dependencies { 
    // debugImplementation because LeakCanary should only run in debug builds. 
    debugImplementation 'com.squareup.leakcanary:leakcanary-android:2.9.1' 
}

internal class MainProcessAppWatcherInstaller : ContentProvider() {
  override fun onCreate(): Boolean {
    val application = context!!.applicationContext as Application
    AppWatcher.manualInstall(application)
    return true
  }
  ...
 }

@UnsupportedAppUsage
private void handleBindApplication(AppBindData data) {
    ...
    // 这边创建application，并会调用到application的attach()方法，最终调用到attachBaseContext()方法
    app = data.info.makeApplication(data.restrictedBackupMode, null);
    ...
    // don't bring up providers in restricted mode; they may depend on the
    // app's custom Application class
    if (!data.restrictedBackupMode) {
        if (!ArrayUtils.isEmpty(data.providers)) {
            //初始化Provider并且调用了Provider的onCreate()方法
            installContentProviders(app, data.providers);
        }
    }
    ...
    try {
        //调用了Application的onCreate()方法
        mInstrumentation.callApplicationOnCreate(app);
    } catch (Exception e) {
        if (!mInstrumentation.onException(app, e)) {
            throw new RuntimeException(
              "Unable to create application " + app.getClass().getName()
              + ": " + e.toString(), e);
        }
    }
    ...
}

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <bool name="leak_canary_watcher_auto_install">false</bool>
</resources>

internal class MainProcessAppWatcherInstaller : ContentProvider() {
  override fun onCreate(): Boolean {
    val application = context!!.applicationContext as Application
    //这边调用初始化的方法
    AppWatcher.manualInstall(application)
    return true
  }
  ...
 }

@JvmOverloads
fun manualInstall(
  application: Application,
  retainedDelayMillis: Long = TimeUnit.SECONDS.toMillis(5),
  watchersToInstall: List<InstallableWatcher> = appDefaultWatchers(application)
) {
  //先判断是否是在主线程，如果不是在主线程则抛出异常
  checkMainThread()
  //判断是否已经初始化了，如果已经初始化了则抛出异常，避免重复初始化
  if (isInstalled) {
    throw IllegalStateException(
      "AppWatcher already installed, see exception cause for prior install call", installCause
    )
  }
  //判断设置的延迟时间是否小于0,这个时间是用来延迟检测被观察的对象是否已被释放，
  //也就是说如果到了这个时间，对象仍没有被释放，那么可能出现了内存泄漏
  check(retainedDelayMillis >= 0) {
    "retainedDelayMillis $retainedDelayMillis must be at least 0 ms"
  }
  this.retainedDelayMillis = retainedDelayMillis
  if (application.isDebuggableBuild) {
    LogcatSharkLog.install()
  }
  //初始化 InternalLeakCanary，并调用了InternalLeakCanary.invoke()方法，
  //这个类是用来检查判断对象是否被回收
  LeakCanaryDelegate.loadLeakCanary(application)
  //开启监听器，也就是appDefaultWatchers(application)方法里面的
  watchersToInstall.forEach {
    it.install()
  }
  // Only install after we're fully done with init.
  installCause = RuntimeException("manualInstall() first called here")
}

fun appDefaultWatchers(
  application: Application,
  reachabilityWatcher: ReachabilityWatcher = objectWatcher
): List<InstallableWatcher> {
  return listOf(
    ActivityWatcher(application, reachabilityWatcher),
    FragmentAndViewModelWatcher(application, reachabilityWatcher),
    RootViewWatcher(reachabilityWatcher),
    ServiceWatcher(reachabilityWatcher)
  )
}

object AppWatcher {
 ...
/**
 * The [ObjectWatcher] used by AppWatcher to detect retained objects.
 * Only set when [isInstalled] is true.
 */
val objectWatcher = ObjectWatcher(
  clock = { SystemClock.uptimeMillis() },
  checkRetainedExecutor = {
    check(isInstalled) {
      "AppWatcher not installed"
    }
    mainHandler.postDelayed(it, retainedDelayMillis)
  },
  isEnabled = { true }
 )
 ...
}

