How To Mock Method Channels In Flutter.

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While testing apps, we sometimes run into situations where the logic being tested requires us to access third party plugins. That calls for mocking the plugin. Here’s how:

In this situation I have a bloc with an event CopyResult which copies text to device’s clipboard and calls a toast with the help of a third party plugin called FlutterToast.

on<CopyResult>(
      (event, emit) async {
        final string = event.result;
        await Clipboard.setData(ClipboardData(text: string));
        Fluttertoast.showToast(
            msg: "Text Copied.",
            toastLength: Toast.LENGTH_SHORT,
            timeInSecForIosWeb: 1,
            backgroundColor: Colors.black,
            webBgColor: "black",
            textColor: Colors.white,
            fontSize: 16.0);

        emit(UrlEncoderDecoderResult(result: string));
      },
    );

Initially, I was testing it like this:

blocTest("onCopyResult the String is copied to clipboard",
        build: () => UrlEncoderDecoderBloc(),
        act: (bloc) => bloc
            .add(const CopyResult("http://example.org/api?foo=some message")),
        wait: const Duration(seconds: 1),
        expect: () => const [
              UrlEncoderDecoderResult(
                  result: "http://example.org/api?foo=some message")
        ]);

Which led me to an error: MissingPluginException(No implementation found for method showToast on channel PonnamKarthik/fluttertoast)

This meant, my bloc was unable to find an implementation for fluttertoast. I used the method `setMockMethodCallHandler` to register a mock handler to handle my request:

blocTest("onCopyResult the String is copied to clipboard",
        build: () {
          MethodChannel channel =
              const MethodChannel('PonnamKarthik/fluttertoast');

          TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
              .setMockMethodCallHandler(channel, (message) {
            // use this space to handle different type of method calls
            // for the method channel, differently.
            // Since I have no use of any return value, I'm returning null.
            return null;
          });

          return UrlEncoderDecoderBloc();
        },
        act: (bloc) => bloc
            .add(const CopyResult("http://example.org/api?foo=some message")),
        wait: const Duration(seconds: 1),
        expect: () => const [
              UrlEncoderDecoderResult(
                  result: "http://example.org/api?foo=some message")
         ]);

More can be found in this wonderful answer by Adam Barth.