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YYLayer中，异步渲染的核心逻辑代码
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| - (void)displayAsyncLayer:(_ASDisplayLayer *)asyncLayer asynchronously:(BOOL)asynchronously | |
| { | |
| ASDisplayNodeAssertMainThread(); | |
| ASDN::MutexLocker l(__instanceLock__); | |
| if (_hierarchyState & ASHierarchyStateRasterized) { | |
| return; | |
| } | |
| // for async display, capture the current displaySentinel value to bail early when the job is executed if another is | |
| // enqueued | |
| // for sync display, just use nil for the displaySentinel and go | |
| // FIXME: what about the degenerate case where we are calling setNeedsDisplay faster than the jobs are dequeuing | |
| // from the displayQueue? Need to not cancel early fails from displaySentinel changes. | |
| ASSentinel *displaySentinel = (asynchronously ? _displaySentinel : nil); | |
| int32_t displaySentinelValue = [displaySentinel increment]; | |
| asdisplaynode_iscancelled_block_t isCancelledBlock = ^{ | |
| return BOOL(displaySentinelValue != displaySentinel.value); | |
| }; | |
| // Set up displayBlock to call either display or draw on the delegate and return a UIImage contents | |
| asyncdisplaykit_async_transaction_operation_block_t displayBlock = [self _displayBlockWithAsynchronous:asynchronously isCancelledBlock:isCancelledBlock rasterizing:NO]; | |
| if (!displayBlock) { | |
| return; | |
| } | |
| ASDisplayNodeAssert(_layer, @"Expect _layer to be not nil"); | |
| // This block is called back on the main thread after rendering at the completion of the current async transaction, or immediately if !asynchronously | |
| asyncdisplaykit_async_transaction_operation_completion_block_t completionBlock = ^(id<NSObject> value, BOOL canceled){ | |
| ASDisplayNodeCAssertMainThread(); | |
| if (!canceled && !isCancelledBlock()) { | |
| UIImage *image = (UIImage *)value; | |
| BOOL stretchable = (NO == UIEdgeInsetsEqualToEdgeInsets(image.capInsets, UIEdgeInsetsZero)); | |
| if (stretchable) { | |
| ASDisplayNodeSetupLayerContentsWithResizableImage(_layer, image); | |
| } else { | |
| _layer.contentsScale = self.contentsScale; | |
| _layer.contents = (id)image.CGImage; | |
| } | |
| [self didDisplayAsyncLayer:self.asyncLayer]; | |
| } | |
| }; | |
| // Call willDisplay immediately in either case | |
| [self willDisplayAsyncLayer:self.asyncLayer]; | |
| if (asynchronously) { | |
| // Async rendering operations are contained by a transaction, which allows them to proceed and concurrently | |
| // while synchronizing the final application of the results to the layer's contents property (completionBlock). | |
| // First, look to see if we are expected to join a parent's transaction container. | |
| CALayer *containerLayer = _layer.asyncdisplaykit_parentTransactionContainer ? : _layer; | |
| // In the case that a transaction does not yet exist (such as for an individual node outside of a container), | |
| // this call will allocate the transaction and add it to _ASAsyncTransactionGroup. | |
| // It will automatically commit the transaction at the end of the runloop. | |
| _ASAsyncTransaction *transaction = containerLayer.asyncdisplaykit_asyncTransaction; | |
| // Adding this displayBlock operation to the transaction will start it IMMEDIATELY. | |
| // The only function of the transaction commit is to gate the calling of the completionBlock. | |
| [transaction addOperationWithBlock:displayBlock priority:self.drawingPriority queue:[_ASDisplayLayer displayQueue] completion:completionBlock]; | |
| } else { | |
| UIImage *contents = (UIImage *)displayBlock(); | |
| completionBlock(contents, NO); | |
| } | |
| } |
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| // 系统回调，用于刷新Content | |
| - (void)display { | |
| super.contents = super.contents; | |
| [self _displayAsync:_displaysAsynchronously]; | |
| } | |
| #pragma mark - Private | |
| - (void)_displayAsync:(BOOL)async { | |
| // 强引用保证不被释放 | |
| __strong id<YYAsyncLayerDelegate> delegate = self.delegate; | |
| // 获取异步渲染任务 | |
| YYAsyncLayerDisplayTask *task = [delegate newAsyncDisplayTask]; | |
| // 无显示block则直接清空内容 | |
| if (!task.display) { | |
| if (task.willDisplay) task.willDisplay(self); | |
| self.contents = nil; | |