class ActivityWatcher(
  private val application: Application,
  private val reachabilityWatcher: ReachabilityWatcher
) : InstallableWatcher {
  //回调，这个是向application.registerActivityLifecycleCallbacks注册activity生命周期回调
  //这边通过监听Activity的onDestroyed来观察它的回收状态
  private val lifecycleCallbacks =
    object : Application.ActivityLifecycleCallbacks by noOpDelegate() {
      override fun onActivityDestroyed(activity: Activity) {
        //一旦activity进入到destory，则开始通过onActivityDestroyed观察它的回收状态
        reachabilityWatcher.expectWeaklyReachable(
          activity, "${activity::class.java.name} received Activity#onDestroy() callback"
        )
      }
    }
  override fun install() {
    application.registerActivityLifecycleCallbacks(lifecycleCallbacks)
  }
  override fun uninstall() {
    application.unregisterActivityLifecycleCallbacks(lifecycleCallbacks)
  }
}

private val fragmentDestroyWatchers: List<(Activity) -> Unit> = run {
  val fragmentDestroyWatchers = mutableListOf<(Activity) -> Unit>()
  //添加android.app.Fragment监听，创建AndroidOFragmentDestroyWatcher，并加入到集合中
  if (SDK_INT >= O) {
    fragmentDestroyWatchers.add(
      AndroidOFragmentDestroyWatcher(reachabilityWatcher)
    )
  }
  //添加 androidx.fragment.app.Fragment监听，
  //通过反射的方式创建AndroidXFragmentDestroyWatcher，并加入到集合中
  getWatcherIfAvailable(
    ANDROIDX_FRAGMENT_CLASS_NAME,
    ANDROIDX_FRAGMENT_DESTROY_WATCHER_CLASS_NAME,
    reachabilityWatcher
  )?.let {
    fragmentDestroyWatchers.add(it)
  }
  //添加android.support.v4.app.Fragment监听
  //通过反射的方式 创建 AndroidSupportFragmentDestroyWatcher，并加入到集合中
  getWatcherIfAvailable(
    ANDROID_SUPPORT_FRAGMENT_CLASS_NAME,
    ANDROID_SUPPORT_FRAGMENT_DESTROY_WATCHER_CLASS_NAME,
    reachabilityWatcher
  )?.let {
    fragmentDestroyWatchers.add(it)
  } 
  fragmentDestroyWatchers
}

private val lifecycleCallbacks =
  object : Application.ActivityLifecycleCallbacks by noOpDelegate() {
    override fun onActivityCreated(
      activity: Activity,
      savedInstanceState: Bundle?
    ) {
      for (watcher in fragmentDestroyWatchers) {
        watcher(activity)
      }
    }
  }

internal class AndroidXFragmentDestroyWatcher(
  private val reachabilityWatcher: ReachabilityWatcher
) : (Activity) -> Unit {
  private val fragmentLifecycleCallbacks = object : FragmentManager.FragmentLifecycleCallbacks() {
    override fun onFragmentCreated(
      fm: FragmentManager,
      fragment: Fragment,
      savedInstanceState: Bundle?
    ) {
      ViewModelClearedWatcher.install(fragment, reachabilityWatcher)
    }
    override fun onFragmentViewDestroyed(
      fm: FragmentManager,
      fragment: Fragment
    ) {
      val view = fragment.view
      if (view != null) {
        reachabilityWatcher.expectWeaklyReachable(
          view, "${fragment::class.java.name} received Fragment#onDestroyView() callback " +
          "(references to its views should be cleared to prevent leaks)"
        )
      }
    }
    override fun onFragmentDestroyed(
      fm: FragmentManager,
      fragment: Fragment
    ) {
      reachabilityWatcher.expectWeaklyReachable(
        fragment, "${fragment::class.java.name} received Fragment#onDestroy() callback"
      )
    }
  }
  override fun invoke(activity: Activity) {
    if (activity is FragmentActivity) {
      val supportFragmentManager = activity.supportFragmentManager
      supportFragmentManager.registerFragmentLifecycleCallbacks(fragmentLifecycleCallbacks, true)
      ViewModelClearedWatcher.install(activity, reachabilityWatcher)
    }
  }
}

