总结iOS实现渐变颜色的三种方法

在iOS开发过程中有的时候会需要用到渐变的颜色，这篇文章总结了三种方法来实现，有需要的朋友们下面来一起看看吧。

一、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：

colors 渐变的颜色

locations 渐变颜色的分割点

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)代表水平方向渐变,(0,0)(0,1.0)代表竖直方向渐变

 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，但存在一定的局限性，比如无法自定义整个渐变区域的形状，如环形、曲线形的渐变。

二、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。

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.

 //创建CGContextRef
 UIGraphicsBeginImageContext(self.view.bounds.size);
 CGContextRef gc = UIGraphicsGetCurrentContext();

 //创建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.

 //创建CGContextRef
 UIGraphicsBeginImageContext(self.view.bounds.size);
 CGContextRef gc = UIGraphicsGetCurrentContext();

 //创建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];
}

三、以CAShapeLayer作为layer的mask属性

CALayer的mask属性可以作为遮罩让layer显示mask遮住(非透明)的部分；CAShapeLayer为CALayer的子类，通过path属性可以生成不同的形状，将CAShapeLayer对象用作layer的mask属性的话，就可以生成不同形状的图层。

故生成颜色渐变有以下几个步骤：

1、生成一个imageView(也可以为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;
}

总结

以上就是这篇文章的全部内容了，希望本文的内容对各位iOS开发者们能有所帮助，如果有疑问大家可以留言交流。

基础教程