iOS自学之NSOperation、NSOperationQueue、Background

　　iOS中多线程编程主要分为NSThread、NSOperation和GCD，今天主要记录下自己在学习NSOperation中的点滴～如有不对的地方帮忙指出下，PS：人生第一次写blog，各位看官请轻虐，谢啦～

　　NSOperation是abstract类，不能直接使用，可以使用CocoTouch提供的NSBlockOperaion和NSInvocationOperation，也可以自己实现subclass。NSOperation可以理解为一个独立的任务，没有调度功能，真正利用NSOperation实现多线程的关键是NSOperationQueue，当NSOperation 添加到NSOperationQueue后，NSOperationQueue就会给队列中的NSOperation分配线程并调度。

1、NSOperationQueue为队列中的NSOperation分配不同的线程，测试代码如下：

 - (void)viewDidLoad {
     [super viewDidLoad];
     // Do any additional setup after loading the view, typically from a nib.
     NSLog(@"main Thread is %p",[NSThread mainThread]);
     [self testOperationWithQueue];
 }
 - (void)testOperationWithQueue
 {
     NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
     NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:];
         NSLog(@"blockOperation1:current Thread is %p",[NSThread currentThread]);
     }];
     NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:];
         NSLog(@"blockOperation2:current Thread is %p",[NSThread currentThread]);
     }];
     NSBlockOperation *blockOperation3 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:];
         NSLog(@"blockOperation3:current Thread is %p",[NSThread currentThread]);
     }];
     [operationQueue addOperation:blockOperation1];
     [operationQueue addOperation:blockOperation2];
     [operationQueue addOperation:blockOperation3];
 }

 打印结果：
 -- ::22.747 TestNSOperation[:] main Thread is 0x7fc279f28130
 -- ::31.288 TestNSOperation[:] blockOperation1:current Thread is 0x7fc279e0b8a0
 -- ::31.288 TestNSOperation[:] blockOperation3:current Thread is 0x7fc279d661c0
 -- ::31.288 TestNSOperation[:] blockOperation2:current Thread is 0x7fc279e064c0

从打印结果可以看出blockOperation1、blockOperation2、blockOperation3运行于不同的线程，是同时运行的，不需要等其他NSOperation，这也表明NSOperationQueue默认是并发执行。如果想要serial执行，则可以设置operationQueue.maxConcurrentOperationCount = 1;

　　2、不用NSOperationQueue，也可以直接调用NSOperation的start方法，但此时NSOepration运行在当前线程上,测试代码如下：

 - (void)testOperationNotQueue
 {
     NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"blockOperation1:current Thread is %p",[NSThread currentThread]);
     }];
     NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"blockOperation2:current Thread is %p",[NSThread currentThread]);
     }];
     NSBlockOperation *blockOperation3 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"blockOperation3:current Thread is %p",[NSThread currentThread]);
     }];
     [blockOperation1 start];
     [blockOperation2 start];
     [blockOperation3 start];
 }

 //打印结果
 -- ::20.497 TestNSOperation[:] main Thread is 0x7fb562427fd0
 -- ::20.498 TestNSOperation[:] blockOperation1:current Thread is 0x7fb562427fd0
 -- ::20.499 TestNSOperation[:] blockOperation2:current Thread is 0x7fb562427fd0
 -- ::20.499 TestNSOperation[:] blockOperation3:current Thread is 0x7fb562427fd0

　　从打印结果可以看出blockOperation都运行在mainThread上。

　　3、NSOperation还提供了cancel功能，cancel不是强制把你的代码stop掉，只是改变了NSOperation内部的状态，该功能只能cancel掉Ready、Finish状态的NSOperation，不能cancel掉正在executing的NSOperation，测试代码如下：

 - (void)testCancelOperation
 {
     NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
     NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"start blockOperation1!");
         [NSThread sleepForTimeInterval:];
         if(blockOperation1.isCancelled)
         {
             NSLog(@"blockOperation2 cancelled!");
             return;
         }
         NSLog(@"blockOperation1:current Thread is %p",[NSThread currentThread]);
     }];
     NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"blockOperation2:current Thread is %p",[NSThread currentThread]);
     }];
     NSBlockOperation *blockOperation3 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"blockOperation3:current Thread is %p",[NSThread currentThread]);
     }];
     operationQueue.maxConcurrentOperationCount = ;
     [operationQueue addOperation:blockOperation1];
     [operationQueue addOperation:blockOperation2];
     [operationQueue addOperation:blockOperation3];
     [NSThread sleepForTimeInterval:];
     NSLog(@"cancel blockOperation1");
     [blockOperation1 cancel];
     NSLog(@"cancel blockOperation2");
     [blockOperation2 cancel];
     NSLog(@"cancel blockOperation3");
     [blockOperation3 cancel];
 }
 打印结果：
 -- ::26.311 TestNSOperation[:] main Thread is 0x7ff02ae24e70
 -- ::26.312 TestNSOperation[:] start blockOperation1!
 -- ::28.318 TestNSOperation[:] cancel blockOperation1
 -- ::28.318 TestNSOperation[:] cancel blockOperation2
 -- ::28.318 TestNSOperation[:] cancel blockOperation3
 -- ::36.315 TestNSOperation[:] blockOperation1:current Thread is 0x7ff02af1dec0

