WindowSpy.kt

import android.annotation.TargetApi
import android.os.Build.VERSION_CODES
import android.view.View
import android.view.Window

object WindowSpy {

  /**
   * Installs a listener for new [Window] instances. This should only be called once, any new call
   * will uninstall the previous listener.
   *
   * Replaces WindowManagerGlobal.mViews in the WindowManagerGlobal singleton instances with an
   * [ArrayList] subclass that overrides [java.util.ArrayList.add] to also notify [block], if
   * the new root view is a DecorView that has a non null window.
   *
   * This is a no-op before API 19.
   */
  @TargetApi(VERSION_CODES.LOLLIPOP)
  fun onWindowAdded(block: (Window) -> Unit) {
    try {
      onWindowAddedThrowing(block)
    } catch (ignored: Throwable) {
    }
  }

  private fun onWindowAddedThrowing(block: (Window) -> Unit) {
    val windowManagerGlobalClass = Class.forName("android.view.WindowManagerGlobal")
    val windowManagerGlobalInstance =
      windowManagerGlobalClass.getDeclaredMethod("getInstance").invoke(null)

    val decorViewClass = try {
      Class.forName("com.android.internal.policy.DecorView")
    } catch (ignored: ClassNotFoundException) {
      // In some later release of Android 6 / API 23 DecorView was moved out of PhoneWindow.
      // https://cs.android.com/android/_/android/platform/frameworks/base/+
      // /8804af2b63b0584034f7ec7d4dc701d06e6a8754
      Class.forName("com.android.internal.policy.PhoneWindow\$DecorView")
    }

    val mWindowField = try {
      decorViewClass.getDeclaredField("mWindow").apply { isAccessible = true }
    } catch (ignored: NoSuchFieldException) {
      // In Android 6 / API 23 PhoneWindow.DecorView became an inner static class. Before, it had
      // a direct reference to its outer PhoneWindow class.
      // https://cs.android.com/android/_/android/platform/frameworks/base/+
      // /0daf2102a20d224edeb4ee45dd4ee91889ef3e0c
      decorViewClass.getDeclaredField("this$0").apply { isAccessible = true }
    }

    val mViewsField = windowManagerGlobalClass.getDeclaredField("mViews").apply { isAccessible = true }

    @Suppress("UNCHECKED_CAST")
    val rootViews = mViewsField[windowManagerGlobalInstance] as ArrayList<View>

    mViewsField[windowManagerGlobalInstance] = object : ArrayList<View>(rootViews) {
      override fun add(element: View): Boolean {
        // The Window class is actually not core to root views / the window manager, it's more of
        // a convenience helper that is sometimes used.
        // The only implementation of Window is PhoneWindow and its only ever used combined with
        // DecorView. DecorView is used for activities and dialogs, and has a mWindow field. Toasts
        // can have any view as their root view and don't use a window.
        if (decorViewClass.isInstance(element)) {
          (mWindowField[element] as Window?)?.let { block(it) }
        }
        return super.add(element)
      }
    }
  }
}