1. 看了一遍 CoreAnimation Advanced ... 
2. YYKit那篇界面流畅的文章
3. VVeboTableView 源码
4. CoreText 文本渲染
5. YYAsyncLayer

1. 自定义UILabel，内部使用CoreText文本绘制
2. 子线程异步完成文本的绘制，并渲染得到图像
3. 最终将图像设置给UILabel的backing layer显示
4. 涉及到的特殊文本的高亮、点击效果，通过预先正则式切割，然后保存高亮文本出现的frame
5. 将绘制渲染的代码，全部放到子线程

1. 开始滚动时，保存一个状态
2. 停止滚动时，保存一个状态，并且计算出最终停止时的indexpath
3. 过滤掉中间快速滚动过的indexpath
4. 将最终停止出现的可见indexpath保存到一个数组，并额外添加附近的三个indexpath
5. 最终列表停止后，将保存在数组中的indexpath，挨个取出对应的cell，进行内容的绘制

@implementation XXXLabel
- (void)asyncDraw {
    /**
     *  渲染生成图像的过程，全部都在子线程异步完成
     */
    XZHDispatchQueueAsyncBlockWithQOSBackgroud(^{
        CGSize size = self.frame.size;
        size.height += 10;
        UIGraphicsBeginImageContextWithOptions(size, ![self.backgroundColor isEqual:[UIColor clearColor]], 0);
        CGContextRef context = UIGraphicsGetCurrentContext();
        if (context==NULL) {return;}
        if (![self.backgroundColor isEqual:[UIColor clearColor]]) {
            [self.backgroundColor set];
            CGContextFillRect(context, CGRectMake(0, 0, size.width, size.height));
        }
        CGContextSetTextMatrix(context,CGAffineTransformIdentity);
        CGContextTranslateCTM(context,0,size.height);
        CGContextScaleCTM(context,1.0,-1.0);
        NSString *md5 = [_text xzh_MD5];
        CTFrameRef ctFrame = CTFrameForKey(md5);
        CTFontRef font;
        CTFramesetterRef framesetter;
        CGRect rect = CGRectMake(0, 5,(size.width),(size.height-5));
        if (!_highlighting && ctFrame) {
            [self drawWithCTFrame:ctFrame inRect:rect context:context];
        } else {
            UIColor* textColor = self.textColor;
            CGFloat minimumLineHeight = self.font.pointSize,maximumLineHeight = minimumLineHeight, linespace = self.lineSpace;
            font = CTFontCreateWithName((__bridge CFStringRef)self.font.fontName, self.font.pointSize,NULL);
            CTLineBreakMode lineBreakMode = kCTLineBreakByWordWrapping;
            CTTextAlignment alignment = CTTextAlignmentFromUITextAlignment(self.textAlignment);
            CTParagraphStyleRef style = CTParagraphStyleCreate((CTParagraphStyleSetting[6]){
                {kCTParagraphStyleSpecifierAlignment, sizeof(alignment), &alignment},
                {kCTParagraphStyleSpecifierMinimumLineHeight,sizeof(minimumLineHeight),&minimumLineHeight},
                {kCTParagraphStyleSpecifierMaximumLineHeight,sizeof(maximumLineHeight),&maximumLineHeight},
                {kCTParagraphStyleSpecifierMaximumLineSpacing, sizeof(linespace), &linespace},
                {kCTParagraphStyleSpecifierMinimumLineSpacing, sizeof(linespace), &linespace},
                {kCTParagraphStyleSpecifierLineBreakMode,sizeof(CTLineBreakMode),&lineBreakMode}
            },6);
            NSDictionary* attributes = [NSDictionary dictionaryWithObjectsAndKeys:(__bridge id)font,(NSString*)kCTFontAttributeName,
                                        textColor.CGColor,kCTForegroundColorAttributeName,
                                        style,kCTParagraphStyleAttributeName,
                                        nil];
            NSMutableAttributedString *attributedStr = [[NSMutableAttributedString alloc] initWithString:_text
                                                                                              attributes:attributes];
            CFAttributedStringRef attributedString = (__bridge CFAttributedStringRef)[self highlightText:attributedStr];
            framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attributedString);
            CGMutablePathRef path = CGPathCreateMutable();
            CGPathAddRect(path, NULL, rect);
            CTFrameRef ctFrame = CTFramesetterCreateFrame(framesetter,
                                                        CFRangeMake(0, _text.length),
                                                        path,
                                                        NULL);
            CacheCTFrameWithKey(ctFrame, md5);
            [self drawWithCTFrame:ctFrame inRect:rect context:context];
            //CFRelease(ctFrame);
        }
