多线程实现之Operation Queues

一、Operation Queues简介

Cocoa operations 为我们提供了一种面向对象的方式的来封装异步任务。包含Cocoa框架中的NSOperation和NSOperationQueue两个类，Operation通常与OperationQueue相结合来使用，是多线程的一种实现方式。其实现方式一是创建新线程，二是使用GCD的方式实现，相对于GCD可操作性高，是GCD的一种替代方式。

二、NSOperation介绍

NSOperation是一个抽象类，表示一个独立的计算单元。因为是抽象类我们不能直接使用，苹果为我们实现了两个类既NSInvocationOperation和NSBlockOperation。我们也可以自定义自己的Operation通过继承NSOperation类。

1、NSOperation的方法和属性

NSOperation定义的属性如下：

  • completionBlock 该属性是任务完成后调用的block
  • cancelled, executing, finished, concurrent, asynchronous, ready 这些是Operation的相关状态
  • name Operation的名字
  • queuePriority 执行的优先级 有以下几种：
  • NSOperationQueuePriorityVeryLow
  • NSOperationQueuePriorityLow
  • NSOperationQueuePriorityNormal
  • NSOperationQueuePriorityHigh
  • NSOperationQueuePriorityVeryHigh

NSOperation定义的方法有如下：

  • start 启动任务
  • main 真正执行任务的的方法，自定义Operation必须实现该方法
  • cancel 取消任务
  • addDependency 添加依赖方法，添加依赖任务会在依赖任务完成后才执行
  • removeDependency 移除依赖
  • waitUnitlFinished 该方法会阻塞当前线程直到该操作任务完成。一个Operation决不能自身调用改方法，也不能在加入OperationQueue之前调用。

2、创建Operation

使用NSInvocationOperation创建

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NSInvocationOperation *invocationOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(invocationOperationTask) object:nil];
[invocationOperation start];

使用NSBlockOperation创建

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NSBlockOperation *blockOperation = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"blockOperation thread =%@ ", [NSThread currentThread]);
        NSLog(@"blockOperation task...");
    }];
[blockOperation start];

自定义Operation必须实现initialization和main方法，Operation默认按synchronous方式执行，若要按asynchronous方式则必须实现start, isExecuting, isFinished, isConcurrent方法并且维护Executing, Finished, Concurrent等状态值的更新。

下面是自定义异步Operation实现：

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#import "MyOperation.h"

@interface MyOperation(){
    BOOL finished;
    BOOL executing;
}

- (void)completeOperation;

@end

@implementation MyOperation

- (id)init {
    self = [super init];
    if (self) {
        executing = NO;
        finished = NO;
    }
    return self;
}

- (void)start{
    if ([self isCancelled]) {
        // Must move the operation to the finished state if it is canceled.
        [self willChangeValueForKey:@"isFinished"];
        finished = YES;
        [self didChangeValueForKey:@"isFinished"];
        NSLog(@"has cancelled ...");
        return;
    }

    // If the operation is not canceled, begin executing the task.
    [self willChangeValueForKey:@"isExecuting"];
    [NSThread detachNewThreadSelector:@selector(main) toTarget:self withObject:nil];
    executing = YES;
    [self didChangeValueForKey:@"isExecuting"];

}

- (void)main {
    @try {
        // Do the main work of the operation here.

        NSLog(@"do task ....");


        [self completeOperation];
    }
    @catch(...) {
        // Do not rethrow exceptions.
        NSLog(@"do task exception....");
    }
}

- (void)completeOperation {
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];

    executing = NO;
    finished = YES;

    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

- (BOOL)isConcurrent {
    return YES;
}

- (BOOL)isAsynchronous{
    return YES;
}

- (BOOL)isExecuting {
    return executing;
}

- (BOOL)isFinished {
    return finished;
}

@end

三、使用NSOperationQueue管理Operation队列

简单的Operation不没有什么特别大的用处。在实际的项目中往往使用NSOperationQueue管理一连串的Operation来完成复杂任务。可以NSOperationQueue中的-(void)addOperation:(NSOperation *)方法将Operation加入队列中。

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NSInvocationOperation *invocationOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(invocationOperationTask) object:nil];

NSBlockOperation *blockOperation = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"blockOperation thread =%@ ", [NSThread currentThread]);
        NSLog(@"blockOperation task...");
    }];

MyOperation *myOperation = [[MyOperation alloc] init];

NSOperationQueue *myOperationQueue = [[NSOperationQueue alloc] init];
//将Operation加入到队列
[myOperationQueue addOperation:myOperation];
[myOperationQueue addOperation:invocationOperation];
[myOperationQueue addOperation:blockOperation];

也可以使用addOperationWithBlock方法快速添加一个Operation到OperationQueue中

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[myOperationQueue addOperationWithBlock:^{
    NSLog(@"addOperationWithBlock thread =%@ ",     [NSThread currentThread]);
    NSLog(@"addOperationWithBlock task...");
}];

加入到OperationQueue中的Operation会按自动执行。可以设置Operation的优先级既queuePriority来改变Operation执行的优先级。使用addDependency来添加依赖关系，变相控制Operation的执行顺序。扩展一下上面的代码：

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NSOperationQueue *myOperationQueue = [[NSOperationQueue alloc] init];
[myOperationQueue setName:@"myqueue"];

NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"blockOperation3 task start...");
    sleep(3);
    NSLog(@"blockOperation1 task finished...");
}];

NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"blockOperation2 task start...");
    sleep(3);
    NSLog(@"blockOperation2 task finished...");
}];

NSBlockOperation *blockOperation3 = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"blockOperation3 task start...");
    sleep(3);
    NSLog(@"blockOperation3 task finished...");
}];
[blockOperation3 addDependency:blockOperation1];

[myOperationQueue addOperation:blockOperation1];
[myOperationQueue addOperation:blockOperation2];
[myOperationQueue addOperation:blockOperation3];

NSLog(@"main thread ...");

我们为blockOperation3添加依赖blockOperation1，blockOperation3会在blockOperation1执行结束后才会启动。结果如下：

operation-dependency

在上文中我们介绍过waitUntilFinished方法下面我们就看一下它的用法，改变一下Operation加入队列的代码：

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[myOperationQueue addOperation:blockOperation1];
[blockOperation1 waitUntilFinished];
[myOperationQueue addOperation:blockOperation2];
[myOperationQueue addOperation:blockOperation3];

我们插入了一行[blockOperation1 waitUntilFinished]代码，并将blockOperation3的依赖设置为blockOperation2。waitUntilFinished会阻塞当前线程直到blockOperation1执行完毕。

operation-dependency

另外OperationQueue还有方法

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- (void)addOperations:(NSArray<NSOperation *> *)ops
    waitUntilFinished:(BOOL)wait

通过改方法可以将一组Operation加入到队列中，wait为YES时表示需要等待任务执行结束会阻塞当前线程，为NO时则直接返回。修改代码如下：

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NSOperationQueue *myOperationQueue = [[NSOperationQueue alloc] init];
[myOperationQueue setName:@"myqueue"];

NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"blockOperation1 task start...");
    sleep(3);
    NSLog(@"blockOperation1 task finished...");
}];

NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"blockOperation2 task start...");
    sleep(3);
    NSLog(@"blockOperation2 task finished...");
}];

NSBlockOperation *blockOperation3 = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"blockOperation3 task start...");
    sleep(3);
    NSLog(@"blockOperation3 task finished...");
}];
[blockOperation3 addDependency:blockOperation2];

[myOperationQueue addOperations:[NSArray arrayWithObjects:blockOperation1, blockOperation2, blockOperation3, nil] waitUntilFinished:YES];

运行结果如下：

operation-dependency

我们可以取消一个Operation当Operation还没有执行的时候，只需要调用cancel方法或者调用OperationQueue中的cancelAllOperations取消队列中所有没有执行的Operation。

四、总结

通过NSOperation和NSOpertionQueue可以实现多线程的功能。我们可以通过继承NSOperation来自定义Operation来完成复杂的功能。NSOperationQueue和GCD都是苹果为我们提供的多线程实现方式，简化了我们处理复杂业务的难度。NSOperationQueue相对与GCD操作性更高，我们可以根据实际需要来选择。

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