简书地址:http://www.jianshu.com/p/a5baa43b71c8
本文的中文注释代码demo更新在我的github上。
在研究iOS UI性能优化上,异步绘制一直是一个离不开的话题。最近在研究Facebook的开源框架AsyncDisplayKit的时候,找到了YYKit作者所实现的YYAsyncLayer。从这个项目了解异步绘制的方法。
YYAsyncLayer项目较为简单,一共就三个文件:
其中YYTransaction涉及了runloop的内容,具体runloop的了解,可以从作者的另一篇文章深入理解RunLoop了解。
以下将分别介绍下面3个文件。
YYSentinel使用原子性操作函数,进行计数。
/** 线程安全的计数器 */
@interface YYSentinel : NSObject
/** 当前计数 */
@property (readonly) int32_t value;
/** 原子性增加值 @return 新值 */
- (int32_t)increase;
@end
#import <libkern/OSAtomic.h>
@implementation YYSentinel {
int32_t _value;
}
- (int32_t)value {
return _value;
}
- (int32_t)increase {
//使用OSAtomic增加值
return OSAtomicIncrement32(&_value);
}
@end
YYTransaction的逻辑也并不复杂:将target和相应selector存入一个set中(重写hash与isEqual用于set判断),并且在runloop中注册kCFRunLoopBeforeWaiting与kCFRunLoopExit事件,将优先级定义为0,即在Core Animation执行完毕后,执行相应的display方法,去模拟Core Animation的绘制机制,进行相应异步绘制的方法。
YYTransaction.h声明
@interface YYTransaction : NSObject
/** 创建和返回一个transaction通过一个定义的target和selector @param target 执行target,target会在runloop结束前被retain @param selector target的selector @return 1个新的transaction,或者有错误时返回nil */
+ (YYTransaction *)transactionWithTarget:(id)target selector:(SEL)selector;
/** 加入transaction到runloop */
- (void)commit;
@end
YYTransaction.m实现
@interface YYTransaction()
@property (nonatomic, strong) id target;
@property (nonatomic, assign) SEL selector;
@end
static NSMutableSet *transactionSet = nil;
//runloop循环的回调
static void YYRunLoopObserverCallBack(CFRunLoopObserverRef observer, CFRunLoopActivity activity, void *info) {
if (transactionSet.count == 0) return;
NSSet *currentSet = transactionSet;
//获取完上一次需要执行的方法后,将所有方法清空
transactionSet = [NSMutableSet new];
//遍历set。执行里面的selector
[currentSet enumerateObjectsUsingBlock:^(YYTransaction *transaction, BOOL *stop) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
[transaction.target performSelector:transaction.selector];
#pragma clang diagnostic pop
}];
}
static void YYTransactionSetup() {
static dispatch_once_t onceToken;
//gcd只运行一次
dispatch_once(&onceToken, ^{
transactionSet = [NSMutableSet new];
CFRunLoopRef runloop = CFRunLoopGetMain();
CFRunLoopObserverRef observer;
//注册runloop监听,在等待与退出前进行
observer = CFRunLoopObserverCreate(CFAllocatorGetDefault(),
kCFRunLoopBeforeWaiting | kCFRunLoopExit,
true, // repeat
0xFFFFFF, // after CATransaction(2000000)
YYRunLoopObserverCallBack, NULL);
//将监听加在所有mode上
CFRunLoopAddObserver(runloop, observer, kCFRunLoopCommonModes);
CFRelease(observer);
});
}
@implementation YYTransaction
+ (YYTransaction *)transactionWithTarget:(id)target selector:(SEL)selector{
if (!target || !selector) return nil;
YYTransaction *t = [YYTransaction new];
t.target = target;
t.selector = selector;
return t;
}
- (void)commit {
if (!_target || !_selector) return;
//初始化runloop监听
YYTransactionSetup();
//添加行为到set中
[transactionSet addObject:self];
}
//hash值返回
- (NSUInteger)hash {
long v1 = (long)((void *)_selector);
long v2 = (long)_target;
return v1 ^ v2;
}
//isEqual返回
- (BOOL)isEqual:(id)object {
if (self == object) return YES;
if (![object isMemberOfClass:self.class]) return NO;
YYTransaction *other = object;
return other.selector == _selector && other.target == _target;
}
@end
YYAsyncLayer为了异步绘制而继承CALayer的子类。通过使用Core Graphic相关方法,在子线程中绘制内容Context,绘制完成后,回到主线程对layer.contents进行直接显示。控制了渲染线程的数量以及通过原子计数YYSentinel控制了取消异步渲染的内容。通过delegate回调,可以使得不同的delegate对象在block中绘制需要的内容。
YYAsyncLayer.h声明
/** YYAsyncLayer是异步渲染的CALayer子类 */
@interface YYAsyncLayer : CALayer
//是否异步渲染
@property BOOL displaysAsynchronously;
@end
/** YYAsyncLayer's的delegate协议,一般是uiview。必须实现这个方法 */
@protocol YYAsyncLayerDelegate <NSObject>
@required
//当layer的contents需要更新的时候,返回一个新的展示任务
- (YYAsyncLayerDisplayTask *)newAsyncDisplayTask;
@end
/** YYAsyncLayer在后台渲染contents的显示任务类 */
@interface YYAsyncLayerDisplayTask : NSObject
/** 这个block会在异步渲染开始的前调用,只在主线程调用。 */
@property (nullable, nonatomic, copy) void (^willDisplay)(CALayer *layer);
/** 这个block会调用去显示layer的内容 */
@property (nullable, nonatomic, copy) void (^display)(CGContextRef context, CGSize size, BOOL(^isCancelled)(void));
/** 这个block会在异步渲染结束后调用,只在主线程调用。 */
@property (nullable, nonatomic, copy) void (^didDisplay)(CALayer *layer, BOOL finished);
@end
YYAsyncLayer.m实现
/// Global display queue, used for content rendering.
