关于线程包活和释放

创建线程：

 for (int i = 0; i < 1000000; ++i) {
        //总结：test At: xcode8,ios 9.3.4
        //1,当用CFRunLoopRun(),然后调用CFRunLoopStop，此方法是后果会输出current thread，thread dealloc，current thread，thread dealloc ...所以不会用内存问题
        //2,当用 [runLoop run];,然后调用CFRunLoopStop,此方法会current thread，current thread，... 最后输出[NSThread start]: Thread creation failed with error 35.然后app卡住，然后app crash. 内存不会暴增。但是线程无法销毁
        //3,当用 [runLoop runMode:NSRunLoopCommonModes beforeDate:[NSDate distantFuture]];,然后调用CFRunLoopStop,此方法会。[ViewController performSelector:onThread:withObject:waitUntilDone:modes:]: target thread exited while waiting for the perform' crash。是因为 [runLoop runMode:NSRunLoopCommonModes beforeDate:[NSDate distantFuture]]; 无法阻塞线程，所以线程很快执行完run 方法。然后线程exit，导致奔溃（在一个退出的线程，当然这个时候线程没有释放，执行方法奔溃）
        NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
        [thread start];
        [self performSelector:@selector(stopThread) onThread:thread withObject:nil waitUntilDone:YES];

    }

- (void)stopThread {
    CFRunLoopStop(CFRunLoopGetCurrent());//该方法可以释放掉该线程。
}

- (void)run {
    @autoreleasepool {
        NSLog(@"current thread = %@", [NSThread currentThread]);
        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        if (!self.emptyPort) {
            self.emptyPort = [NSMachPort port];
        }
        [runLoop addPort:self.emptyPort forMode:NSDefaultRunLoopMode];
        // 下面这两种写法都不可取
//        [runLoop run];
//        [runLoop runMode:NSRunLoopCommonModes beforeDate:[NSDate distantFuture]];
        CFRunLoopRun();//通过这种方法开启runloop可以使线程常驻后台并且线程可以释放掉。
        NSLog(@"run over");
    }
}