        CGContextSetTextMatrix(context,CGAffineTransformIdentity);
        CGContextTranslateCTM(context,0,size.height);
        CGContextScaleCTM(context,1.0,-1.0);
        UIImage *screenShotimage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        dispatch_async(dispatch_get_main_queue(), ^{
//            if (font) {CFRelease(font);}
//            if (framesetter) {CFRelease(framesetter);}
            if (_highlighting) {
                _highlightImageView.image = nil;
                if (_highlightImageView.width!=screenShotimage.size.width) {
                    _highlightImageView.width = screenShotimage.size.width;
                }
                if (_highlightImageView.height!=screenShotimage.size.height) {
                    _highlightImageView.height = screenShotimage.size.height;
                }
                _highlightImageView.image = screenShotimage;
            } else {
                if (_labelImageView.width!=screenShotimage.size.width) {
                    _labelImageView.width = screenShotimage.size.width;
                }
                if (_labelImageView.height!=screenShotimage.size.height) {
                    _labelImageView.height = screenShotimage.size.height;
                }
                _highlightImageView.image = nil;
                _labelImageView.image = nil;
                _labelImageView.image = screenShotimage;
            }
//            [self debugDraw];//绘制可触摸区域
        });
    });
}
- (void)drawWithCTFrame:(CTFrameRef)frame
                 inRect:(CGRect)rect
                context:(CGContextRef)ctx
{
    if (NULL == frame) {return;}
    if (NULL == ctx) {return;}
    CFArrayRef lines = CTFrameGetLines(frame);
    NSInteger numberOfLines = CFArrayGetCount(lines);
    CGPoint lineOrigins[numberOfLines];
    CTFrameGetLineOrigins(frame, CFRangeMake(0, numberOfLines), lineOrigins);
    for (CFIndex lineIndex = 0; lineIndex < numberOfLines; lineIndex++) {
        CGPoint lineOrigin = lineOrigins[lineIndex];
        lineOrigin = CGPointMake(CGFloat_ceil(lineOrigin.x), CGFloat_ceil(lineOrigin.y));
        CGContextSetTextPosition(ctx, lineOrigin.x, lineOrigin.y);
        CTLineRef line = CFArrayGetValueAtIndex(lines, lineIndex);
        CGFloat descent = 0.0f;
        CGFloat ascent = 0.0f;
        CGFloat lineLeading;
        CTLineGetTypographicBounds((CTLineRef)line, &ascent, &descent, &lineLeading);
        CGFloat flushFactor = NSTextAlignmentLeft;
        CGFloat penOffset;
        CGFloat y;
        penOffset = (CGFloat)CTLineGetPenOffsetForFlush(line, flushFactor, rect.size.width);
        y = lineOrigin.y - descent - self.font.descender;
        CGContextSetTextPosition(ctx, penOffset, y);
        CTLineDraw(line, ctx);
        if (!_highlighting && (self.superview != nil)) {
            CFArrayRef runs = CTLineGetGlyphRuns(line);
            for (int j = 0; j < CFArrayGetCount(runs); j++) {
                CGFloat runAscent;
                CGFloat runDescent;
                CTRunRef run = CFArrayGetValueAtIndex(runs, j);
                NSDictionary* attributes = (__bridge NSDictionary*)CTRunGetAttributes(run);
                if (!CGColorEqualToColor((__bridge CGColorRef)([attributes valueForKey:@"CTForegroundColor"]), self.textColor.CGColor)
                    && _clickRangeFramesDict!=nil) {
                    CFRange range = CTRunGetStringRange(run);
                    CGRect runRect;
                    runRect.size.width = CTRunGetTypographicBounds(run, CFRangeMake(0,0), &runAscent, &runDescent, NULL);
                    float offset = CTLineGetOffsetForStringIndex(line, range.location, NULL);
                    float height = runAscent;
                    runRect = CGRectMake(lineOrigin.x + offset, (self.height+5)-y-height+runDescent/2, runRect.size.width, height);
                    NSRange nRange = NSMakeRange(range.location, range.length);
                    [_clickRangeFramesDict setValue:[NSValue valueWithCGRect:runRect] forKey:NSStringFromRange(nRange)];
                }
            }
        }
    }
}
@end

