代碼同步到主線程執(zhí)行的三種方法如下：

// 1.NSThread
[self performSelectorOnMainThread:@selector(updateUI) withObject:nil waitUntilDone:NO];

- (void)updateUI {
    // UI更新代碼
    self.alert.text = @"Thanks!";
}

// 2.NSOperationQueue
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
    // UI更新代碼
    self.alert.text = @"Thanks!";
    }];

// 3.GCD
dispatch_async(dispatch_get_main_queue(), ^{
   // UI更新代碼
   self.alert.text = @"Thanks!";
});

看下面的例子： 以下代碼有什么問(wèn)題？如何修復(fù)？

@interface TTWaitController : UIViewController

@property (strong, nonatomic) UILabel *alert;

@end

@implementation TTWaitController

- (void)viewDidLoad
{
    CGRect frame = CGRectMake(20, 200, 200, 20);
    self.alert = [[UILabel alloc] initWithFrame:frame];
    self.alert.text = @"Please wait 10 seconds...";
    self.alert.textColor = [UIColor whiteColor];
    [self.view addSubview:self.alert];

    NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];
    [waitQueue addOperationWithBlock:^{
        [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
        self.alert.text = @"Thanks!";
    }];
}

@end

@implementation TTAppDelegate

- (BOOL)application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.rootViewController = [[TTWaitController alloc] init];
    [self.window makeKeyAndVisible];
    return YES;
}

這段代碼是想提醒用戶等待10s，10s后在標(biāo)簽上顯示“Thanks”，但多線程代碼部分NSOperationQueue的addOperationWithBlock函數(shù)不能保證block里面的語(yǔ)句是在主線程中運(yùn)行的，UILabel顯示文字屬于UI更新，必須要在主線程進(jìn)行，否則會(huì)有未知的操作，無(wú)法在界面上及時(shí)正常顯示。

解決方法是將UI更新的代碼寫(xiě)在主線程上即可，代碼同步到主線程上主要有三種方法：NSThread、NSOperationQueue和GCD，三個(gè)層次的多線程都可以獲取主線程并同步。

NSThread級(jí)主線程同步：performSelectorOnMainThread

NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];
[waitQueue addOperationWithBlock:^{
    [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
    // 同步到主線程
    [self performSelectorOnMainThread:@selector(updateUI) withObject:nil waitUntilDone:NO];
}];

/**
 * UI更新函數(shù)
 */
- (void)updateUI {
    self.alert.text = @"Thanks!";
}

NSOperationQueue級(jí)主線程同步：[NSOperationQueue mainQueue]

NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];
[waitQueue addOperationWithBlock:^{
    [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
    // 同步到主線程
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        self.alert.text = @"Thanks!";
    }];
}];

GCD級(jí)主線程同步：dispatch_get_main_queue

NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];
[waitQueue addOperationWithBlock:^{
    [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
    // 同步到主線程
    dispatch_async(dispatch_get_main_queue(), ^{
        self.alert.text = @"Thanks!";
    });
}];

參考：https://blog.csdn.net/cordova/article/details/54933729