Difference between Activity Context and Application Context

Question

This has me stumped, I was using this in Android 2.1-r8 SDK:

ProgressDialog.show(getApplicationContext(), ....);

and also in

Toast t = Toast.makeText(getApplicationContext(),....);

using getApplicationContext() crashes both ProgressDialog and Toast .... which lead me to this question:

What is the actual differences between a activity context and application context, despite sharing the wording 'Context'?

Answer 1

They are both instances of Context, but the application instance is tied to the lifecycle of the application, while the Activity instance is tied to the lifecycle of an Activity. Thus, they have access to different information about the application environment.

If you read the docs at getApplicationContext it notes that you should only use this if you need a context whose lifecycle is separate from the current context. This doesn't apply in either of your examples.

The Activity context presumably has some information about the current activity that is necessary to complete those calls. If you show the exact error message, might be able to point to what exactly it needs.

But in general, use the activity context unless you have a good reason not to.

Answer 2

I found this table super useful for deciding when to use different types of Contexts:

1. An application CAN start an Activity from here, but it requires that a new task be created. This may fit specific use cases, but can create non-standard back stack behaviors in your application and is generally not recommended or considered good practice.
2. This is legal, but inflation will be done with the default theme for the system on which you are running, not what’s defined in your application.
3. Allowed if the receiver is null, which is used for obtaining the current value of a sticky broadcast, on Android 4.2 and above.

Original article here.

Answer 3

This obviously is deficiency of the API design. In the first place, Activity Context and Application context are totally different objects, so the method parameters where context is used should use ApplicationContext or Activity directly, instead of using parent class Context. In the second place, the doc should specify which context to use or not explicitly.

Answer 4

The reason I think is that ProgressDialog is attached to the activity that props up the ProgressDialog as the dialog cannot remain after the activity gets destroyed so it needs to be passed this(ActivityContext) that also gets destroyed with the activity whereas the ApplicationContext remains even after the activity gets destroyed.

Answer 5

I think when everything need a screen to show ( button, dialog,layout...) we have to use context activity, and everything doesn't need a screen to show or process ( toast, service telelphone,contact...) we could use a application context

Answer 6

Use getApplicationContext() if you need something tied to a Context that itself will have global scope.

If you use Activity, then the new Activity instance will have a reference, which has an implicit reference to the old Activity, and the old Activity cannot be garbage collected.