#pragma mark - UIScrollViewDelegate
// 【开始滚动时】、清除缓存的所有cell
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{
    //1. 标记正在滚动ing
    _isScrolling = YES;
    //2. 清除之前保存的绘制cell的indexPath
    [_drawableIndexPaths removeAllObjects];
}
// 【手指离开屏幕】、如果【最终停止的indexpath】与【当前indexpath】相差超过指定行数
//那么只在目标滚动范围的前后指定3行的cell进行内容数据的绘制
- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView
                     withVelocity:(CGPoint)velocity
              targetContentOffset:(inout CGPoint *)targetContentOffset
{
    NSIndexPath *curIndexPath = [_tableView xzh_firstVisbledCellIndexPath];
    CGPoint stoppedPoint = CGPointMake(0, targetContentOffset->y);
    NSIndexPath *stopedIndexPath = [_tableView indexPathForRowAtPoint:stoppedPoint];
    NSLog(@"curIndexPath = %@, stopedIndexPath = %@", curIndexPath, stopedIndexPath);
    NSInteger skipCount = 8;
    /**
     *  如果 【滚动前的row】 距离 【滚动停止时的row】，超过了 skipCount
     *  - (1) 则忽略中间的 skipCount个 cell的绘制
     *  - (2) 只在停止滚动的【前后】指定的 3行 cell进行绘制
     */
    BOOL isOverSkipCount = labs(stopedIndexPath.row - curIndexPath.row) > skipCount;
    if (isOverSkipCount) {
        NSArray *stoppedVisbleIndexpaths = [_tableView indexPathsForRowsInRect:CGRectMake(0,
                                                                                          targetContentOffset->y,
                                                                                          _tableView.width,
                                                                                          _tableView.height)];
        NSMutableArray *mutableIndexPaths = [NSMutableArray arrayWithArray:stoppedVisbleIndexpaths];
        if (velocity.y > 0) {
            NSIndexPath *idx = [mutableIndexPaths lastObject];
            if ((idx.row + 3) < _tweetList.count) {
                NSIndexPath *next1 = [idx xzh_nextRow];
                NSIndexPath *next2 = [next1 xzh_nextRow];
                NSIndexPath *next3 = [next2 xzh_nextRow];
                [mutableIndexPaths addObject:next1];
                [mutableIndexPaths addObject:next2];
                [mutableIndexPaths addObject:next3];
            }
        } else {
            NSIndexPath *idx = [mutableIndexPaths firstObject];
            if ((idx.row - 3) >= 0) {
                NSIndexPath *prev1 = [idx xzh_previousRow];
                NSIndexPath *prev2 = [prev1 xzh_previousRow];
                NSIndexPath *prev3 = [prev2 xzh_previousRow];
                [mutableIndexPaths addObject:prev1];
                [mutableIndexPaths addObject:prev2];
                [mutableIndexPaths addObject:prev3];
            }
        }
        [_drawableIndexPaths addObjectsFromArray:mutableIndexPaths];
    } else {
        /**
         *  走到这里，不会走scrollview下面的几个delegate函数，
         *  所以，直接标记停止滚动，并绘制当前scrollview的可见区域的subviews
         */
        _isScrolling = NO;
        [self drawVisbledCells];
    }
}
// 【是否允许滚动到顶部】
- (BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView{
    _isScrolling = YES;
    return YES;
}
// 【已经滚动到顶部】
- (void)scrollViewDidScrollToTop:(UIScrollView *)scrollView{
    _isScrolling = NO;
    [self drawVisbledCells];
}
// 【停止滚动】
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    _isScrolling = NO;
    // 开始绘制当前可见的cell
    [self drawVisbledCells];
}

