标签：
CAShapeLayer
CAShapeLayer是一个通过矢量图形而不是bitmap来绘制的图层子类。你指定诸如颜色和线宽等属性，用CGPath来定义想要绘制的图 形，最后CAShapeLayer就自动渲染出来了。当然，你也可以用Core Graphics直接向原始的CALyer的内容中绘制一个路径，相比直下，使用CAShapeLayer有以下一些优点：
渲染快速。CAShapeLayer使用了硬件加速，绘制同一图形会比用Core Graphics快很多。
高效使用内存。一个CAShapeLayer不需要像普通CALayer一样创建一个寄宿图形，所以无论有多大，都不会占用太多的内存。
不会被图层边界剪裁掉。一个CAShapeLayer可以在边界之外绘制。你的图层路径不会像在使用Core Graphics的普通CALayer一样被剪裁掉（如我们在第二章所见）。
不会出现像素化。当你给CAShapeLayer做3D变换时，它不像一个有寄宿图的普通图层一样变得像素化。
#import "DrawingView.h"#import @interface ViewController ()@property (nonatomic, weak) IBOutlet UIView *containerView;@end@implementation ViewController- (void)viewDidLoad{ [super viewDidLoad]; //create path UIBezierPath *path = [[UIBezierPath alloc] init]; [path moveToPoint:CGPointMake(175, 100)]; ? [path addArcWithCenter:CGPointMake(150, 100) radius:25 startAngle:0 endAngle:2*M_PI clockwise:YES]; [path moveToPoint:CGPointMake(150, 125)]; [path addLineToPoint:CGPointMake(150, 175)]; [path addLineToPoint:CGPointMake(125, 225)]; [path moveToPoint:CGPointMake(150, 175)]; [path addLineToPoint:CGPointMake(175, 225)]; [path moveToPoint:CGPointMake(100, 150)]; [path addLineToPoint:CGPointMake(200, 150)]; //create shape layer CAShapeLayer *shapeLayer = [CAShapeLayer layer]; shapeLayer.strokeColor = [UIColor redColor].CGColor; shapeLayer.fillColor = [UIColor clearColor].CGColor; shapeLayer.lineWidth = 5; shapeLayer.lineJoin = kCALineJoinRound; shapeLayer.lineCap = kCALineCapRound; shapeLayer.path = path.CGPath; //add it to our view [self.containerView.layer addSublayer:shapeLayer];}@end
//define path parametersCGRect rect = CGRectMake(50, 50, 100, 100);CGSize radii = CGSizeMake(20, 20);UIRectCorner corners = UIRectCornerTopRight | UIRectCornerBottomRight | UIRectCornerBottomLeft;//create pathUIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:rect byRoundingCorners:corners cornerRadii:radii];@interface ViewController ()@property (nonatomic, weak) IBOutlet UIView *labelView;@end@implementation ViewController- (void)viewDidLoad{ [super viewDidLoad]; //create a text layer CATextLayer *textLayer = [CATextLayer layer]; textLayer.frame = self.labelView.bounds; [self.labelView.layer addSublayer:textLayer]; //set text attributes textLayer.foregroundColor = [UIColor blackColor].CGColor; textLayer.alignmentMode = kCAAlignmentJustified; textLayer.wrapped = YES; //choose a font UIFont *font = [UIFont systemFontOfSize:15]; //set layer font CFStringRef fontName = (__bridge CFStringRef)font.fontName; CGFontRef fontRef = CGFontCreateWithFontName(fontName); textLayer.font = fontRef; textLayer.fontSize = font.pointSize; CGFontRelease(fontRef); //choose some text NSString *text = @"Lorem ipsum dolor sit amet, consectetur adipiscing \ elit. Quisque massa arcu, eleifend vel varius in, facilisis pulvinar \ leo. Nunc quis nunc at mauris pharetra condimentum ut ac neque. Nunc elementum, libero ut porttitor dictum, diam odio congue lacus, vel \ fringilla sapien diam at purus. Etiam suscipit pretium nunc sit amet \ lobortis"; //set layer text textLayer.string = text;}@end textLayer.contentsScale = [UIScreen mainScreen].scale;这样就解决了这个问题（如图6.3）
@implementation ViewController- (void)viewDidLoad{ [super viewDidLoad]; //create a text layer CATextLayer *textLayer = [CATextLayer layer]; textLayer.frame = self.labelView.bounds; textLayer.contentsScale = [UIScreen mainScreen].scale; [self.labelView.layer addSublayer:textLayer]; //set text attributes textLayer.alignmentMode = kCAAlignmentJustified; textLayer.wrapped = YES; //choose a font UIFont *font = [UIFont systemFontOfSize:15]; //choose some text NSString *text = @"Lorem ipsum dolor sit amet, consectetur adipiscing \ elit. Quisque massa arcu, eleifend vel varius in, facilisis pulvinar \ leo. Nunc quis nunc at mauris pharetra condimentum ut ac neque. Nunc \ elementum, libero