Hunting Memory Leaks in Android: LeakCanary, Profiler, and Production Monitoring

// build.gradle.kts (app module)
dependencies {
    // debugImplementation: LeakCanary should only run in debug builds
    debugImplementation("com.squareup.leakcanary:leakcanary-android:2.14")
}

// Confirm it is running: filter Logcat on the LeakCanary tag.
// D LeakCanary: LeakCanary is running and ready to detect leaks

// Watching an object LeakCanary does not know about:
class ImageCache : Closeable {
    override fun close() {
        // After close(), nothing should hold this object
        AppWatcher.objectWatcher.watch(this, "ImageCache was closed")
    }
}

// A leak trace, from the LeakCanary documentation:
//
// ┬───
// │ GC Root: System class
// │
// ├─ android.provider.FontsContract class
// │    ↓ static FontsContract.sContext
// ├─ com.example.leakcanary.ExampleApplication instance
// │    Leaking: NO (Application is a singleton)
// │    ↓ ExampleApplication.leakedViews
// │                         ~~~~~~~~~~~
// ├─ java.util.ArrayList instance
// │    ↓ ArrayList.elementData
// │                ~~~~~~~~~~~
// ├─ java.lang.Object[] array
// │    ↓ Object[].[0]
// │               ~~~
// ├─ android.widget.TextView instance
// │    Leaking: YES (View.mContext references a destroyed activity)
// │    ↓ TextView.mContext
// ╰→ com.example.leakcanary.MainActivity instance
//
// The cause is the first underlined reference,
// ExampleApplication.leakedViews: an object that lives as long as
// the process, holding a view that belongs to one Activity.

// PATTERN 1: Fragment view fields kept after onDestroyView()
// BAD: the binding holds the whole view tree while the
// Fragment waits on the back stack
class ProfileFragment : Fragment() {
    private var binding: FragmentProfileBinding? = null

    // FIX: clear view fields when the view is destroyed
    override fun onDestroyView() {
        super.onDestroyView()
        binding = null
    }
}

// PATTERN 2: an Activity stored in an object that outlives it
// BAD: the ViewModel survives a configuration change,
// the Activity it points at does not
class MainViewModel : ViewModel() {
    lateinit var activity: Activity
}
// FIX: pass data down. Never store an Activity, a View or a
// Context from one in a ViewModel or a singleton.

// PATTERN 3: a listener that is never unregistered
// BAD: the singleton now holds the Fragment
object EventBus {
    private val listeners = mutableListOf<EventListener>()
    fun register(listener: EventListener) { listeners.add(listener) }
    fun unregister(listener: EventListener) { listeners.remove(listener) }
}

class MyFragment : Fragment(), EventListener {
    override fun onStart() {
        super.onStart()
        EventBus.register(this)
    }

    // FIX: unregister at the matching end of the lifecycle
    override fun onStop() {
        super.onStop()
        EventBus.unregister(this)
    }
}

// On launch: how many recent deaths were low memory kills?
fun lowMemoryKills(context: Context): Int {
    val activityManager =
        context.getSystemService(ActivityManager::class.java)

    // pid 0 and maxNum 0 mean "do not filter": all records
    val exits = activityManager.getHistoricalProcessExitReasons(
        context.packageName, 0, 0
    )

    return exits.count {
        it.reason == ApplicationExitInfo.REASON_LOW_MEMORY
    }
}