1. dispatch_queue_t 的缓存pool
2. CTFrameRef 的缓存

- Pool
    - (1) QOS_CLASS_USER_INITIATED Dispatch Context 对象
        - 缓存的dispatch_queue_t 实例1
        - 缓存的dispatch_queue_t 实例1
        - ....
        - 缓存的dispatch_queue_t 实例n
    - (2) QOS_CLASS_DEFAULT Dispatch Context 对象
        - 缓存的dispatch_queue_t 实例1
        - 缓存的dispatch_queue_t 实例1
        - ....
        - 缓存的dispatch_queue_t 实例n
    - (3) QOS_CLASS_UTILITY Dispatch Context 对象
        - 缓存的dispatch_queue_t 实例1
        - 缓存的dispatch_queue_t 实例1
        - ....
        - 缓存的dispatch_queue_t 实例n
    - (4) QOS_CLASS_BACKGROUND Dispatch Context 对象
        - 缓存的dispatch_queue_t 实例1
        - 缓存的dispatch_queue_t 实例1
        - ....
        - 缓存的dispatch_queue_t 实例n

// 创建pool
void XZHDispatchQueuePoolCreate();


// 将一个绘制代码block，分配到一个QOS下的缓存queue
- (void)test2 {
   XZHDispatchQueueAsyncBlockWithQOSUserInteractive(^{
        NSLog(@"task1 : %@", [NSThread currentThread]);
    });
    XZHDispatchQueueAsyncBlockWithQOSUserInitiated(^{
        NSLog(@"task2 : %@", [NSThread currentThread]);
    });
    XZHDispatchQueueAsyncBlockWithQOSUtility(^{
        NSLog(@"task3 : %@", [NSThread currentThread]);
    });
    XZHDispatchQueueAsyncBlockWithQOSBackgroud(^{
        NSLog(@"task4 : %@", [NSThread currentThread]);
    });


// 不再使用的时候，全部废弃掉。但是我觉得还是不要废弃，全局就使用这一个缓存池
void XZHDispatchQueuePoolRelease();

- (void)asyncDraw {
    /**
     *  渲染生成图像的过程，全部都在子线程异步完成
     */
    XZHDispatchQueueAsyncBlockWithQOSBackgroud(^{
        ..........................     
        // 对文本的md5
         NSString *md5 = [_text xzh_MD5];
        // 使用MD5从内存缓存取出CTFrameRef
         CTFrameRef ctFrame = CTFrameForKey(md5);
        // 判断是否使用缓存的CTFrameRef，进行直接绘制
        if (!_highlighting && ctFrame) {
            // 使用缓存的CTFrame进行绘制，不必再进行文本的解析、渲染
            [self drawWithCTFrame:ctFrame inRect:rect context:context];
        } else {
            // 重新走解析CTFrameRef的流程
            CTFrameRef ctFrame = ............;
            // 将CTFrameRef实例，缓存起来，避免重复对同一段文本进行解析
            CacheCTFrameWithKey(ctFrame, md5);
            ...........
      }
}

- (1) CAShapeLayer 矢量绘图
- (2) CATextLayer 文本绘制
- (3) CATransformLayer 形变绘制
- (4) CAGradientLayer 渐变绘制
- (5) CAReplicatorLayer 重复多个样式的绘制
- (6) CAScrollLayer 类似ScollView
- (7) CATiledLayer 切割大图为n个小图，按需加载
- (8) CAEmitterLayer 不常用
- (9) CAEAGLLayer 不常用
- (10) AVPlayerLayer 播放视频，不算为一种高效layer