| if (task.didDisplay) task.didDisplay(self, YES); | |
| return; | |
| } | |
| // 区分异步渲染跟同步渲染 | |
| if (async) { | |
| if (task.willDisplay) task.willDisplay(self); | |
| // 线程安全计数器 | |
| YYSentinel *sentinel = _sentinel; | |
| int32_t value = sentinel.value; | |
| // 取消渲染Block，判断计数器是否有增加，很巧妙的实现 | |
| BOOL (^isCancelled)() = ^BOOL() { | |
| return value != sentinel.value; | |
| }; | |
| CGSize size = self.bounds.size; | |
| BOOL opaque = self.opaque; | |
| CGFloat scale = self.contentsScale; | |
| CGColorRef backgroundColor = (opaque && self.backgroundColor) ? CGColorRetain(self.backgroundColor) : NULL; | |
| if (size.width < 1 || size.height < 1) { | |
| CGImageRef image = (__bridge_retained CGImageRef)(self.contents); | |
| self.contents = nil; | |
| if (image) { | |
| // 异步线程释放，减轻主线程压力 | |
| dispatch_async(YYAsyncLayerGetReleaseQueue(), ^{ | |
| CFRelease(image); | |
| }); | |
| } | |
| if (task.didDisplay) task.didDisplay(self, YES); | |
| CGColorRelease(backgroundColor); | |
| return; | |
| } | |
| dispatch_async(YYAsyncLayerGetDisplayQueue(), ^{ | |
| if (isCancelled()) { | |
| CGColorRelease(backgroundColor); | |
| return; | |
| } | |
| UIGraphicsBeginImageContextWithOptions(size, opaque, scale); | |
| CGContextRef context = UIGraphicsGetCurrentContext(); | |
| if (opaque) { | |
| CGContextSaveGState(context); { | |
| if (!backgroundColor || CGColorGetAlpha(backgroundColor) < 1) { | |
| CGContextSetFillColorWithColor(context, [UIColor whiteColor].CGColor); | |
| CGContextAddRect(context, CGRectMake(0, 0, size.width * scale, size.height * scale)); | |
| CGContextFillPath(context); | |
| } | |
| if (backgroundColor) { | |
| CGContextSetFillColorWithColor(context, backgroundColor); | |
| CGContextAddRect(context, CGRectMake(0, 0, size.width * scale, size.height * scale)); | |
| CGContextFillPath(context); | |
| } | |
| } CGContextRestoreGState(context); | |
| CGColorRelease(backgroundColor); | |
| } | |
| task.display(context, size, isCancelled); | |
| if (isCancelled()) { | |
| // 每走一步，都对Cancel判断一次，尽可能减少渲染次数 | |
| UIGraphicsEndImageContext(); | |
| dispatch_async(dispatch_get_main_queue(), ^{ | |
| if (task.didDisplay) task.didDisplay(self, NO); | |
| }); | |
| return; | |
| } | |
| UIImage *image = UIGraphicsGetImageFromCurrentImageContext(); | |
| UIGraphicsEndImageContext(); | |
| if (isCancelled()) { | |
| dispatch_async(dispatch_get_main_queue(), ^{ | |
| if (task.didDisplay) task.didDisplay(self, NO); | |
| }); | |
| return; | |
| } | |
| dispatch_async(dispatch_get_main_queue(), ^{ | |
| if (isCancelled()) { | |
| if (task.didDisplay) task.didDisplay(self, NO); | |
| } else { | |
| self.contents = (__bridge id)(image.CGImage); | |
| if (task.didDisplay) task.didDisplay(self, YES); | |
| } | |
| }); | |
| }); | |
| } else { | |
| // 同步渲染前先增加计数，标明这是一次新渲染，旧的未完成的渲染都Cancel掉 | |
| [_sentinel increase]; | |
| if (task.willDisplay) task.willDisplay(self); | |
| UIGraphicsBeginImageContextWithOptions(self.bounds.size, self.opaque, self.contentsScale); | |
| CGContextRef context = UIGraphicsGetCurrentContext(); | |
| if (self.opaque) { | |
| CGSize size = self.bounds.size; | |
| size.width *= self.contentsScale; | |
| size.height *= self.contentsScale; | |
| CGContextSaveGState(context); { | |
| if (!self.backgroundColor || CGColorGetAlpha(self.backgroundColor) < 1) { | |
| CGContextSetFillColorWithColor(context, [UIColor whiteColor].CGColor); | |
| CGContextAddRect(context, CGRectMake(0, 0, size.width, size.height)); | |
| CGContextFillPath(context); | |
| } | |
| if (self.backgroundColor) { | |
| CGContextSetFillColorWithColor(context, self.backgroundColor); | |
| CGContextAddRect(context, CGRectMake(0, 0, size.width, size.height)); | |
| CGContextFillPath(context); | |
| } | |
| } CGContextRestoreGState(context); | |
| } | |
| task.display(context, self.bounds.size, ^{return NO;}); | |
| UIImage *image = UIGraphicsGetImageFromCurrentImageContext(); | |
| UIGraphicsEndImageContext(); | |
| self.contents = (__bridge id)(image.CGImage); | |
| if (task.didDisplay) task.didDisplay(self, YES); | |
| } | |
| } | |
| - (void)_cancelAsyncDisplay { | |
| // 增加计数，标明取消之前的渲染 | |
| [_sentinel increase]; | |
| } |
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YYKit 中用计数器来中断刷新过程的思路，应该是来自ASDK。