YYAsyncLayer主要作用是異步繪圖，可以提高性能，平常應(yīng)用時，比如像微信朋友圈的列表內(nèi)容就可以用這個框架實現(xiàn)。先看個例子：

YYTextShadow *shadow = [YYTextShadow new];//陰影
shadow.offset = CGSizeMake(0,1);
shadow.color = [UIColor blackColor];
shadow.radius = 5;

NSMutableAttributedString *attributedStr = [[NSMutableAttributedString alloc]initWithString:@"Shadow"];
attributedStr.textShadow = shadow;

YYLabel *label = [YYLabel new];
label.attributedText = attributedStr;
[self.view addSubview: label];

1. YYAsyncLayer繼承于CALayer，繪圖時調(diào)用display，通過子類重寫CALayer實現(xiàn)自定義繪制：

@interface YYAsyncLayer : CALayer

@implementation YYAsyncLayer
...
- (void)display {
    super.contents = super.contents;
    [self _displayAsync:_displaysAsynchronously];//_displaysAsynchronously默認YES
}

- (void)_displayAsync:(BOOL)async {
    __strong id<YYAsyncLayerDelegate> delegate = self.delegate;
    YYAsyncLayerDisplayTask *task = [delegate newAsyncDisplayTask];//調(diào)用代理
    if (!task.display) {
        if (task.willDisplay) task.willDisplay(self);//準備繪制的工作
        self.contents = nil;//清除上次繪制內(nèi)容
        if (task.didDisplay) task.didDisplay(self, YES);//繪制完成后的工作
        return;
    }
    ...
    if (async) {
        if (task.willDisplay) task.willDisplay(self);//準備繪制的工作
        ...
        //子線程
        dispatch_async(YYAsyncLayerGetDisplayQueue(), ^{
            ...
            UIGraphicsBeginImageContextWithOptions(size, opaque, scale);//開始合成圖片
            ...
            task.display(context, size, isCancelled);//調(diào)用繪制
            ...
            UIImage *image = UIGraphicsGetImageFromCurrentImageContext();//使用圖片達到效果，可以減少圖層的層級
            ...
            //主線程
            dispatch_async(dispatch_get_main_queue(), ^{
                if (isCancelled()) {
                    if (task.didDisplay) task.didDisplay(self, NO);
                } else {
                    self.contents = (__bridge id)(image.CGImage);//顯示的內(nèi)容
                    if (task.didDisplay) task.didDisplay(self, YES);//繪制完成后的工作
                }
            });
        });
}

2. 然后調(diào)用代理協(xié)議，而代理就是YYLabel：

@implementation YYLabel
...
- (YYAsyncLayerDisplayTask *)newAsyncDisplayTask {
    ...
    YYAsyncLayerDisplayTask *task = [YYAsyncLayerDisplayTask new];
    
    task.willDisplay = ^(CALayer *layer) { ... }

    task.display = ^(CGContextRef context, CGSize size, BOOL (^isCancelled)(void)) {
        ...
        [drawLayout drawInContext:context size:size point:point view:nil layer:nil debug:debug cancel:isCancelled];//繪制
    };

    task.didDisplay = ^(CALayer *layer, BOOL finished) { ... }

    return task;
}

@implementation YYTextLayout
...
- (void)drawInContext:(CGContextRef)context
                 size:(CGSize)size
                point:(CGPoint)point
                 view:(UIView *)view
                layer:(CALayer *)layer
                debug:(YYTextDebugOption *)debug
                cancel:(BOOL (^)(void))cancel{
    @autoreleasepool {
        ...
        if (self.needDrawShadow && context) {
            if (cancel && cancel()) return;
            YYTextDrawShadow(self, context, size, point, cancel);//繪制陰影
        }
        ...
    }
}

3. 完成后回到第1步，返回主線程進行顯示self.contents = (__bridge id)(image.CGImage);。

使用框架創(chuàng)建任務(wù)時，會根據(jù)Runloop狀態(tài)分發(fā)任務(wù)，然后會調(diào)用setNeedsDisplay，接著執(zhí)行display進行異步繪制。最終顯示的文字內(nèi)容其實是UIImage。

• YYAsyncLayerGetDisplayQueue

在上文出現(xiàn)了YYAsyncLayerGetDisplayQueue，用它來調(diào)度子線程，內(nèi)部進行了處理：

static dispatch_queue_t YYAsyncLayerGetDisplayQueue() {
#ifdef YYDispatchQueuePool_h
    return YYDispatchQueueGetForQOS(NSQualityOfServiceUserInitiated);
#else
...
}

dispatch_queue_t YYDispatchQueueGetForQOS(NSQualityOfService qos) {
    return YYDispatchContextGetQueue(YYDispatchContextGetForQOS(qos));
}

通過YYDispatchContextGetForQOS，根據(jù)NSQualityOfService控制最大線程數(shù)，防止線程數(shù)使用過大：

static YYDispatchContext *YYDispatchContextGetForQOS(NSQualityOfService qos) {
    static YYDispatchContext *context[5] = {0};
    switch (qos) {
        case NSQualityOfServiceUserInteractive: {
            static dispatch_once_t onceToken;
            dispatch_once(&onceToken, ^{
                int count = (int)[NSProcessInfo processInfo].activeProcessorCount;
                count = count < 1 ? 1 : count > MAX_QUEUE_COUNT ? MAX_QUEUE_COUNT : count;
                context[0] = YYDispatchContextCreate("com.ibireme.yykit.user-interactive", count, qos);
            });
            return context[0];
        } break;
        case NSQualityOfServiceUserInitiated: { ... } break;
        case NSQualityOfServiceUtility: { ... } break;
        case NSQualityOfServiceBackground: { ... } break;
        default: { ... } break;
    }
}

并可以對已經(jīng)創(chuàng)建過的隊列進行復用，從而減少占用 CPU 資源：

static dispatch_queue_t YYDispatchContextGetQueue(YYDispatchContext *context) {
    int32_t counter = OSAtomicIncrement32(&context->counter);
    if (counter < 0) counter = -counter;
    void *queue = context->queues[counter % context->queueCount];//獲取已經(jīng)創(chuàng)建過的隊列進行復用，減少cpu資源占用
    return (__bridge dispatch_queue_t)(queue);
}