子线程更新UI的方法

子线程中不能直接更新UI，如果直接更新的话会发生崩溃
所以要在主线程中更新UI，总计三种回到主线程更新UI的方式

1、NSThread级主线程同步：performSelectorOnMainThread

NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];

[waitQueue addOperationWithBlock:^
{
    [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
    // 同步到主线程
    [self performSelectorOnMainThread:@selector(updateUI) withObject:nil waitUntilDone:NO];
}];

UI更新方法

- (void)updateUI 
{
    self.text = @"Thanks!";
}

2、NSOperationQueue级主线程同步：[NSOperationQueue mainQueue]

NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];

[waitQueue addOperationWithBlock:^
{
    [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];

    // 同步到主线程
    [[NSOperationQueue mainQueue] addOperationWithBlock:^
    {
        self.text = @"Thanks!";
    }];
}];

3、GCD级主线程同步：dispatch_get_main_queue

NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];

[waitQueue addOperationWithBlock:^
{
    [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:3]];

    // 同步到主线程
    dispatch_async(dispatch_get_main_queue(), ^
    {
        self.text = @"Thanks!";
    });
}];