当调用活动上下文或应用程序上下文？ [英] When to call activity context OR application context?

问题描述

目前已经有很多张贴关于什么这两个背景是。但我仍然没有得到它完全正确

There has been a lot of posting about what these two contexts are.. But I'm still not getting it quite right

据我所知，到目前为止： 每一个都是它的类的实例，这意味着一些程序员建议你使用 this.getApplicationContext（）尽可能经常为了不漏出的内存。这是因为其他这（获得活动实例上下文）指向一个活动正被每个使用者倾斜手机或离开应用程序等等。这显然垃圾收集器（GC）没有赶上，所以时间销毁使用了太多的内存。

As I understand it so far: Each is an instance of its class which means that some programmers recommend you to use this.getApplicationContext() as often as possible in order to not "leak" out any memory. This is because the other this (getting the Activity instance context) points to an Activity that is being destroyed each time the user tilts the phone or leave the app etc.. Which apparently the Garbage Collector (GC) doesn't catch and therefore uses too much memory..

不过，任何人都可以，请拿出一些真正好的编码例子，这将是正确的选择使用这（获得的电流活动）实例和应用程序上下文将是无用/错了吗？

But can anyone please come up with some really good coding examples where it would be the right thing to use this (getting the context of the current Activity instance) and the application context will be useless/wrong?

推荐答案

getApplicationContext（）几乎总是错的。 <一href="http://stackoverflow.com/questions/5228160/what-exactly-does-using-the-application-context-mean/5228494#5228494">Ms. Hackborn （等等）已经非常明确，你的只有的使用 getApplicationContext（）时，你知道的为什么你使用 getApplicationContext（）只有当你的需要的使用 getApplicationContext（）

getApplicationContext() is almost always wrong. Ms. Hackborn (among others) have been very explicit that you only use getApplicationContext() when you know why you are using getApplicationContext() and only when you need to use getApplicationContext().

要坦率地说，有些程序员使用 getApplicationContext（）（或 getBaseContext（），在较小程度），因为他们的Java经验是有限的。他们实现一个内部类（例如，一个 OnClickListener 为按钮在活动），需要一个上下文。而不是使用 MyActivity.this 以获得在外部类'这，他们用 getApplicationContext （）或 getBaseContext（）获得上下文对象。

To be blunt, "some programmers" use getApplicationContext() (or getBaseContext(), to a lesser extent) because their Java experience is limited. They implement an inner class (e.g., an OnClickListener for a Button in an Activity) and need a Context. Rather than using MyActivity.this to get at the outer class' this, they use getApplicationContext() or getBaseContext() to get a Context object.

您的只有的使用 getApplicationContext（）当你的知道的你需要一个上下文的东西可能更长寿比其他任何可能的上下文在您的处置你。方案包括：

You only use getApplicationContext() when you know you need a Context for something that may live longer than any other likely Context you have at your disposal. Scenarios include:

• 使用 getApplicationContext（）如果你需要的东西绑在上下文这本身也会有全局作用域。我用 getApplicationContext（），例如， WakefulIntentService ，对静态 WakeLock 来用于服务。由于该 WakeLock 是静态的，而我需要一个上下文来获得在电源管理器以创建它，它是最安全的方法是使用 getApplicationContext（）。

• Use getApplicationContext() if you need something tied to a Context that itself will have global scope. I use getApplicationContext(), for example, in WakefulIntentService, for the static WakeLock to be used for the service. Since that WakeLock is static, and I need a Context to get at PowerManager to create it, it is safest to use getApplicationContext().

使用 getApplicationContext（）当你绑定到服务从活动，如果你想通过 ServiceConnection （即句柄绑定）活动的通过 onRetainNonConfigurationInstance实例（）。机器人内部通过这些 ServiceConnections 跟踪绑定，并持有引用到上下文创造的绑定。如果从活动结合，那么新的活动实例都会有一个参考 ServiceConnection 里面有一个隐含的引用老活动，和老活动不能垃圾收集。

Use getApplicationContext() when you bind to a Service from an Activity, if you wish to pass the ServiceConnection (i.e., the handle to the binding) between Activity instances via onRetainNonConfigurationInstance(). Android internally tracks bindings via these ServiceConnections and holds references to the Contexts that create the bindings. If you bind from the Activity, then the new Activity instance will have a reference to the ServiceConnection which has an implicit reference to the old Activity, and the old Activity cannot be garbage collected.

有些开发人员使用的应用程序自定义子类为自己的全球数据，他们通过检索getApplicationContext（）。这当然是可能的。我preFER静态数据成员，如果没有别的原因，你只能拥有的一个的自定义应用程序对象。我建立使用自定义应用程序对象，一个应用程序，并发现它是痛苦的。 <一href="http://stackoverflow.com/questions/3826905/singletons-vs-application-context-in-android/3827166#3827166">Ms. Hackborn也同意这一立场。

Some developers use custom subclasses of Application for their own global data, which they retrieve via getApplicationContext(). That's certainly possible. I prefer static data members, if for no other reason than you can only have one custom Application object. I built one app using a custom Application object and found it to be painful. Ms. Hackborn also agrees with this position.

下面有原因的没有的使用 getApplicationContext（）无论你走到哪里：

Here are reasons why not to use getApplicationContext() wherever you go:

• 这不是一个完整的上下文，支持一切活动一样。各种各样的事情，你会尝试做这个上下文将失败，<一个href="http://groups.google.com/group/android-developers/browse_thread/thread/7a648edddccf6f7d">mostly有关GUI 。

• It's not a complete Context, supporting everything that Activity does. Various things you will try to do with this Context will fail, mostly related to the GUI.

它可以创建内存泄漏，如果上下文从 getApplicationContext（）保存到所创造的东西就可以了你的电话，你不清理。随着活动，如果持有到的东西，一旦活动被垃圾回收，一切冲出来了。该应用程序的对象仍然是流程的生命周期。

It can create memory leaks, if the Context from getApplicationContext() holds onto something created by your calls on it that you don't clean up. With an Activity, if it holds onto something, once the Activity gets garbage collected, everything else flushes out too. The Application object remains for the lifetime of your process.

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