1. CAGradientLayer實現(xiàn)漸變
CAGradientLayer是CALayer的一個特殊子類寒跳，用于生成顏色漸變的圖層，使用較為方便咕别，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 ? ?顏色漸變的方向收叶，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(xiàn)漸變標間簡單直觀菇夸，但存在一定的局限性琼富，比如無法自定義整個漸變區(qū)域的形狀，如環(huán)形庄新、曲線形的漸變鞠眉。
2. Core Graphics相關方法實現(xiàn)漸變
iOS Core Graphics中有兩個方法用于繪制漸變顏色，CGContextDrawLinearGradient可以用于生成線性漸變择诈，CGContextDrawRadialGradient用于生成圓半徑方向顏色漸變械蹋。函數(shù)可以自定義path，無論是什么形狀都可以羞芍，原理都是用來做Clip哗戈，所以需要在CGContextClip函數(shù)前調(diào)用CGContextAddPath函數(shù)把CGPathRef加入到Context中。
另外一個需要注意的地方是漸變的方向荷科，方向是由兩個點控制的唯咬，點的單位就是坐標。因此需要正確從CGPathRef中找到正確的點步做，方法當然有很多種看具體實現(xiàn)副渴，本例中，我就是簡單得通過調(diào)用CGPathGetBoundingBox函數(shù)全度，返回CGPathRef的矩形區(qū)域煮剧，然后根據(jù)這個矩形取兩個點，讀者可以根據(jù)自行需求修改具體代碼将鸵。
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(jù)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(chuàng)建CGContextRef
UIGraphicsBeginImageContext(self.view.bounds.size);
CGContextRef?gc?=?UIGraphicsGetCurrentContext();
//創(chuàng)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中獲取圖像勉盅，并顯示在界面上
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(chuàng)建CGContextRef
UIGraphicsBeginImageContext(self.view.bounds.size);
CGContextRef?gc?=?UIGraphicsGetCurrentContext();
//創(chuàng)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，并顯示在界面上
UIImage?*img?=?UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
UIImageView?*imgView?=?[[UIImageView?alloc]?initWithImage:img];
[self.view?addSubview:imgView];
}
3. 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)的部分顶掉；CAShapeLayer為CALayer的子類草娜，通過path屬性可以生成不同的形狀，將CAShapeLayer對象用作layer的mask屬性的話痒筒，就可以生成不同形狀的圖層宰闰。故生成顏色漸變有以下幾個步驟：
生成一個imageView(也可以為layer)茬贵，image的屬性為顏色漸變的圖片
生成一個CAShapeLayer對象，根據(jù)path屬性指定所需的形狀
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;
}