private val listener = OnRootViewAddedListener { rootView ->
  val trackDetached = when(rootView.windowType) {
    PHONE_WINDOW -> {
      when (rootView.phoneWindow?.callback?.wrappedCallback) {
        // Activities are already tracked by ActivityWatcher
        // activity不需要重复注册
        is Activity -> false
        //监听dialog，这边还需要判断下资源文件里面的配置信息是否是打开的
        is Dialog -> {
          // Use app context resources to avoid NotFoundException
          // https://github.com/square/leakcanary/issues/2137
          val resources = rootView.context.applicationContext.resources
          resources.getBoolean(R.bool.leak_canary_watcher_watch_dismissed_dialogs)
        }
        // Probably a DreamService
        // 其他情况都打开
        else -> true
      }
    }
    // Android widgets keep detached popup window instances around.
    POPUP_WINDOW -> false
    TOOLTIP, TOAST, UNKNOWN -> true
  }
  if (trackDetached) {
    rootView.addOnAttachStateChangeListener(object : OnAttachStateChangeListener {
      val watchDetachedView = Runnable {
        reachabilityWatcher.expectWeaklyReachable(
          rootView, "${rootView::class.java.name} received View#onDetachedFromWindow() callback"
        )
      }
      override fun onViewAttachedToWindow(v: View) {
        //当显示时移除内存监听器
        mainHandler.removeCallbacks(watchDetachedView)
      }
      override fun onViewDetachedFromWindow(v: View) {
        //当view从界面上移除时时，就执行监听，通过Handler.post()
        mainHandler.post(watchDetachedView)
      }
    })
  }
}
override fun install() {
  //使用三方库的Curtains来对view进行状态的监听
  Curtains.onRootViewsChangedListeners += listener
}

//存放即将stop的service
private val servicesToBeDestroyed = WeakHashMap<IBinder, WeakReference<Service>>()
private val activityThreadClass by lazy { Class.forName("android.app.ActivityThread") }
//通过反射的方式调用ActivityThread里面的currentActivityThread方法，也就是取到ActivityThread对象
private val activityThreadInstance by lazy {
  activityThreadClass.getDeclaredMethod("currentActivityThread").invoke(null)!!
}
//通过反射方式从ActivityThread取出mServices
private val activityThreadServices by lazy {
  val mServicesField =
    activityThreadClass.getDeclaredField("mServices").apply { isAccessible = true }
  @Suppress("UNCHECKED_CAST")
  mServicesField[activityThreadInstance] as Map<IBinder, Service>
}
override fun install() {
  //做一些检查
  checkMainThread()
  check(uninstallActivityThreadHandlerCallback == null) {
    "ServiceWatcher already installed"
  }
  check(uninstallActivityManager == null) {
    "ServiceWatcher already installed"
  }
  try {
    //hook ActivityThread里面的mH，也就是Handler，并替换里面的mCallBack
    swapActivityThreadHandlerCallback { mCallback ->
      //还原hook的mCallback，uninstall()时使用
      uninstallActivityThreadHandlerCallback = {
        swapActivityThreadHandlerCallback {
          mCallback
        }
      }
      //返回需要替换的mCallback，在这里面监听handler消息，
      Handler.Callback { msg ->
        // https://github.com/square/leakcanary/issues/2114
        // On some Motorola devices (Moto E5 and G6), the msg.obj returns an ActivityClientRecord
        // instead of an IBinder. This crashes on a ClassCastException. Adding a type check
        // here to prevent the crash.
        if (msg.obj !is IBinder) {
          return@Callback false
        }
        //如果收到了Service的Stop的消息，表示service要结束了，
        //此时调用onServicePreDestroy，将当前service加入到servicesToBeDestroyed集合里面去
        if (msg.what == STOP_SERVICE) {
          val key = msg.obj as IBinder
          activityThreadServices[key]?.let {
            onServicePreDestroy(key, it)
          }
        }
        //继续处理消息
        mCallback?.handleMessage(msg) ?: false
      }
    }
    //这边hook ActivityManagerService
    swapActivityManager { activityManagerInterface, activityManagerInstance ->
      uninstallActivityManager = {
        swapActivityManager { _, _ ->
          activityManagerInstance
        }
      }
      //使用动态代理的方式
      Proxy.newProxyInstance(
        activityManagerInterface.classLoader, arrayOf(activityManagerInterface)
      ) { _, method, args ->
        //如果执行serviceDoneExecuting方法时，则调用onServiceDestroyed()
        //方法来观察service的内存回收情况
        if (METHOD_SERVICE_DONE_EXECUTING == method.name) {
          val token = args!![0] as IBinder
          if (servicesToBeDestroyed.containsKey(token)) {
            onServiceDestroyed(token)
          }
        }
        try {
          if (args == null) {
            method.invoke(activityManagerInstance)
          } else {
            method.invoke(activityManagerInstance, *args)
          }
        } catch (invocationException: InvocationTargetException) {
          throw invocationException.targetException
        }
      }
    }
  } catch (ignored: Throwable) {
    SharkLog.d(ignored) { "Could not watch destroyed services" }
  }
}

