示在界面上

UIImage *img = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

UIImageView *imgView = [[UIImageView alloc] initWithImage:img];

[view addSubview:imgView];

return view;

}