Question

每次我的应用程序从挂起模式唤醒时，我正在记录（写入文件）backgroundTimeRemaining值，就在我开始使用到期处理程序的UIApplication的后台任务之前，就像这样（在我的方法中发出网络请求））：

if([[UIApplication sharedApplication] applicationState] == UIApplicationStateBackground){

        [Logger logIntoFileNamed:@"log" withContent:[NSString stringWithFormat:@" state %ld, background time remaining %.2f",(long)[[UIApplication sharedApplication] applicationState],[[UIApplication sharedApplication] backgroundTimeRemaining ]] andPrettyTime:true];

        UIApplication*    app = [UIApplication sharedApplication];

        __block UIBackgroundTaskIdentifier task = [app beginBackgroundTaskWithExpirationHandler:^{

            [app endBackgroundTask:task];
            task = UIBackgroundTaskInvalid;

            [Logger logIntoFileNamed:@"log" withContent:@" expiration handler executed " andPrettyTime:true];
        }];
    }

根据文档，如果应用程序位于前台，则backgroundTimeRemaining的值可能很大：

当应用程序在前台运行时，此属性中的值仍然适当大。

但这不是一个例子。我的方法被执行，因为应用程序状态等于UIApplicationStateBackground。我在这里缺少什么？

Answer 1

是的，根据文件：

如果app在，则backgroundTimeRemaining的值可能很大前景。

在提供正确的applicationDidEnterBackground之前，backgroundTimeRemaining需要一些时间在州之间。这应该在~180左右开始。使用dispatch_async在异步线程中运行它，并过滤掉Foreground值（＆gt; 180）。

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    if([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)])
    {
        NSLog(@"Multitasking Supported");

        __block UIBackgroundTaskIdentifier background_task;
        background_task = [application beginBackgroundTaskWithExpirationHandler:^ {

            //Clean up code. Tell the system that we are done.
            [application endBackgroundTask: background_task];
            background_task = UIBackgroundTaskInvalid;
        }];

        //To make the code block asynchronous
        dispatch_async(dispatch_get_main_queue(), ^{

            //### background task starts
            NSLog(@"Running in the background\n");
            while(TRUE)
            {
                //#### Filter Foreground Values
                if ([[UIApplication sharedApplication] backgroundTimeRemaining] < 180) {
                    NSLog(@"Background time Remaining: %f",[[UIApplication sharedApplication] backgroundTimeRemaining]);
                    [NSThread sleepForTimeInterval:1]; //wait for 1 sec
                }

            }

            //#### background task ends
            //Clean up code. Tell the system that we are done.
            [application endBackgroundTask: background_task];
            background_task = UIBackgroundTaskInvalid;
        });
    }
    else
    {
        NSLog(@"Multitasking Not Supported");
    }

}

Answer 2

这可能是由于您记录backgroundTimeRemaining的方式。在获得后台任务后尝试输入：

UIApplication *app = [UIApplication sharedApplication];
long appState = (long)app.applicationState;

if (appState == UIApplicationStateBackground) {
   __block UIBackgroundTaskIdentifier task = [app beginBackgroundTaskWithExpirationHandler:^{

     [app endBackgroundTask:task];
     task = UIBackgroundTaskInvalid;
     [Logger logIntoFileNamed:@"log" withContent:@" expiration handler executed " andPrettyTime:true];
  }];

  NSTimeInterval backgroundTimeRemaining = app.backgroundTimeRemaining;
  [Logger logIntoFileNamed:@"log" withContent:[NSString stringWithFormat:@" state %ld, background time remaining %.2f", appState, backgroundTimeRemaining] andPrettyTime:true];
}

为什么backgroundTimeRemaining提供DBL_MAX，即使应用程序状态是UIApplicationStateBackground？

2 个答案: