Drawing Gradients（绘制渐变）

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绘制过程:

创建上下文环境－CGContextRef UIGraphicsGetCurrentContext (void); //CGContextRef currentContext = UIGraphicsGetCurrentContext();

保存状态－void CGContextSaveGState (CGContextRef c); // CGContextSaveGState(currentContext);

创建颜色渐变对象:

创建色彩空间范围－CGColorSpaceRef CGColorSpaceCreateDeviceRGB(void); // CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

标识颜色构成的数组，包括渐变中的起始颜色，结束颜色，每种颜色包括四个部分:red,green,blue,alpha -CGFloat colorComponents[8]

标识颜色位置的数组－ CGFloat colorIndices[2]

创建颜色渐变－CGGradientRef CGGradientCreateWithColorComponents(

CGColorSpaceRef space,

const CGFloat components[],

const CGFloat locations[],

size_t count

);

释放色彩空间－void CGColorSpaceRelease(CGColorSpaceRef cs); // CGColorSpaceRelease(colorSpace);

绘制渐变:

- void CGContextDrawLinearGradient(

CGContextRef context,

CGGradientRef gradient,

CGPoint startPoint, //渐变的起点

CGPoint endPoint, //渐变的终点

CGGradientDrawingOptions options

//绘制选项:KCGGradientDrawsAfterEndLocation | KCGGradientDrawsBeforeStartLocation | 0

//当起点，终点不在边界范围内时，边界颜色的延伸处理

);

释放色渐变－void CGGradientRelease(CGGradientRef gradient); // CGGradientRelease(gradient);

恢复初始状态－void CGContextRestoreGState (CGContextRef c); //CGContextRestoreGState(currentContext);

- (void)drawRect:(CGRect)rect{

CGContextRef currentContext = UIGraphicsGetCurrentContext();

CGContextSaveGState(currentContext);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

//颜色构成

UIColor *startColor = [UIColor blueColor];

CGFloat *startColorComponents =(CGFloat *)CGColorGetComponents([startColor CGColor]); // components: 包括red,green,blue,alpha

UIColor *endColor = [UIColor greenColor];

CGFloat *endColorComponents =(CGFloat *)CGColorGetComponents([endColor CGColor]);

CGFloat colorComponents[8] = {

/* Four components of the blue color (RGBA) */

startColorComponents[0],

startColorComponents[1],

startColorComponents[2],

startColorComponents[3], /* First color = blue */

/* Four components of the green color (RGBA) */

endColorComponents[0],

endColorComponents[1],

endColorComponents[2],

endColorComponents[3], /* Second color = green */

};

CGFloat colorIndices[2] = { //标识颜色

0.0f, /* Color 0 in the colorComponents array */

1.0f, /* Color 1 in the colorComponents array */

};

CGGradientRef gradient =CGGradientCreateWithColorComponents

(colorSpace,

(const CGFloat *)&colorComponents,

(const CGFloat *)&colorIndices,

2);

CGColorSpaceRelease(colorSpace);

//开始绘制

CGPoint startPoint, endPoint;

CGRect screenBounds = [[UIScreen mainScreen] bounds];

/* 渐变的起点，终点决定了渐变的方向，如横向渐变，斜向渐变 */

/* 渐变一

startPoint = CGPointMake(0.0f,screenBounds.size.height / 2.0f);

endPoint = CGPointMake(screenBounds.size.width,startPoint.y);

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

/* 渐变二

startPoint = CGPointMake(120,260);

endPoint = CGPointMake(200.0f,220);

CGContextDrawLinearGradient (currentContext,gradient,startPoint,endPoint,

kCGGradientDrawsBeforeStartLocation |

kCGGradientDrawsAfterEndLocation);

// 渐变三

startPoint = CGPointMake(120,260);

endPoint = CGPointMake(200.0f,220);

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

CGGradientRelease(gradient);

CGContextRestoreGState(currentContext);

}

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