ObjectWatcher:
@Synchronized override fun expectWeaklyReachable(
  watchedObject: Any,
  description: String
) {
  //这边在前面初始化的时候默认传进来是true，因此会继续往下执行下面的代码
  if (!isEnabled()) {
    return
  }
  //这边会先先处理下，将被回收的对象从watchedObjects这个待观察的集合里面移除
  removeWeaklyReachableObjects()
  //创建一个随机的UUID，用来当做待观察对象的key，方便从watchedObjects这个map取值
  val key = UUID.randomUUID()
    .toString()
  // 获取当前的时间，自系统开机到现在的一个时间
  val watchUptimeMillis = clock.uptimeMillis()
  //创建弱引用对象，也就是WeakReference
  val reference =
    KeyedWeakReference(watchedObject, key, description, watchUptimeMillis, queue)
  SharkLog.d {
    "Watching " +
      (if (watchedObject is Class<*>) watchedObject.toString() else "instance of ${watchedObject.javaClass.name}") +
      (if (description.isNotEmpty()) " ($description)" else "") +
      " with key $key"
  }
  //把对象放入到待观察集合里面去
  watchedObjects[key] = reference
  //执行延时操作，默认延时5秒，去检查对象是否回收
  checkRetainedExecutor.execute {
    moveToRetained(key)
  }
}

private fun removeWeaklyReachableObjects() {
  // WeakReferences are enqueued as soon as the object to which they point to becomes weakly
  // reachable. This is before finalization or garbage collection has actually happened.
  var ref: KeyedWeakReference?
  //通过迭代遍历的方式，判断queue里面时候有被回收的对象
  do {
    ref = queue.poll() as KeyedWeakReference?
    if (ref != null) {
      //如果对象被回收了 ，则讲弱引用从待观察的map里面移除
      watchedObjects.remove(ref.key)
    }
  } while (ref != null)
}

@Synchronized private fun moveToRetained(key: String) {
  removeWeaklyReachableObjects()
  val retainedRef = watchedObjects[key]
  if (retainedRef != null) {
    retainedRef.retainedUptimeMillis = clock.uptimeMillis()
    onObjectRetainedListeners.forEach { it.onObjectRetained() }
  }
}

AppWatcher
  -> manualIstall()
    -> LeakCanaryDelegate.loadLeakCanary(application)
      -> InternalLeakCanary
       -> invoke()

internal object InternalLeakCanary : (Application) -> Unit, OnObjectRetainedListener {
    ...
    override fun invoke(application: Application) {
      _application = application
      checkRunningInDebuggableBuild()
      //添加监听，因为InternalLeakCanary实现了OnObjectRetainedListener接口，因此直接传this
      AppWatcher.objectWatcher.addOnObjectRetainedListener(this)
      ...
    }
    ...
}

internal object InternalLeakCanary : (Application) -> Unit, OnObjectRetainedListener {
    ...
    //回调到这个方法，然后又调用了scheduleRetainedObjectCheck
    override fun onObjectRetained() = scheduleRetainedObjectCheck()
    fun scheduleRetainedObjectCheck() {
      //先判断heapDumpTrigger是否有初始化了，
      if (this::heapDumpTrigger.isInitialized) {
        heapDumpTrigger.scheduleRetainedObjectCheck()
      }
    }
    ...   
}

