[Android] Fix mEditable NullPointerException in TextInputPlugin

[eggfly]

Fix:

• [Android] NullPointerException in TextInputPlugin.setTextInputEditingState flutter#94794
• NullPointerException in TextInputPlugin.clearTextInputClient(TextInputPlugin.java:527) flutter#92241

Our app also faced this problem in our internal crash report system, see the root cause from below comments.

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[eggfly]

I thought a lot about it, there are two MethodChannel invokes to Platform thread called: 'TextInput.setClient' and 'TextInput.setEditingState', the setEditingState is always called after _attach.

https://github.com/flutter/flutter/blob/ffcd32ebb68fbfe3c9788a3bf1a09d3ebe28887f/packages/flutter/lib/src/services/text_input.dart#L1495

  /// This method actually notifies the embedding of the client. It is utilized
  /// by [attach] and by [_handleTextInputInvocation] for the
  /// `TextInputClient.requestExistingInputState` method.
  void _attach(TextInputConnection connection, TextInputConfiguration configuration) {
    assert(connection != null);
    assert(connection._client != null);
    assert(configuration != null);
    assert(_debugEnsureInputActionWorksOnPlatform(configuration.inputAction));
    _channel.invokeMethod<void>(
      'TextInput.setClient',
      <dynamic>[ connection._id, configuration.toJson() ],
    );
    _currentConnection = connection;
    _currentConfiguration = configuration;
  }

https://github.com/flutter/flutter/blob/ffcd32ebb68fbfe3c9788a3bf1a09d3ebe28887f/packages/flutter/lib/src/services/text_input.dart#L1666

  void _setEditingState(TextEditingValue value) {
    assert(value != null);
    _channel.invokeMethod<void>(
      'TextInput.setEditingState',
      value.toJSON(),
    );
  }

engine/shell/platform/android/io/flutter/plugin/editing/TextInputPlugin.java

Line 408 in 2962099

void setTextInputClient(int client, TextInputChannel.Configuration configuration) {

  @VisibleForTesting
  void setTextInputClient(int client, TextInputChannel.Configuration configuration) {
    // Call notifyViewExited on the previous field.
    notifyViewExited();
    inputTarget = new InputTarget(InputTarget.Type.FRAMEWORK_CLIENT, client);

    if (mEditable != null) {
      mEditable.removeEditingStateListener(this);
    }
    mEditable =
        new ListenableEditingState(
            configuration.autofill != null ? configuration.autofill.editState : null, mView);   // <---- here 
    // more code ..
  }

So there's impossible to leave the mEditable member's value is null in TextInputPlugin class.

BUT UNLESS: there's another FlutterView object?

engine/shell/platform/android/io/flutter/embedding/android/FlutterView.java

Line 1080 in e11a257

public void attachToFlutterEngine(@NonNull FlutterEngine flutterEngine) {

  public void attachToFlutterEngine(@NonNull FlutterEngine flutterEngine) {
    Log.v(TAG, "Attaching to a FlutterEngine: " + flutterEngine);
    if (isAttachedToFlutterEngine()) {
      if (flutterEngine == this.flutterEngine) {
        // We are already attached to this FlutterEngine
        Log.v(TAG, "Already attached to this engine. Doing nothing.");
        return;
      }

      // Detach from a previous FlutterEngine so we can attach to this new one.
      Log.v(
          TAG,
          "Currently attached to a different engine. Detaching and then attaching"
              + " to new engine.");
      detachFromFlutterEngine();
    }

    this.flutterEngine = flutterEngine;

    // Instruct our FlutterRenderer that we are now its designated RenderSurface.
    FlutterRenderer flutterRenderer = this.flutterEngine.getRenderer();
    isFlutterUiDisplayed = flutterRenderer.isDisplayingFlutterUi();
    renderSurface.attachToRenderer(flutterRenderer);
    flutterRenderer.addIsDisplayingFlutterUiListener(flutterUiDisplayListener);

    // Initialize various components that know how to process Android View I/O
    // in a way that Flutter understands.
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
      mouseCursorPlugin = new MouseCursorPlugin(this, this.flutterEngine.getMouseCursorChannel());
    }
    textInputPlugin = 
        new TextInputPlugin(
            this,
            this.flutterEngine.getTextInputChannel(),
            this.flutterEngine.getPlatformViewsController());   // <---- here new instance

  @VisibleForTesting
  void setTextInputClient(int client, TextInputChannel.Configuration configuration) {
    // Call notifyViewExited on the previous field.
    notifyViewExited();
    inputTarget = new InputTarget(InputTarget.Type.FRAMEWORK_CLIENT, client);

    if (mEditable != null) {
      mEditable.removeEditingStateListener(this);
    }
    mEditable =
        new ListenableEditingState(
            configuration.autofill != null ? configuration.autofill.editState : null, mView);
    this.configuration = configuration;
    updateAutofillConfigurationIfNeeded(configuration);

    // setTextInputClient will be followed by a call to setTextInputEditingState.
    // Do a restartInput at that time.
    mRestartInputPending = true;
    unlockPlatformViewInputConnection();
    lastClientRect = null;
    mEditable.addEditingStateListener(this);
  }

If we try to attach an old engine (reuse engine) to a new FlutterView instance and call setTextInputEditingState or call clearTextInputClient, the mEditable maybe null. It will crash.

• So I think we can pre-create a mEditable default object in the constructor of TextInputPlugin, give it a default client_id and mView. @LongCatIsLooong @blasten

[blasten]

LGTM % nits