#import <QuartzCore/QuartzCore.h>
#import <UIKit/UIKit.h>
@interface CALayerSub : CALayer
@end
@implementation CALayerSub
- (void)drawInContext:(CGContextRef)ctx {
    NSLog(@"thread = %@", [NSThread currentThread]);
    // 绘制一个图像
    //CGContextDrawImage(ctx, self.bounds, [UIImage imageNamed:@"demo"].CGImage);
    // 绘制一个椭圆
    CGContextAddEllipseInRect(ctx, self.bounds);
    CGContextSetFillColorWithColor(ctx, [UIColor orangeColor].CGColor);
    CGContextFillPath(ctx);
    NSLog(@"thread = %@", [NSThread currentThread]);
}
@end

#import "ViewController.h"
#import "CALayerSub.h"
@interface ViewController ()
@end
@implementation ViewController
- (void)test1 {
    CALayerSub *layer = [CALayerSub layer];
    layer.drawsAsynchronously = YES;
    layer.frame = CGRectMake(50, 100, 200, 100);
    [self.view.layer addSublayer:layer];
    [layer setNeedsDisplay];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    [self test1];
}
@end

2017-03-09 18:21:48.673 XZHAsyncLayerDemo[21807:1380124] thread = <NSThread: 0x60000006acc0>{number = 1, name = main}
2017-03-09 18:21:48.674 XZHAsyncLayerDemo[21807:1380124] thread = <NSThread: 0x60000006acc0>{number = 1, name = main}

@implementation ViewController
- (void)test1 {
    CALayerSub *layer = [CALayerSub layer];
//    layer.drawsAsynchronously = YES;
    layer.frame = CGRectMake(50, 100, 200, 100);
    layer.borderWidth = 1;
    layer.contents = (__bridge id)([UIImage imageNamed:@"demo"].CGImage);
    dispatch_async(dispatch_get_main_queue(), ^{
        [self.view.layer addSublayer:layer];
    });
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
          [self test1];
    });
}
@end

@interface ViewController () {
@property (weak, nonatomic) IBOutlet UIImageView *imageview;
@end
@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.imageview.image = [UIImage imageNamed:@"demo"];
}
- (void)testScreenShotAsync {
    CALayer *layer = self.imageview.layer;
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        UIImage *image = [layer xzh_screenShot];//网上找到一个CALayer数据渲染的代码
        CALayer *bottomLayer = [CALayer layer];
        bottomLayer.contentsScale = [UIScreen mainScreen].scale;
        bottomLayer.frame = CGRectMake(50, 200, 200, 150);
        bottomLayer.borderWidth = 1;
        bottomLayer.borderColor = [UIColor redColor].CGColor;
        bottomLayer.contents = (__bridge id)(image.CGImage);
        dispatch_async(dispatch_get_main_queue(), ^{
            [self.view.layer addSublayer:bottomLayer];
        });
    });
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
      [self testScreenShotAsync];
}
@end

- (void)test1 {
    CALayerSub *layer = [CALayerSub layer];
//    layer.drawsAsynchronously = YES; 注释不注释不影响
    layer.frame = CGRectMake(50, 100, 200, 100);
    [self.view.layer addSublayer:layer];
    for (int i = 0; i < 10; i++) {
        layer.backgroundColor = [UIColor randomColor].CGColor;
//        [layer display]; 会强制重绘10次
        [layer setNeedsDisplay]; // 只会进行最后的一次重绘
    }
}

2017-03-09 19:16:35.469 XZHAsyncLayerDemo[22421:1428266] thread = <NSThread: 0x6080000680c0>{number = 1, name = main}

2017-03-09 19:17:28.803 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.803 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.803 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.804 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.804 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.804 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.804 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.804 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.805 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}
2017-03-09 19:17:28.805 XZHAsyncLayerDemo[22472:1429927] thread = <NSThread: 0x60000006a3c0>{number = 1, name = main}

