Core Animation － CATextLayer和富文本

1.CATextLayer
为什么要存在CATextLayer呢？它的意义是什么？我们可以用UILabel来写文字，但两者不是一码事，CATextLayer的渲染速度比UILabel快的多，而UILabel的精髓就是用图层代理把字符串用Core Graphics写入如层，CATextLayer则不用，它以涂层的形式几乎包含了UILabel所有的特性，并有额外的新特性。

//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.Lorem ipsum dolor sit amet, consectetur adipiscing \ elit.Lorem ipsum dolor sit amet, consectetur adipiscing \ elit.Lorem ipsum dolor sit amet, consectetur adipiscing \ elit.Lorem ipsum dolor sit amet, consectetur adipiscing \ elit.";
 textLayer.string = text;
 //单写上面的会发现文字显示像素不高，那是因为没有支持retain显示屏，前面说过，要支持retain，需要设置contentsScale
 textLayer.contentsScale = [UIScreen mainScreen].scale;

2.富文本
这是一个在编程中经常用到的东西,使用富文本需要添加库 #import

//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, eLorem ipsum dolor sit amet, consectetur adipiscing \ elit. Quisque massa arcu, eLorem ipsum dolor sit amet, consectetur adipiscing \ elit. Quisque massa arcu, eLorem ipsum dolor sit amet, consectetur adipiscing \ elit. Quisque massa arcu, e";
    //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;