ios实现颜色渐变的几种方法

demo地址：https://github.com/xiaochaofeiyu/YSCAnimation
有用的话求个star，欢迎建议讨论。

1. CAGradientLayer实现渐变

CAGradientLayer是CALayer的一个特殊子类，用于生成颜色渐变的图层，使用较为方便，下面介绍下它的相关属性：

1. colors 渐变的颜色
2. locations 渐变颜色的分割点
3. startPoint&endPoint 颜色渐变的方向，范围在(0,0)与(1.0,1.0)之间，如(0,0)(1.0,0)表明水平方向渐变,(0,0)(0,1.0)表明竖直方向渐变

CAGradientLayer *gradientLayer = [CAGradientLayer layer];
    gradientLayer.colors = @[(__bridge id)[UIColor redColor].CGColor, (__bridge id)[UIColor yellowColor].CGColor, (__bridge id)[UIColor blueColor].CGColor];
    gradientLayer.locations = @[@0.3, @0.5, @1.0];
    gradientLayer.startPoint = CGPointMake(0, 0);
    gradientLayer.endPoint = CGPointMake(1.0, 0);
    gradientLayer.frame = CGRectMake(0, 100, 300, 100);
    [self.view.layer addSublayer:gradientLayer];

CAGradientLayer实现渐变标间简单直观，但存在必定的局限性，好比没法自定义整个渐变区域的形状，如环形、曲线形的渐变。

2. Core Graphics相关方法实现渐变

iOS Core Graphics中有两个方法用于绘制渐变颜色，CGContextDrawLinearGradient能够用于生成线性渐变，CGContextDrawRadialGradient用于生成圆半径方向颜色渐变。函数能够自定义path，不管是什么形状均可以，原理都是用来作Clip，因此须要在CGContextClip函数前调用CGContextAddPath函数把CGPathRef加入到Context中。
另一个须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。所以须要正确从CGPathRef中找到正确的点，方法固然有不少种看具体实现，本例中，我就是简单得经过调用CGPathGetBoundingBox函数，返回CGPathRef的矩形区域，而后根据这个矩形取两个点，读者能够根据自行需求修改具体代码。

1-> 线性渐变

- (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), CGRectGetMidY(pathRect));
    CGPoint endPoint = CGPointMake(CGRectGetMaxX(pathRect), CGRectGetMidY(pathRect));
    
    CGContextSaveGState(context);
    CGContextAddPath(context, path);
    CGContextClip(context);
    CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);
    CGContextRestoreGState(context);
    
    CGGradientRelease(gradient);
    CGColorSpaceRelease(colorSpace);
}

- (void)viewDidLoad 
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    //建立CGContextRef
    UIGraphicsBeginImageContext(self.view.bounds.size);
    CGContextRef gc = UIGraphicsGetCurrentContext();
    
    //建立CGMutablePathRef
    CGMutablePathRef path = CGPathCreateMutable();
    
    //绘制Path
    CGRect rect = CGRectMake(0, 100, 300, 200);
    CGPathMoveToPoint(path, NULL, CGRectGetMinX(rect), CGRectGetMinY(rect));
    CGPathAddLineToPoint(path, NULL, CGRectGetMidX(rect), CGRectGetMaxY(rect));
    CGPathAddLineToPoint(path, NULL, CGRectGetWidth(rect), CGRectGetMaxY(rect));
    CGPathCloseSubpath(path);
    
    //绘制渐变
    [self drawLinearGradient:gc path:path startColor:[UIColor greenColor].CGColor endColor:[UIColor redColor].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];
    [self.view addSubview:imgView];
}

2-> 圆半径方向渐变

- (void)drawRadialGradient:(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);
    CGPoint center = CGPointMake(CGRectGetMidX(pathRect), CGRectGetMidY(pathRect));
    CGFloat radius = MAX(pathRect.size.width / 2.0, pathRect.size.height / 2.0) * sqrt(2);
    
    CGContextSaveGState(context);
    CGContextAddPath(context, path);
    CGContextEOClip(context);
    
