Flutterで一時的にAndroidの戻る操作を無効にする方法

MainActivityのonBackPressedをオーバーライドしたら対応できました。

class MainActivity: FlutterFragmentActivity() {
    private var backDisabled = false

    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)
        val mychannel = MethodChannel(flutterEngine.dartExecutor.binaryMessenger, "com.xxx/channel")
        mychannel.setMethodCallHandler {
                call, result ->
            if (call.method == "setBackDisabled") {
                backDisabled = call.arguments as? Boolean ?: false
            }
        }
    }
    override fun onBackPressed() {
        if (!backDisabled) {
            super.onBackPressed()
        }
    }
}

後はFlutter側でよんであげるだけ

  static const MethodChannel _channel =
      MethodChannel('com.xxx/channel');

  static Future<void> setBackDisabled(bool disabled) async {
    await _channel.invokeMethod('setBackDisabled', disabled);
  }