//全局显示线程,给content渲染用
static dispatch_queue_t YYAsyncLayerGetDisplayQueue() {
//如果存在YYDispatchQueuePool
#ifdef YYDispatchQueuePool_h
return YYDispatchQueueGetForQOS(NSQualityOfServiceUserInitiated);
#else
#define MAX_QUEUE_COUNT 16
static int queueCount;
static dispatch_queue_t queues[MAX_QUEUE_COUNT];
static dispatch_once_t onceToken;
static int32_t counter = 0;
dispatch_once(&onceToken, ^{
//处理器数量,最多创建16个serial线程
queueCount = (int)[NSProcessInfo processInfo].activeProcessorCount;
queueCount = queueCount < 1 ? 1 : queueCount > MAX_QUEUE_COUNT ? MAX_QUEUE_COUNT : queueCount;
if ([UIDevice currentDevice].systemVersion.floatValue >= 8.0) {
for (NSUInteger i = 0; i < queueCount; i++) {
dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_USER_INITIATED, 0);
queues[i] = dispatch_queue_create("com.ibireme.yykit.render", attr);
}
} else {
for (NSUInteger i = 0; i < queueCount; i++) {
queues[i] = dispatch_queue_create("com.ibireme.yykit.render", DISPATCH_QUEUE_SERIAL);
dispatch_set_target_queue(queues[i], dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0));
}
}
});
//循环获取相应的线程
int32_t cur = OSAtomicIncrement32(&counter);
if (cur < 0) cur = -cur;
return queues[(cur) % queueCount];
#undef MAX_QUEUE_COUNT
#endif
}
//释放线程
static dispatch_queue_t YYAsyncLayerGetReleaseQueue() {
#ifdef YYDispatchQueuePool_h
return YYDispatchQueueGetForQOS(NSQualityOfServiceDefault);
#else
return dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
#endif
}
@implementation YYAsyncLayerDisplayTask
@end
@implementation YYAsyncLayer {
//计数,用于取消异步绘制
YYSentinel *_sentinel;
}
#pragma mark - Override
+ (id)defaultValueForKey:(NSString *)key {
if ([key isEqualToString:@"displaysAsynchronously"]) {
return @(YES);
} else {
return [super defaultValueForKey:key];
}
}
- (instancetype)init {
self = [super init];
static CGFloat scale; //global
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
scale = [UIScreen mainScreen].scale;
});
self.contentsScale = scale;
_sentinel = [YYSentinel new];
_displaysAsynchronously = YES;
return self;
}
- (void)dealloc {
[_sentinel increase];
}
//需要重新渲染的时候,取消原来没有完成的异步渲染
- (void)setNeedsDisplay {
[self _cancelAsyncDisplay];
[super setNeedsDisplay];
}
/** 重写展示方法,设置contents内容 */
- (void)display {
super.contents = super.contents;
[self _displayAsync:_displaysAsynchronously];
}
#pragma mark - Private
- (void)_displayAsync:(BOOL)async {
//获取delegate对象,这边默认是CALayer的delegate,持有它的uiview
__strong id<YYAsyncLayerDelegate> delegate = self.delegate;
//delegate的初始化方法
YYAsyncLayerDisplayTask *task = [delegate newAsyncDisplayTask];
//没有展示block,就直接调用其他两个block返回
if (!task.display) {
if (task.willDisplay) task.willDisplay(self);
self.contents = nil;
if (task.didDisplay) task.didDisplay(self, YES);
return;
}
//异步
if (async) {
//先调用willdisplay
if (task.willDisplay) task.willDisplay(self);
//获取计数
YYSentinel *sentinel = _sentinel;
int32_t value = sentinel.value;
//用计数判断是否已经取消
BOOL (^isCancelled)() = ^BOOL() {
return value != sentinel.value;
};
CGSize size = self.bounds.size;
BOOL opaque = self.opaque;
CGFloat scale = self.contentsScale;
//长宽<1,直接清除contents内容
if (size.width < 1 || size.height < 1) {
//获取contents内容
CGImageRef image = (__bridge_retained CGImageRef)(self.contents);
//清除内容
self.contents = nil;
//如果是图片就release图片
if (image) {
dispatch_async(YYAsyncLayerGetReleaseQueue(), ^{
CFRelease(image);
});
}
//已经展示完成block,finish为yes
if (task.didDisplay) task.didDisplay(self, YES);
return;
}
//异步线程调用
dispatch_async(YYAsyncLayerGetDisplayQueue(), ^{
//是否取消
if (isCancelled()) return;
//创建Core Graphic bitmap context
