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Best practices of appropriate Context in Android

Published On: 10 January 2017.By .
  • Mobile

You’re testing your new app and everything is looking great when suddenly you start seeing some lag in your once buttery transitions. Is it your imagination or are images taking longer to load now too? As the day progresses you notice the app become slower and slower until right after you get into bed, the app crashes.

 

Sometimes the problem is Context.  What is Context.???

 

Context is probably the most used element in Android applications…it may also be the most misused.

Context objects are so common and get passed around so frequently, it can be easy to create a situation you didn’t intend.  Loading resources, launching a new Activity, obtaining a system service, getting internal file paths, and creating views all require a Context (and that’s not even getting started on the full list!) to accomplish the task.  What I’d like to do is provide for you some insights on how Context works alongside some tips that will (hopefully) allow you to leverage it more effectively in your applications.

 

Context Type

Depending on the Android application component, the Context you have access to varies slightly:

 

Application – is a singleton instance running in your application process.  It can be accessed via methods like getApplication() from an Activity or Service, and getApplicationContext() from any other object that inherits from Context.  Regardless of where or how it is accessed, you will always receive the same instance from within your process.

Activity/Service – inherit from ContextWrapper which implements the same API, but proxies all of its method calls to a hidden internal Context instance, also known as its base context.  Whenever the framework creates a new Activity or Service instance, it also creates a new ContextImpl instance to do all of the heavy liftings that either component will wrap.  Each Activity or Service, and their corresponding base context are unique per-instance.

BroadcastReceiver – is not a Context in and of itself, but the framework passes a Context to it in onReceive() each time a new broadcast event comes in.  This instance is a ReceiverRestrictedContext with two main functions disabled; calling registerReceiver() and bindService().  These two functions are not allowed from within an existing BroadcastReceiver.onReceive().  Each time a receiver processes a broadcast, the Contexthanded to it is a new instance.

ContentProvider – is also not a Context but is given one when created that can be accessed via getContext().  If the ContentProvider is running locally to the caller (i.e. same application process), then this will actually return the same Application singleton.  However, if the two are in separate processes, this will be a newly created instance representing the package the provider is running in.

Saved References

The first issue we need to address comes from saving a reference to a Context in an object or class that has a lifecycle that extends beyond that of the instance you saved.  For example, creating a custom singleton that requires a Context to load resources or access a ContentProvider, and saving a reference to the current Activity or Service in that singleton.

Bad Singleton

public class CustomManager {

  private static CustomManager sInstance;

  public static CustomManager getInstance(Context context) {

    if (sInstance == null) {
      sInstance = new CustomManager(context);
    }
    return sInstance;
  }

  private Context mContext;
  
  private CustomManager(Context context) {
    mContext = context;
  }
}

The problem here is we don’t know where that Context came from, and it is not safe to hold a reference to the object if it ends up being an Activity or a Service.  This is a problem because a singleton is managed by a single static reference inside the enclosing class.  This means that our object, and ALL the other objects referenced by it, will never be garbage collected.  If this Context were an Activity, we would effectively hold hostage in memory all the views and other potentially large objects associated with it; creating a leak.

To protect against this, we modify the singleton to always reference the application context:

Better Singleton

public class CustomManager {

  private static CustomManager sInstance;

  public static CustomManager getInstance(Context context) {

    if (sInstance == null) {
      //Always pass in the Application Context
      sInstance = new CustomManager(context.getApplicationContext());
    }

    return sInstance;
  }

  private Context mContext;

  private CustomManager(Context context) {
    mContext = context;
  }
}

Now it doesn’t matter where our Context came from, because the reference we are holding is safe.  The application context is itself a singleton, so we aren’t leaking anything by creating another static reference to it.  Another great example of places where this can crop up is saving references to a Context from inside a running background thread or a pending Handler.

So why can’t we always just reference the application context?  Take the middleman out of the equation, as it were, and never have to worry about creating leaks?  The answer, as I alluded to in the introduction, is because one Context is not equal to another.

Context Capabilities

The common actions you can safely take with a given Context object depends on where it came from originally.  Below is a table of the common places an application will receive a Context, and in each case what it is useful for:

Application Activity Service ContentProvider BroadcastReceiver
Show a Dialog NO YES NO NO NO
Start an Activity NO1 YES NO1 NO1 NO1
Layout Inflation NO2 YES NO2 NO2 NO2
Start a Service YES YES YES YES YES
Bind to a Service YES YES YES YES NO
Send a Broadcast YES YES YES YES YES
Register BroadcastReceiver YES YES YES YES NO3
Load Resource Values YES YES YES YES YES
  1. An application CAN start an Activity from here, but it requires that a new task is 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.

 

 

 

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