@implementation CoreTextDemoVC {
    UIImageView *_labelImageView;
}
- (void)viewDidLoad {
    [super viewDidLoad];
    [self drawText2_3];
}
- (void)drawText2_3 {
    CGRect rect = CGRectMake(10, 100, 300, 300);
    _labelImageView = [[UIImageView alloc] initWithFrame:rect];
    _labelImageView.layer.borderWidth = 1;
    _labelImageView.contentMode = UIViewContentModeScaleAspectFit;
    _labelImageView.clipsToBounds = YES;
    [self.view addSubview:_labelImageView];
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        UIGraphicsBeginImageContextWithOptions(rect.size, NO, 0);
        CGContextRef context = UIGraphicsGetCurrentContext();
        [[UIColor whiteColor] set];
        CGContextFillRect(context, rect);
        CGContextSetTextMatrix(context, CGAffineTransformIdentity);
        CGContextTranslateCTM(context, 0, rect.size.height);
        CGContextScaleCTM(context, 1.0, -1.0);
        NSMutableAttributedString *attrString = [[NSMutableAttributedString alloc] initWithString:@"iOS程序在启动时会创建一个主线程，而在一个线程只能执行一件事情，如果在主线程执行某些耗时操作，例如加载网络图片，下载资源文件等会阻塞主线程（导致界面卡死，无法交互），所以就需要使用多线程技术来避免这类情况。iOS中有三种多线程技术 NSThread，NSOperation，GCD，这三种技术是随着IOS发展引入的，抽象层次由低到高，使用也越来越简单。"];
        CTFramesetterRef frameSetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attrString);
        CGMutablePathRef path = CGPathCreateMutable();
        CGPathAddEllipseInRect(path, NULL, CGRectMake(0, 0, rect.size.width, rect.size.height));
        [[UIColor redColor]set];
        CGContextFillEllipseInRect(context, CGRectMake(0, 0, rect.size.width, rect.size.height));
        CTFrameRef frame = CTFramesetterCreateFrame(frameSetter, CFRangeMake(0, [attrString length]), path, NULL);
        CTFrameDraw(frame, context);
        CFRelease(frame);
        CFRelease(path);
        CFRelease(frameSetter);
CGContextSetTextMatrix(context,CGAffineTransformIdentity);
        CGContextTranslateCTM(context, 0, rect.size.height);
        CGContextScaleCTM(context, 1.0, -1.0);
        UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        dispatch_async(dispatch_get_main_queue(), ^{
            _labelImageView.image = img;
        });
    });
}
@end

1. UIView 的 backing layer 设置为 YYAsyncLayer
2. 修改UIView对象的text、textColor、textSize...等等，需要对内容进行重新绘制
3. [UIView setNeedsDisplay] or [UIView对象.layer setNeedsDisplay] 触发某一个属性值修改后的重绘
4. 将 {UI对象, text, setNeedsDisplay}、{UI对象, textColor, setNeedsDisplay}、{UI对象, textSize, setNeedsDisplay} 分别打包成Transaction对象
5. -[Transaction commit]
6. 将当前执行commit的Transaction对象，使用一个暂时的Set容器保存
7. 等到runloop即将休息的时候，在将当前Set容器保存的所有的Transaction对象，提交给runloop暂存，runloop进入休眠
8. runloop在下一个轮回唤醒时，执行上一次设置的所有的Transaction对象的操作
9. 被遍历的某一个Transaction对象的制定的SEL的消息被发送，一般是执行 [CALayer setNeedsDisplay]
10. -[YYAsyncLayer setNeedsDisplay] 被调用
11. -[YYAsyncLayer display] 被调用
12. -[YYAsyncLayer _displayAsync:是否异步绘制] 被调用
13. 给当前重绘任务创建一个新的Task：YYAsyncLayerDisplayTask *task = [YYAsyncLayer对象.delegate newAsyncDisplayTask]; 
14. task.willDisplay(); 告诉外界即将开始绘制
15. 构造传递给外界用来判断是否结束取消此次绘制的 isCancelled block，捕获局部counter值和counter对象
16. 开始异步子线程
17. 创建CGContext画布、以及初始化
18. if (task.didDisplay) task.didDisplay(self, 当前绘制是否结束);
19. 完成绘制，UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
20. YYAsyncLayer对象.contents = (__bridge id)(image.CGImage);