# ActivityLifecycleMonitor

`public interface ActivityLifecycleMonitor`
``

android.support.test.runner.lifecycle.ActivityLifecycleMonitor

Interface for tests to use when they need to query the activity lifecycle state.

Activity lifecycle changes occur only on the UI thread - therefore listeners registered with an ActivityLifecycleMonitor should expect to be invoked on the UI thread. The direct query methods can only be called on the UI thread because otherwise they would not be able to return consistent responses.

Retrieve instances of the monitor through `ActivityLifecycleMonitorRegistry`.

Detecting these lifecycle states requires support from Instrumentation, therefore do not expect an instance to be present under any arbitrary instrumentation.

## Summary

### Public methods
`abstract void` | `addLifecycleCallback(ActivityLifecycleCallback callback)` Adds a new callback that will be notified when lifecycle changes occur.
`abstract Collection<Activity>` | `getActivitiesInStage(Stage stage)` Returns all activities in a given stage of their lifecycle.
`abstract Stage` | `getLifecycleStageOf(Activity activity)` Returns the current lifecycle stage of a given activity.
`abstract void` | `removeLifecycleCallback(ActivityLifecycleCallback callback)` Removes a previously registered lifecycle callback.

## Public methods

### addLifecycleCallback

    void addLifecycleCallback (ActivityLifecycleCallback callback)

Adds a new callback that will be notified when lifecycle changes occur.

Implementors will not hold a strong ref to the callback, the code which registers callbacks is responsible for this. Code which registers callbacks should responsibly remove their callback when it is no longer needed.

Callbacks are executed on the main thread of the application, and should take care not to block or otherwise perform expensive operations as it will directly impact the application.

Parameters
`callback` | `ActivityLifecycleCallback`: an ActivityLifecycleCallback

### getActivitiesInStage

    Collection<Activity> getActivitiesInStage (Stage stage)

Returns all activities in a given stage of their lifecycle.

This method can only return a consistant and correct answer from the main thread, therefore callers should always invoke it from the main thread and implementors are free to throw an exception if the call is not made on the main thread.

Implementors should ensure this method returns a consistant response if called from a lifecycle callback also registered with this monitor (eg: it would be horriblely wrong if a callback sees PAUSED and calls this method with the PAUSED and does not see its activity in the response.

Callers should be aware that the monitor implementation may not hold strong references to the Activities in the application. Therefore stages which are considered end stages or eligible for garbage collection on low memory situations may not return an instance of a particular activity if it has been garbage collected.

Parameters
`stage` | `Stage`: the stage to query for.

Returns
`Collection<Activity>` | a snapshot Collection of activities in the given stage. This collection may be empty.

Throws
`IllegalStateException` | if called from outside the main thread.

### getLifecycleStageOf

    Stage getLifecycleStageOf (Activity activity)

Returns the current lifecycle stage of a given activity.

This method can only return a consistant and correct answer from the main thread, therefore callers should always invoke it from the main thread and implementors are free to throw an exception if the call is not made on the main thread.

Implementors should ensure this method returns a consistant response if called from a lifecycle callback also registered with this monitor (eg: it would be horriblely wrong if a callback sees PAUSED and calls this method with the same activity and gets RESUMED.

Parameters
`activity` | `Activity`: an activity in this application.

Returns
`Stage` | the lifecycle stage this activity is in.

Throws
`IllegalArgumentException` | if activity is unknown to the monitor.
`NullPointerException` | if activity is null.
`IllegalStateException` | if called off the main thread.

### removeLifecycleCallback

    void removeLifecycleCallback (ActivityLifecycleCallback callback)

Removes a previously registered lifecycle callback.

Parameters
`callback` | `ActivityLifecycleCallback`

- ## Interfaces

  - ActivityLifecycleCallback
  - ActivityLifecycleMonitor
  - ApplicationLifecycleCallback
  - ApplicationLifecycleMonitor

- ## Classes

  - ActivityLifecycleMonitorRegistry
  - ApplicationLifecycleMonitorRegistry

- ## Enums

  - ApplicationStage
  - Stage