当你从你的Android当前运行的应用程序列表中刷下一个应用程序时，应用程序和数据到底会发生什么变化？我们调查时请继续阅读。

今天的问答环节是由Android爱好者提供的，Android爱好者是Stack Exchange的一个分支，Stack Exchange是一个由社区驱动的问答网站分组。

问题

Android发烧友读者Eldarerathis对应用程序刷卡功能很好奇，它允许你通过home按钮调出正在运行的应用程序列表，然后向右刷卡，大概是为了关闭它们：

The recent apps list in Ice Cream Sandwich added the ability to swipe apps out of the list, thereby di**issing them permanently (and as far as I know this is a vanilla function, not a CM/custom ROM one). The documentation and platform highlights don’t appear to cover the under-the-hood workings of this functionality, but I’m curious to know what the system is actually doing.

Further adding to my curiosity, I decided to do a quick test: I started up Music on a CM9 install, then backed out of it. I then checked the recent apps list and saw it was indeed there (and in the proper state, based on the thumbnail). I then went into Settings->Applicati*** and force stopped the Music app, but it was still listed in the recent list, leading me to believe it’s not connected to processes lingering in the background.

Realizing now that Music may have been a poor choice, I also tested with the USA Today app. This exhibited basically the same behavior, and it seemed like it was forced to “relaunch” after the force stop (which makes sense) though the thumbnail in the recent apps list didn’t reflect this (cached, I’m guessing?).

So, what actually happens at the OS level when you swipe an app out of the recent list? Does it simply clear the app’s data out of RAM and garbage collect it, destroying its saved state?

当你从列表中刷下应用程序时到底发生了什么？

答案

Android爱好者贡献者Austin Mills提供了一些见解：

从最近的应用程序列表中刷出应用程序是普通的，是的，没有很好的文档记录。这一直是各种Android论坛上大量讨论的话题……这里的一些评论似乎最好地描述了这种共识：这种行为类似于关闭应用程序，但并不完全相同——一般来说（对于没有明确定义后退按钮处理的应用程序），这与回击次数足够多是一样的从应用程序中退出的次数。

这个链接有更多关于细节的细节，但是总体上你可以把它看作是退出应用程序。

我认为，对于音乐应用程序，它会启动服务，因此虽然任务本身（Music app/UI）可能关闭，但该服务仍在后台运行，这样您的音乐不会因为内存管理原因而突然停止。这可能影响到你所看到的。

随后，参加了问答圈的生活，埃尔达拉提斯带着自己的一些研究回来，总结出答案：

I appear to have found the magical search terms that led to some explanati*** from Google employees. Specifically, I found a couple of different places where Dianne Hackborn explains what happens when you swipe something out of the recent list. The first is a comment on one of her Google+ posts:

[W]hat specifically happens when you swipe away a recent task is it: (1) kills any background or empty processes of the application (see here for what this means), and (2) uses the new API to tell any services of the application about the task being removed so it can do whatever it thinks is appropriate.

She also notes in a blog comment:

Actually, removing an entry in recent tasks will kill any background processes that exist for the process. It won’t directly causes services to stop, however there is an API for them to find out the task was removed to decide if they want this to mean they should stop. This is so that removing say the recent task of an e-mail app won’t cause it to stop checking for e-mail.

If you really want to completely stop an app, you can long press on recent tasks to go to app info, and hit force stop there. Force stop is a complete kill of the app — all processes are killed, all services stopped, all notificati*** removed, all alarms removed, etc. The app is not allowed to launch again until explicitly requested.

So, it looks like the summary is that swiping an app out of the list will first kill all background processes for the app, then use onTaskRemoved to notify the app that the background task was removed. At that point it looks like it’s up to the app to decide what happens, so I guess there technically isn’t a hard-and-fast rule about what happens to the app beyond that point.

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