在iOS開發過程中有的時候會需要用到漸變的顏色，這篇文章總結了三種方法來實現，有需要的朋友們下面來一起看看吧。

一、CAGradientLayer實現漸變

CAGradientLayer是CALayer的一個特殊子類，用於生成顏色漸變的圖層，使用較為方便

下面介紹下它的相關屬性：

      colors 漸變的顏色

      locations 漸變顏色的分割點

      startPoint&endPoint 顏色漸變的方向，范圍在(0,0)與(1.0,1.0)之間，如(0,0)(1.0,0)代表水平方向漸變,(0,0)(0,1.0)代表豎直方向漸變

 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實現漸變標間簡單直觀，但存在一定的局限性，比如無法自定義整個漸變區域的形狀，如環形、曲線形的漸變。

二、Core Graphics相關方法實現漸變

iOS Core Graphics中有兩個方法用於繪制漸變顏色，CGContextDrawLinearGradient可以用於生成線性漸變，CGContextDrawRadialGradient用於生成圓半徑方向顏色漸變。函數可以自定義path，無論是什麼形狀都可以，原理都是用來做Clip，所以需要在CGContextClip函數前調用CGContextAddPath函數把CGPathRef加入到Context中。
另外一個需要注意的地方是漸變的方向，方向是由兩個點控制的，點的單位就是坐標。因此需要正確從CGPathRef中找到正確的點，方法當然有很多種看具體實現，本例中，我就是簡單得通過調用CGPathGetBoundingBox函數，返回CGPathRef的矩形區域，然後根據這個矩形取兩個點，讀者可以根據自行需求修改具體代碼。

1-> 線性漸變

- (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);

 //具體方向可根據需求修改
 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.

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

 //創建CGMutablePathRef
 CGMutablePathRef path = CGPathCreateMutable();

 //繪制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);

 //繪制漸變
 [self drawLinearGradient:gc path:path startColor:[UIColor greenColor].CGColor endColor:[UIColor redColor].CGColor];

 //注意釋放CGMutablePathRef
 CGPathRelease(path);

 //從Context中獲取圖像，並顯示在界面上
 UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
 UIGraphicsEndImageContext();

 UIImageView *imgView = [[UIImageView alloc] initWithImage:img];
 [self.view addSubview:imgView];
}

2-> 圓半徑方向漸變

- (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.

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

 //創建CGMutablePathRef
 CGMutablePathRef path = CGPathCreateMutable();

 //繪制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);

 //繪制漸變
 [self drawRadialGradient:gc path:path startColor:[UIColor greenColor].CGColor endColor:[UIColor redColor].CGColor];

 //注意釋放CGMutablePathRef
 CGPathRelease(path);

 //從Context中獲取圖像，並顯示在界面上
 UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
 UIGraphicsEndImageContext();

 UIImageView *imgView = [[UIImageView alloc] initWithImage:img];
 [self.view addSubview:imgView];
}

三、以CAShapeLayer作為layer的mask屬性

CALayer的mask屬性可以作為遮罩讓layer顯示mask遮住(非透明)的部分；CAShapeLayer為CALayer的子類，通過path屬性可以生成不同的形狀，將CAShapeLayer對象用作layer的mask屬性的話，就可以生成不同形狀的圖層。

故生成顏色漸變有以下幾個步驟：

     1、生成一個imageView(也可以為layer)，image的屬性為顏色漸變的圖片

     2、生成一個CAShapeLayer對象，根據path屬性指定所需的形狀

     3、將CAShapeLayer對象賦值給imageView的mask屬性

- (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;
}

總結

以上就是這篇文章的全部內容了，希望本文的內容對各位iOS開發者們能有所幫助，如果有疑問大家可以留言交流。