ios – 为什么backgroundTimeRemaining给出DBL_MAX,即使应用程序状态是UIApplicationStateBackground？

概述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){ 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app在前台,backgroundTimeRemaining的值可能很大：

While the app is running in the foreground,the value in this property
remains suitably large.

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.
我在这里错过了什么？

解决方法 是的,根据文件：

A value of backgroundTimeRemaining can be big if app is in the
foreground.

在提供正确的backgroundTimeRemaining之前,applicationDIDEnterBackground需要在状态之间进行一些时间.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(> 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");    }}

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