从打印结果可以看出不能cancel掉blockOperation1，而blockOperation2和blockOperation3则被成功地取消了。

　　4、NSOpertion还可以设置依赖，这一功能解决了需要按一定次序执行的situation，不多说直接上代码:

 - (void)testOperationWithDependency
 {
     NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
     NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"start blockOperation1");
     }];
     NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:];
         NSLog(@"start blockOperation2");
     }];
     NSBlockOperation *blockOperation3 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:];
         NSLog(@"start blockOperation3");;
     }];
     [blockOperation1 addDependency:blockOperation2];
     [blockOperation2 addDependency:blockOperation3];
     [operationQueue addOperation:blockOperation1];
     [operationQueue addOperation:blockOperation2];
     [operationQueue addOperation:blockOperation3];
 }
 打印结果：
 -- ::06.649 TestNSOperation[:] main Thread is 0x7fdaf9712a00
 -- ::09.654 TestNSOperation[:] start blockOperation3
 -- ::11.660 TestNSOperation[:] start blockOperation2
 -- ::11.660 TestNSOperation[:] start blockOperation1

从打印的结果可以看出只有当依赖的Oepration执行完之后才开始执行自己。

　　5、当NSOperation添加到NSOperationQueue中，即使进入background（超过10min）NSOperation也会执行，真的不可思议啊，不是app进入到后台后如果不开定位服务、循环播放无声音乐、VOIP就最多运行10min吗？怎么超过10min的NSOperation还能执行啊，麻烦知道的网友告知一下，万分感谢～测试代码如下：

 - (void)testBackgroundOperation
 {
     NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
     NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:*];
         NSLog(@"blockOperation1:current Thread is %p",[NSThread currentThread]);
     }];
     [operationQueue addOperation:blockOperation1];
 }
 打印结果：
 -- ::40.511 TestNSOperation[:] main Thread is 0x7faf41c0da30
 -- ::43.095 TestNSOperation[:] applicationDidEnterBackground!
 -- ::40.526 TestNSOperation[:] blockOperation1:current Thread is 0x7faf41e02f90

从打印结果可以看出，进入后台11min后，blockOperation1执行了，这是为什么呀！！！！

　　6、疑问

　　1、NSOperationQueue具体如何管理NSOperation的，网上也没找到相关文章，希望知道的朋友给个链接；2、测试中发现当NSOperationQueue是局部变量，只要NSOperation添加到NSOperationQueue中，即使程序运行到超出NSOperationQueue的生命周期外（NSOperationQueue变量自动释放了），NSOperation依然可以执行，比如如下代码，operationQueue应该是释放了，但blockOperation1、blockOperation2依然运行，其实这2个疑问都是关于NSOperationQueue如何管理NSOperation的，再次恳求知道的网友给个链接，谢谢哈～

 - (void)viewDidLoad {
     [super viewDidLoad];
     // Do any additional setup after loading the view, typically from a nib.
     NSLog(@"main Thread is %p",[NSThread mainThread]);
     [self testOperationLeaveLifeCycle];
     NSLog(@"leave operationQueue lifecycle");
 }
 - (void)testOperationLeaveLifeCycle
 {
     NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
     NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:];
         NSLog(@"blockOperation1:current Thread is %p",[NSThread currentThread]);
     }];
     NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
         [NSThread sleepForTimeInterval:];
         NSLog(@"blockOperation2:current Thread is %p",[NSThread currentThread]);
     }];
     [operationQueue addOperation:blockOperation1];
     [operationQueue addOperation:blockOperation2];
 }
 打印结果：
 -- ::49.649 TestNSOperation[:] main Thread is 0x7ff7e27241b0
 -- ::49.650 TestNSOperation[:] leave operationQueue lifecycle
 -- ::59.652 TestNSOperation[:] blockOperation1:current Thread is 0x7ff7e2517d20
 -- ::09.653 TestNSOperation[:] blockOperation2:current Thread is 0x7ff7e2608a50

备注：上述所有的测试代码放到github上了，共勉～https://github.com/iOSGeek0829/testNSOperation