ios - UIWindow makeKeyAndVisible 和后台任务

Question

我已将后台任务功能添加到我的应用程序中。这是我的应用委托(delegate)

// AppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
    [self setWindow:[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]]];
    [self.window setBackgroundColor:[UIColor blackColor]];

    if (application.applicationState != UIApplicationStateBackground) {        
        // Application is launch in because user tap the app icon or from springboard

        if ([application respondsToSelector:@selector(setMinimumBackgroundFetchInterval:)]) {
            [application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
        }
    } else {
        // Application is launch in background mode
    }

    RootViewController *rootViewController = [[RootViewController alloc] initByDevice];
    [self.window setRootViewController:rootViewController];
    [self.window makeKeyAndVisible];

    return YES;
}

- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {

    [[DataManager instance] updateDataWithMaxAttempt:5 block:^(BOOL success, NSArray *newData) {
        if (success) {
            if ([newData count] > 0) {
                completionHandler(UIBackgroundFetchResultNewData);
            } else {
                completionHandler(UIBackgroundFetchResultNoData);
            }
        } else {
            completionHandler(UIBackgroundFetchResultNoData);
        }
    }];
}

这是我的 Root View Controller

// RootViewController.m

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];

    NSLog(@"Did appear");
    // Do something that I want it to happen only when the application is visible to user
}

当用户点击应用程序图标时，应用程序会像我预期的那样工作。我在控制台中看到“确实出现了”，之后发生了一些事情，就像我预期的那样。

但是当应用程序唤醒执行后台任务(用户不可见)时，RootViewController 的 viewDidAppear 仍然会因为这一行而被调用

[self.window makeKeyAndVisible];

通过调用 makeKeyAndVisible，它使 RootViewController 可见，即使它只是为了后台任务而唤醒。要解决此问题，我必须将此行移动到 applicationDidBecomeActive

- (void)applicationDidBecomeActive:(UIApplication *)application {
    [self.window makeKeyAndVisible];
}

因此，RootViewController 的 viewDidAppear 现在仅在应用程序处于前台(对用户可见)时才被调用。但是，我担心的是，当应用程序在设备内存中(事件或非事件)时，applicationDidBecomeActive 将被调用多次。

• 用户启动应用
• 应用程序从 springboard 再次激活
• 设备解锁
• 结束通话

如果我在应用程序生命周期中多次调用 UIWindow's makeKeyAndVisible 会有什么问题吗？

Answer 1

您可以轻松确保 makeKeyAndVisible 只发生一次:包装它。

if (!self.window.keyWindow) {
    [self.window makeKeyAndVisible;
} else if (self.window.hidden) {
    self.window.hidden = NO;
}