    CGContextDrawRadialGradient(context, gradient, center, 0, center, radius, 0);
    
    CGContextRestoreGState(context);
    
    CGGradientRelease(gradient);
    CGColorSpaceRelease(colorSpace);
}

- (void)viewDidLoad 
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    //建立CGContextRef
    UIGraphicsBeginImageContext(self.view.bounds.size);
    CGContextRef gc = UIGraphicsGetCurrentContext();
    
    //建立CGMutablePathRef
    CGMutablePathRef path = CGPathCreateMutable();
    
    //绘制Path
    CGRect rect = CGRectMake(0, 100, 300, 200);
    CGPathMoveToPoint(path, NULL, CGRectGetMinX(rect), CGRectGetMinY(rect));
    CGPathAddLineToPoint(path, NULL, CGRectGetMidX(rect), CGRectGetMaxY(rect));
    CGPathAddLineToPoint(path, NULL, CGRectGetWidth(rect), CGRectGetMaxY(rect));
    CGPathAddLineToPoint(path, NULL, CGRectGetWidth(rect), CGRectGetMinY(rect));
    CGPathCloseSubpath(path);
    
    //绘制渐变
    [self drawRadialGradient:gc path:path startColor:[UIColor greenColor].CGColor endColor:[UIColor redColor].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];
    [self.view addSubview:imgView];
}

3. 以CAShapeLayer做为layer的mask属性

CALayer的mask属性能够做为遮罩让layer显示mask遮住(非透明)的部分；CAShapeLayer为CALayer的子类，经过path属性能够生成不一样的形状，将CAShapeLayer对象用做layer的mask属性的话，就能够生成不一样形状的图层。故生成颜色渐变有如下几个步骤：

1. 生成一个imageView(也能够为layer)，image的属性为颜色渐变的图片
2. 生成一个CAShapeLayer对象，根据path属性指定所需的形状
3. 将CAShapeLayer对象赋值给imageView的mask属性

- (void)viewDidLoad
{
    [super viewDidLoad];
        
    [self.view addSubview:self.firstCircle];
    _firstCircle.frame = CGRectMake(0, 0, 200, 200);
    _firstCircle.center = CGPointMake(CGRectGetWidth(self.view.bounds) / 2.0, CGRectGetHeight(self.view.bounds) / 2.0);
    CGFloat firsCircleWidth = 5;
    self.firstCircleShapeLayer = [self generateShapeLayerWithLineWidth:firsCircleWidth];
    _firstCircleShapeLayer.path = [self generateBezierPathWithCenter:CGPointMake(100, 100) radius:100].CGPath;
    _firstCircle.layer.mask = _firstCircleShapeLayer;
} 

- (CAShapeLayer *)generateShapeLayerWithLineWidth:(CGFloat)lineWidth
{
    CAShapeLayer *waveline = [CAShapeLayer layer];
    waveline.lineCap = kCALineCapButt;
    waveline.lineJoin = kCALineJoinRound;
    waveline.strokeColor = [UIColor redColor].CGColor;
    waveline.fillColor = [[UIColor clearColor] CGColor];
    waveline.lineWidth = lineWidth;
    waveline.backgroundColor = [UIColor clearColor].CGColor;
    
    return waveline;
}

- (UIBezierPath *)generateBezierPathWithCenter:(CGPoint)center radius:(CGFloat)radius
{
    UIBezierPath *circlePath = [UIBezierPath bezierPathWithArcCenter:center radius:radius startAngle:0 endAngle:2*M_PI clockwise:NO];
    
    return circlePath;
}

- (UIImageView *)firstCircle
{
    if (!_firstCircle) {
        self.firstCircle = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"circleBackground"]];
        _firstCircle.layer.masksToBounds = YES;
        _firstCircle.alpha = 1.0;
    }
    
    return _firstCircle;
}

做者：小超飞鱼
连接：http://www.jianshu.com/p/3e0e25fd9b85
來源：简书
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