The question, How can I get the current Activity? has been asked dozens of times on Stackoverflow and other sites and there are many proposed approaches. However, all of them have drawbacks in one form or another.
In this posting, I am assuming that there is no solution provided for this in Android's APIs, e.g., something like: Application.getTask().getRootActivity().
Wouldn't it be nice if there was :-)?
So, to be clear, I'm not asking for an answer to How can I get the current Activity?
Instead, I am asking for the reason that such a capability has not been provided. Given that each running app has a task (assuming that the task hasn't been emptied) and each such task has a root Activity, it would seem to be easy to provide access to that root Activity.
The fact that that such access is not provided, when it is so clearly desired, implies to me that there is something fundamental about the Android architecture that I don't understand.
What is it that I'm missing? Why is this information not provided by the Android APIs?
For background, here is a section summarizing some of the approaches that have been proposed. I found the following two links particularly informative (each of the approaches below is presented at one or both of the links).
Links
- How to get current foreground activity context in android?
- Android: How can I get the current foreground activity (from a service)?
Approaches
- Static Hook
- Reflection
- ActivityManager
- Other (Instrumentation, AccessibilityService, UsageStatsManager)`
ActivityManager
The ActivityManager approach only provides the name of the Activity class, not the current Activity instance. E.g., for a Context instance c:
c.getSystemService().getActivityManager()
.getAppTasks().get(0).getTaskInfo()
.topActivity().getClassName()
Reflection
My favorite is reflection, as proposed by _AZ, but that approach is fragile, given that it relies on internals. What I would like to see from Android is this approach provided via a standard API that developers could then safely rely on.
Static Hook
The most common approach is using a static hook to save a reference to the currently running Activity. The hook can be either per-Activity or per-Application. Memory leaks can be avoided by saving/destroying the hook's value (e.g., in onCreate()/onDestroy(), onStart()/onStop(), onPause()/onResume()). However, issues can arise when multiple Activities are involved (e.g., due to overlapping lifecycles -- see below).
I implemented a static hook approach which does the following (to be perfectly transparent, I haven't implemented #1 yet -- I am currently using a per-Activity static hook, which is a bug).
- Provides a class that extends Application to provide the hook. The hook contains a Stack; each node in the stack is a simple ActivityInfo class which contains a reference to an Activity instance as well as the state of that instance (CREATED, STARTED, RESUMED).
- Provides a class called ActivityTracker that extends Activity. I then extend each of my Activities with ActivityTracker. ActivityTracker uses its lifecycle callbacks to push/pop itself to/from the stack and to update its state -- my other Activities don't have to do anything.
In theory, this would allow me to always know the full state of the task's back stack -- the full set of Activities, including the root Activity, as well as their current state. In practice, however, there is a twist -- when one Activity starts another Activity, their lifecycles overlap. During that period, peeking at the stop of the stack can yield an unexpected Activity instance.
From: https://developer.android.com/guide/components/activities/activity-lifecycle.html#soafa, "Coordinating activities":
Here's the order of operations that occur when Activity A starts Acivity B:
- Activity A's onPause() method executes.
- Activity B's onCreate(), onStart(), and onResume() methods execute in sequence. (Activity B now has user focus.)
- Then, if Activity A is no longer visible on screen, its onStop() method executes
Of course, this could be managed also. The bottom line is that we do have a global context available for storing information (the Application) and we do have full information about Activity lifecycle transitions, so with enough effort I believe that this static stack-based approach could probably be made pretty bullet-proof.
But in the End
But in the end it feels like I am simply rewriting code which probably already exists internally for managing an Activity back stack, which is why I ask (in case you've forgotten):
Why is there no Android API for getting the current Activity?
UPDATE
In this update, I'll summarize what I've learned from this thread and my own experiments and research. Hopefully, this summary will be useful to others.
Definitions
I'm going to use the following definitions for "Activity Visibility States", based on the Activity State definitions at https://developer.android.com/guide/components/activities/activity-lifecycle.html.