UIGraphicsBeginImageContextWithOptions(size, opaque, scale);
CGContextRef context = UIGraphicsGetCurrentContext();
//返回context进行展示
task.display(context, size, isCancelled);
//如果取消,停止渲染
if (isCancelled()) {
//结束context,并且展示完成block,finish为no
UIGraphicsEndImageContext();
dispatch_async(dispatch_get_main_queue(), ^{
if (task.didDisplay) task.didDisplay(self, NO);
});
return;
}
//获取当前画布
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
//结束context
UIGraphicsEndImageContext();
//如果取消停止渲染
if (isCancelled()) {
dispatch_async(dispatch_get_main_queue(), ^{
if (task.didDisplay) task.didDisplay(self, NO);
});
return;
}
//返回主线程
dispatch_async(dispatch_get_main_queue(), ^{
//如果取消,停止渲染
if (isCancelled()) {
if (task.didDisplay) task.didDisplay(self, NO);
} else {
//主线程设置contents内容进行展示
self.contents = (__bridge id)(image.CGImage);
//已经展示完成block,finish为yes
if (task.didDisplay) task.didDisplay(self, YES);
}
});
});
} else {
//同步展示,直接increase,停止异步展示
[_sentinel increase];
if (task.willDisplay) task.willDisplay(self);
//直接创建Core Graphic bitmap context
UIGraphicsBeginImageContextWithOptions(self.bounds.size, self.opaque, self.contentsScale);
CGContextRef context = UIGraphicsGetCurrentContext();
task.display(context, self.bounds.size, ^{return NO;});
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//进行展示
self.contents = (__bridge id)(image.CGImage);
if (task.didDisplay) task.didDisplay(self, YES);
}
}
- (void)_cancelAsyncDisplay {
[_sentinel increase];
}
@end
这里举作者在github里写的简单例子:
在设置内容与layoutSubviews
内添加YYTransaction,并且实现了- (YYAsyncLayerDisplayTask *)newAsyncDisplayTask
方法去实现相应的三个block,这样,一个异步绘制的YYLabel就完成了。
@interface YYLabel : UIView
@property NSString *text;
@property UIFont *font;
@end
@implementation YYLabel
- (void)setText:(NSString *)text {
_text = text.copy;
[[YYTransaction transactionWithTarget:self selector:@selector(contentsNeedUpdated)] commit];
}
- (void)setFont:(UIFont *)font {
_font = font;
[[YYTransaction transactionWithTarget:self selector:@selector(contentsNeedUpdated)] commit];
}
- (void)layoutSubviews {
[super layoutSubviews];
[[YYTransaction transactionWithTarget:self selector:@selector(contentsNeedUpdated)] commit];
}
- (void)contentsNeedUpdated {
// do update
[self.layer setNeedsDisplay];
}
#pragma mark - YYAsyncLayer
+ (Class)layerClass {
return YYAsyncLayer.class;
}
- (YYAsyncLayerDisplayTask *)newAsyncDisplayTask {
// capture current state to display task
NSString *text = _text;
UIFont *font = _font;
YYAsyncLayerDisplayTask *task = [YYAsyncLayerDisplayTask new];
task.willDisplay = ^(CALayer *layer) {
//...
};
task.display = ^(CGContextRef context, CGSize size, BOOL(^isCancelled)(void)) {
if (isCancelled()) return;
NSArray *lines = CreateCTLines(text, font, size.width);
if (isCancelled()) return;
for (int i = 0; i < lines.count; i++) {
CTLineRef line = line[i];
CGContextSetTextPosition(context, 0, i * font.pointSize * 1.5);
CTLineDraw(line, context);
if (isCancelled()) return;
}
};
task.didDisplay = ^(CALayer *layer, BOOL finished) {
if (finished) {
// finished
} else {
// cancelled
}
};
return task;
}
@end
YYAsyncLayer内使用YYTransaction在 RunLoop 中注册了一个 Observer,监视的事件和 Core Animation 一样,但优先级比 CA 要低。当 RunLoop 进入休眠前、CA 处理完事件后,YYTransaction 就会执行该 loop 内提交的所有任务。
在YYAsyncLayer中,通过重写CALayer显示display方法,向delegate请求一个异步绘制的任务,并且在子线程中绘制Core Graphic对象,最后再回到主线程中设置layer.contents内容。
附上作者的部分解读:
YYAsyncLayer 是 CALayer 的子类,当它需要显示内容(比如调用了 [layer setNeedDisplay])时,它会向 delegate,也就是 UIView 请求一个异步绘制的任务。在异步绘制时,Layer 会传递一个 BOOL(^isCancelled)() 这样的 block,绘制代码可以随时调用该 block 判断绘制任务是否已经被取消。
1.iOS 保持界面流畅的技巧
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