fun scheduleRetainedObjectCheck(
  delayMillis: Long = 0L
) {
  val checkCurrentlyScheduledAt = checkScheduledAt
  //如果之前的子线程任务来没开始执行则返回，也就是就是下面的backgroundHandler.postDelayed
  if (checkCurrentlyScheduledAt > 0) {
    return
  }
  checkScheduledAt = SystemClock.uptimeMillis() + delayMillis
  //让子线程延时去delayMillis执行，通过上面的调用链进入此方法则delayMillis为0，因此会马上执行
  backgroundHandler.postDelayed({
    //子线程任务开始执行，就将checkScheduledAt设置为0，以便下一次出现内存泄漏时能还能进来
    checkScheduledAt = 0
    checkRetainedObjects()
  }, delayMillis)
}

private fun checkRetainedObjects() {
  ...
  //看下此时待观察的集合里面还有多少个没有被分析的弱引用对象
  //也就是retainedUptimeMillis != -1L 的对象的个数
  //并且在调用retainedObjectCount.get()的时候，还会再调用一次removeWeaklyReachableObjects()，尝试再次移除一遍
  var retainedReferenceCount = objectWatcher.retainedObjectCount
  //如果此时还有泄漏对象，则调用gc，并重新再获取一次数量
  if (retainedReferenceCount > 0) {
    gcTrigger.runGc()
    retainedReferenceCount = objectWatcher.retainedObjectCount
  }
  //这边判断应用是出于前台还是后台，
  //如果是在前台，则个数达到5个时才dump，如果应用在后台则只要有一个都会进行dump
  if (checkRetainedCount(retainedReferenceCount, config.retainedVisibleThreshold)) return
  val now = SystemClock.uptimeMillis()
  val elapsedSinceLastDumpMillis = now - lastHeapDumpUptimeMillis
  //如果这个时间与上次dump的时间过短，则会重新调用scheduleRetainedObjectCheck，
  //并延迟执行，这个延迟的时间 = 60s - 时间差，避免两次dump的时间过短，影响使用
  if (elapsedSinceLastDumpMillis < WAIT_BETWEEN_HEAP_DUMPS_MILLIS) {
    onRetainInstanceListener.onEvent(DumpHappenedRecently)
    showRetainedCountNotification(
      objectCount = retainedReferenceCount,
      contentText = application.getString(R.string.leak_canary_notification_retained_dump_wait)
    )
    scheduleRetainedObjectCheck(
      delayMillis = WAIT_BETWEEN_HEAP_DUMPS_MILLIS - elapsedSinceLastDumpMillis
    )
    return
  }
  dismissRetainedCountNotification()
  val visibility = if (applicationVisible) "visible" else "not visible"
  //这边开始进行dump
  dumpHeap(
    retainedReferenceCount = retainedReferenceCount,
    retry = true,
    reason = "$retainedReferenceCount retained objects, app is $visibility"
  )
}

private fun dumpHeap(
  retainedReferenceCount: Int,
  retry: Boolean,
  reason: String
) {
    ...
    val heapDumpUptimeMillis = SystemClock.uptimeMillis()
    ...
    //执行dump，并计算dump的时间
    durationMillis = measureDurationMillis {
      //执行dump，这边就不分析了，感兴趣可以自行点进去看下
      configProvider().heapDumper.dumpHeap(heapDumpFile)
    }
    ...
    //dump之后处理以分析的对象
    objectWatcher.clearObjectsWatchedBefore(heapDumpUptimeMillis)
}

/**
 * Clears all [KeyedWeakReference] that were created before [heapDumpUptimeMillis] (based on
 * [clock] [Clock.uptimeMillis])
 */
@Synchronized fun clearObjectsWatchedBefore(heapDumpUptimeMillis: Long) {
  //从待观察集合里面过滤出时间小于等于当前dump时间的弱引用
  val weakRefsToRemove =
    watchedObjects.filter { it.value.watchUptimeMillis <= heapDumpUptimeMillis }
  //遍历执行clear，将时间设置为-1，这个在下次触发执行checkRetainedObjects，里面不会重复判断
  weakRefsToRemove.values.forEach { it.clear() }
  //然后从待观察列表里面移除
  watchedObjects.keys.removeAll(weakRefsToRemove.keys)
}