-----------------------------------
Visibility State Definition
-----------------------------------
Not Visible Created+Stopped
Partially Visible Started+Paused
Fully Visible Resumed
-----------------------------------
Issues
The very definition of "Current Activity" is murky. When I use it, I mean the single Activity in the Fully Visible state. At any given instant, there may or may not be such an Activity. In particular, when Activity A starts Activity B, A's onPause() gets called and then B's onCreate(), onStart() and onResume(), followed by A's onStop(). There is a stretch between A's onPause() and B's onResume() where neither is in the Fully Visible state, so there is no Current Activity (as I define it). Of course, there are also situations where a background thread may want to access a Current Activity and there may or may not be an Activity at all, much less a Current Activity.
I've also realized that I may not always need a Current ("Fully Visible") Activity. In many cases, I may simply need a reference to an existing Activity, whether or not it is currently visible. In addition, that reference might be to just any Activity (for situations where I need to pass a generic Activity reference to some API method) or it might be to a specific Activity subclass instance (so that I can trigger some code specific to that Activity subclass).
Finally, there is the need to understand when Activity lifecycle callbacks are called by the main UI looper and how events like configuration changes are handled. For example, if I create a DialogFragment using an Activity intance which is currently in the "Not Visible" state, will it ever get displayed and, if so, when? Along similar lines, it turns out that the onDestroy() and onCreate() methods caused by a configuration change are contained in the same message in the UI's message queue (see Android UI Thread Message Queue dispatch order), so no other messages will be processed between those two callbacks (during a configuration change). Understanding this level of processing seems to be critical, but documentation on it is sorely lacking, if not missing completely.
Approaches
Here is a collection of approaches that can be used to address most of the above situations.
Background
- For discussion, assume Activity A and Activity B, where A creates B.
- Generally speaking, a "global" variable can be created by making it "public static" on pretty much any class. Conceptually, extending the Application class and adding it to the extended class would be good, but if that's too much work it could be included (for instance) in one of the Activity classes.
Generic Activity Reference
- Useful whenever a generic Activity is needed.
- Create a global variable. In both A and B, have onCreate() set it to "this" and onDestroy() set it to null.
Topmost Activity Reference
- Useful whenever you want to access the currently visible Activity.
- Create a global variable. In both A and B, have onResume() set it to "this". This approach works fine unless all Activities exit, in which case you may need to create a separate flag to indicate that situation. (That flag could be the Generic Activity Reference implementation mentioned above.)
Specific Activity Reference
- Useful whenever a handle to a specific Activity subclass instance is needed.
- In both A and B: create a global variable in the Activity subclass itself. Have onCreate() set it to "this and onDestroy() set it to null.
Application Context
- Useful whenever a Context spanning the lifecycle of the entire app is needed or when you don't care about using a specific Activity Context (e.g., to create a Toast from a background thread).
- You can get this from Activity's getApplication() and store it on a static hook.
Handling Configuration Changes
There may be times when you want to stop/start a background thread only across an Activity "session", where I define "session" to include the series of Activity instances which may be created and destroyed due to configuration changes. In my particular case, I have a Bluetooth Chat Activity and an associated background thread to handle the network connection. I don't want to have the connection destroyed and created each time the user rotates the device, so I need to create it only when one doesn't exist and destroy it only if a configuration change isn't underway. The key here is understand when onDestroy() is called due to a configuration change. This can be done with or without fragments. As is often the case, I prefer the non-fragment approach since the fragment approach doesn't seem worth the extra complexity to me.
Approach 1: Without Fragments
In onCreate(), create the background thread if it doesn't exist yet. In onDestroy(), destroy the background thread only if isFinally() returns false.
Approach 2: With Fragments
This works well because the FragmentManager will store fragment instances across configuration changes if setRetainInstance(true) is used. For an excellent example of this, see http://www.androiddesignpatterns.com/2013/04/retaining-objects-across-config-changes.html. The example is for AsyncTasks, but can also be applied to managing a background thread (just create the thread instead of an AsyncTask in the fragment's onCreate() and then destroy the thread in the fragment's onDestroy()).
Closing
Fully understanding these issues requires a deep understanding of how the UI looper processes its message queue -- when Activity callbacks are called, how other messages are interleaved with them, when display updates occur, etc. For instance, if a DialogFragment is created using an instance of a non-visible Activity, will it get displayed at all and, if so, when? Perhaps some day Android will provide a deeper API to Tasks and their associated backstacks, along with documentation describing the UI's message processing and associated mechanisms in more detail. Until then, more "source code and/or ... empirical analysis" :-).
Thanks,
Barry