ut porttitor dictum, diam odio congue lacus, vel \ fringilla sapien diam at purus. Etiam suscipit pretium nunc sit amet \ lobortis"; ? //create attributed string NSMutableAttributedString *string = nil; string = [[NSMutableAttributedString alloc] initWithString:text]; //convert UIFont to a CTFont CFStringRef fontName = (__bridge CFStringRef)font.fontName; CGFloat fontSize = font.pointSize; CTFontRef fontRef = CTFontCreateWithName(fontName, fontSize, NULL); //set text attributes NSDictionary *attribs = @{ (__bridge id)kCTForegroundColorAttributeName:(__bridge id)[UIColor blackColor].CGColor, (__bridge id)kCTFontAttributeName: (__bridge id)fontRef }; [string setAttributes:attribs range:NSMakeRange(0, [text length])]; attribs = @{ (__bridge id)kCTForegroundColorAttributeName: (__bridge id)[UIColor redColor].CGColor, (__bridge id)kCTUnderlineStyleAttributeName: @(kCTUnderlineStyleSingle), (__bridge id)kCTFontAttributeName: (__bridge id)fontRef }; [string setAttributes:attribs range:NSMakeRange(6, 5)]; //release the CTFont we created earlier CFRelease(fontRef); //set layer text textLayer.string = string;}@end
我们已经证实了CATextLayer比UILabel有着更好的性能表现，同时还有额外的布局选项并且在iOS 5上支持富文本。但是与一般的标签比较而言会更加繁琐一些。如果我们真的在需求一个UILabel的可用替代品，最好是能够在Interface Builder上创建我们的标签，而且尽可能地像一般的视图一样正常工作。
我们应该继承UILabel，然后添加一个子图层 CATextLayer并重写显示文本的方法。但是仍然会有由UILabel的-drawRect:方法创建的空寄宿图。而且由于CALayer不支持自 动缩放和自动布局，子视图并不是主动跟踪视图边界的大小，所以每次视图大小被更改，我们不得不手动更新子图层的边界。
我们真正想要的是一个用CATextLayer作为宿主图层的UILabel子类，这样就可以随着视图自动调整大小而且也没有冗余的寄宿图啦。
就 像我们在第一章『图层树』讨论的一样，每一个UIView都是寄宿在一个CALayer的示例上。这个图层是由视图自动创建和管理的，那我们可以用别的图 层类型替代它么？一旦被创建，我们就无法代替这个图层了。但是如果我们继承了UIView，那我们就可以重写+layerClass方法使得在创建的时候 能返回一个不同的图层子类。UIView会在初始化的时候调用+layerClass方法，然后用它的返回类型来创建宿主图层。
清单6.4 演示了一个UILabel子类LayerLabel用CATextLayer绘制它的问题，而不是调用一般的UILabel使用的较慢的 -drawRect：方法。LayerLabel示例既可以用代码实现，也可以在Interface Builder实现，只要把普通的标签拖入视图之中，然后设置它的类是LayerLabel就可以了。
清单6.4 使用CATextLayer的UILabel子类：LayerLabel
#import "LayerLabel.h"#import @implementation LayerLabel+ (Class)layerClass{ //this makes our label create a CATextLayer //instead of a regular CALayer for its backing layer return [CATextLayer class];}- (CATextLayer *)textLayer{ return (CATextLayer *)self.layer;}- (void)setUp{ //set defaults from UILabel settings self.text = self.text; self.textColor = self.textColor; self.font = self.font; //we should really derive these from the UILabel settings too //but that‘s complicated, so for now we‘ll just hard-code them [self textLayer].alignmentMode = kCAAlignmentJustified; ? [self textLayer].wrapped = YES; [self.layer display];}- (id)initWithFrame:(CGRect)frame{ //called when creating label programmatically if (self = [super initWithFrame:frame]) { [self setUp]; } return self;}- (void)awakeFromNib{ //called when creating label using Interface Builder [self setUp];}- (void)setText:(NSString *)text{ super.text = text; //set layer text [self textLayer].string = text;}- (void)setTextColor:(UIColor *)textColor{ super.textColor = textColor; //set layer text color [self textLayer].foregroundColor = textColor.CGColor;}- (void)setFont:(UIFont *)font{ super.font = font; //set layer font CFStringRef fontName = (__bridge CFStringRef)font.fontName; CGFontRef fontRef = CGFontCreateWithFontName(fontName); [self textLayer].font = fontRef; [self textLayer].fontSize = font.pointSize; ? CGFontRelease(fontRef);}@end如果你运行代码，你会发现文本并没有像素化，而我们也没有设置contentsScale属性。把CATextLayer作为宿主图层的另一好处就是视图自动设置了contentsScale属性。
在这个简单的例子中，我们只是实现了UILabel的一部分风格和布局属性，不过稍微再改进一下我们就可以创建一个支持UILabel所有功能甚至更多功能的LayerLabel类（你可以在一些线上的开源项目中找到）。
如果你打算支持iOS 6及以上，基于CATextLayer的标签可能就有有些局限性。但是总得来说，如果想在app里面充分利用CALayer子类，用+layerClass来创建基于不同图层的视图是一个简单可复用的方法。
CA*Layer（CAShapeLayer－－CATextLayer）
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原文地址：http://www.cnblogs.com/jingdizhiwa/p/5482841.html
