Operation Queues

Operation Queues 在GCD之前就有了，其中的某些设计原理因Operation Queues而流行，GCD就是基于这些原理构建的。从iOS4与Mac OSX 10.6开始，Operation Queues在底层就是用GCD来实现的。

GCD是纯C的API，而Operation Queues是Objective-C的对象。

• Operation Queues ：相对 GCD 来说，使用 Operation Queues 会增加一点点额外的开销，但是我们却换来了非常强大的灵活性和功能，我们可以给 operation 之间添加依赖关系、取消一个正在执行的 operation 、暂停和恢复 operation queue 等；
• GCD ：则是一种更轻量级的，以 FIFO 的顺序执行并发任务的方式，使用 GCD 时我们并不关心任务的调度情况，而让系统帮我们自动处理。但是 GCD 的短板也是非常明显的，比如我们想要给任务之间添加依赖关系、取消或者暂停一个正在执行的任务时就会变得非常棘手。

NSOperation

很多执行任务类型的案例都很好的运用了NSOperation
，包括网络请求，图像压缩，自然语言处理或者其他很多需要返回处理后数据的、可重复的、结构化的、相对长时间运行的任务
比如我们经常使用的AFNetworking

@interface AFURLConnectionOperation : NSOperation 
@property (nonatomic, strong) NSSet *runLoopModes;
@property (readonly, nonatomic, strong) NSURLRequest *request;
@property (readonly, nonatomic, strong) NSURLResponse *response;
@property (readonly, nonatomic, strong) NSError *error;
@property (readonly, nonatomic, strong) NSData *responseData;
@property (readonly, nonatomic, copy) NSString *responseString;
...

SDWebImage

extern NSString *const SDWebImageDownloadStartNotification;
extern NSString *const SDWebImageDownloadReceiveResponseNotification;
extern NSString *const SDWebImageDownloadStopNotification;
extern NSString *const SDWebImageDownloadFinishNotification;

继承自NSOperation
@interface SDWebImageDownloaderOperation : NSOperation 
@property (strong, nonatomic, readonly) NSURLRequest *request;
@property (assign, nonatomic) BOOL shouldDecompressImages;
...
- (id)initWithRequest:(NSURLRequest *)request
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock;

@end

子类化NSOperation有两种类型

• 非并发型NSOperation
  子类化一个非并发型operation非常简单，只需要重写main方法即可。其他的一些复杂的操作比如工程配置，KVO通知父类已经帮我们做好了。
  main方法执行结束，该类就被释放了。
• 并发型NSOperation
  至少需要实现以下几个方法

- (void)start;
- (BOOL)isConcurrent;
- (BOOL)isExecuting;
- (BOOL)isFinished;

以AFNetworking为例

@implementation AFURLConnectionOperation
- (instancetype)initWithRequest:(NSURLRequest *)urlRequest {
    NSParameterAssert(urlRequest);

    self = [super init];
    if (!self) {
        return nil;
    }

    _state = AFOperationReadyState;

    self.lock = [[NSRecursiveLock alloc] init];
    self.lock.name = kAFNetworkingLockName;

    self.runLoopModes = [NSSet setWithObject:NSRunLoopCommonModes];

    self.request = urlRequest;

    self.shouldUseCredentialStorage = YES;

    self.securityPolicy = [AFSecurityPolicy defaultPolicy];

    return self;
}
- (void)start {
    [self.lock lock];
    if ([self isCancelled]) {
        [self performSelector:@selector(cancelConnection) onThread:[[self class] networkRequestThread] withObject:nil waitUntilDone:NO modes:[self.runLoopModes allObjects]];
    } else if ([self isReady]) {
        self.state = AFOperationExecutingState;

        [self performSelector:@selector(operationDidStart) onThread:[[self class] networkRequestThread] withObject:nil waitUntilDone:NO modes:[self.runLoopModes allObjects]];
    }
    [self.lock unlock];
}

- (BOOL)isConcurrent {
    return YES;
}
- (BOOL)isExecuting {
    return self.state == AFOperationExecutingState;
}

- (BOOL)isFinished {
    return self.state == AFOperationFinishedState;
}

发出isFinished的kvo通知，代表着opration结束

typedef NS_ENUM(NSInteger,MyOperationState) {
    MyOperationStateReady,
    MyOperationStateExecuting,
    MyOperationStateFinished,
};

@interface MyOperation ()
@property (nonatomic, copy) MyOperationAction action;
@property (nonatomic, assign) MyOperationState state;
@property (nonatomic, readonly,getter=isCancelled) BOOL cancel;
@end

@implementation MyOperation

#pragma mark - Override
- (BOOL)isReady{
    return self.state == MyOperationStateReady;
}

- (BOOL)isConcurrent{
    return YES;
}

- (BOOL)isExecuting{
    return self.state == MyOperationStateExecuting;
}

- (BOOL)isFinished{
    return self.state == MyOperationStateFinished;
}

- (void)cancel{
    [self willChangeValueForKey:@"isCancelled"];
    _cancel = YES;
    [self didChangeValueForKey:@"isCancelled"];
}

//在start中建立一个thread执行任务
- (void)start{
    if([self isReady]){
        [self willChangeValueForKey:@"isExecuting"];
        [self willChangeValueForKey:@"isReady"];
        self.state = MyOperationStateExecuting;
        [self didChangeValueForKey:@"isReady"];
        [self didChangeValueForKey:@"isExecuting"];
        
        [self performSelector:@selector(operationDidStart) onThread:[[self class] threadForMyOperation] withObject:nil waitUntilDone:NO];
    }
}

- (void)operationDidStart{
    if (self.isCancelled) {
        [self willChangeValueForKey:@"isFinished"];
        [self willChangeValueForKey:@"isCancelled"];
        self.state = MyOperationStateFinished;
        [self didChangeValueForKey:@"isCancelled"];
        [self didChangeValueForKey:@"isFinished"];
    }else{
        self.action();
        [self willChangeValueForKey:@"isFinished"];
        [self willChangeValueForKey:@"isExecuting"];
        self.state = MyOperationStateFinished;
        [self didChangeValueForKey:@"isExecuting"];
        [self didChangeValueForKey:@"isFinished"];
    }
}

#pragma mark Create Singleton Thread
+ (void)keepThreadAlive{
    do{
        @autoreleasepool {
            [[NSRunLoop currentRunLoop] run];
        }
    }while (YES);
}

+ (NSThread*)threadForMyOperation{
    static NSThread *_threadInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _threadInstance = [[NSThread alloc] initWithTarget:self selector:@selector(keepThreadAlive) object:nil];
        _threadInstance.name = @"MyOperation.Thread";
        [_threadInstance start];
    });
    
    return _threadInstance;
}
@end

NSOPerationQueue

但是仅仅把计算封装进一个对象而不做其他处理显然没有多大用处，我们还需要NSOperationQueue来大显身手。当operation被添加到队列之后，NSOperationQueue会浏览所有的operation，优先运行那些处于ready状态且优先级较高的操作。

http://nshipster.cn/nsoperation/

//创建队列
NSOperationQueue *queue = [[NSOperationQueue alloc] init];  

//将NSOperation加入队列之后，queue会自动执行该operation
[queue addOperation:operation];

//设置并发数目为2
queue.maxConcurrentOperationCount = 2;

取消任务

// 取消一个任务 
[operation cancel];  

// 取消queue中所有的任务  
[queue cancelAllOperations];

（http://nshipster.cn/nsoperation/）
http://blog.leichunfeng.com/blog/2015/07/29/ios-concurrency-programming-operation-queues/
refrence