Using Asynchronous Windows APIs · React Native for Windows + macOS

This documentation and the underlying platform code is a work in progress.

A common scenario for Native Modules is to call one or more native asynchronous methods from a JS asynchronous method. However it may not be immediately obvious how to properly bridge both asynchronous worlds, which can lead to unstable, difficult to debug code.

This document proposes some best patterns to follow when bridging asynchronous methods from JS to native code for React Native Windows. It assumes you've already familiar with the basics of setting up and writing Native Modules.

The complete source for the examples below are provided within the Native Module Sample in microsoft/react-native-windows-samples.

Writing Native Modules that call Asynchronous Windows APIs

Let's write a native module which uses asynchronous Windows APIs to perform a simple HTTP request. We'll call it SimpleHttpModule and it needs a single, promise-based method named GetHttpResponse that takes a URI string as a parameter and on success returns an object with both the HTTP status code and text content.

In the end, we'll want to call the method from JS as follows:

NativeModules.SimpleHttpModule.GetHttpResponse('https://microsoft.github.io/react-native-windows/')
  .then(result => console.log(result))
  .catch(error => console.log(error));

`SimpleHttpModule` in C#

The native module support for C# supports the common asynchronous programming patterns established in C# using async, await and Task<T>.

To expose the module to JavaScript you need to declare a C# class. To indicate it should be exposed to JavaScript, you annotate with a [ReactModule] attribute like:

namespace NativeModuleSample
{
  [ReactModule]
  class SimpleHttpModule
  {
    // Methods go here.
  }
}

This makes an object available to JavaScript via the expression NativeModules.SimpleHttpModule. By default the JavaScript name will match the C# class name. If you don't want the name of the class to match the name in JavaScript, i.e. you want to access the module via the expression NativeModules.CustomModule. you can pass a custom name like:

  [ReactModule("CustomModule")]
  class SimpleHttpModule
  { 
    ...
  }

Now we want to expose the method that performs the HTTP request. It is recommended, and it is the default, to write these functions asynchronously. Writing asynchronous code in C# is pretty straight-forward and intuitive with the async and await keywords and the Task<T> types.

If you're not familiar with writing asynchronous C# code, see Call asynchronous APIs in C# or Visual Basic and Asynchronous programming that will teach you the concepts if you are not familiar yet.

The function signature for a typical web request, annotated with the [ReactMethod] attribute will look like:

    [ReactMethod]
    public async Task<string> GetHttpResponseAsync(string uri) {
      ...
    }

You are now free to fill in the logic like:

      // Create an HttpClient object
      var httpClient = new HttpClient();

      // Send the GET request asynchronously
      var httpResponseMessage = await httpClient.GetAsync(new Uri(uri));

      var content = await httpResponseMessage.Content.ReadAsStringAsync();
      
      return content;

The code takes the following steps:

Creates a HttpClient.
Asynchronously calls the GetAsync method to make an HTTP request for the URI.
Parses the status code out of the returned HttpResponseMessage object.
Asynchronously parses the content out of the returned HttpResponseMessage object.
returns the content

This code only returns a string. You might want to return a more complex object that contains both the content and the status code. For that you can simply declare a C# struct that will be marshaled to JavaScript like:

  internal struct Result {
    public int statusCode { get; set; }
    public string content { get; set; }
  }

It is recommended to follow JavaScript naming conventions here as of now there is no auto-mapping of names between the common style guides of C# and JS

To return the value you'll of course have to update the signature of the method from returning a string to the Result:

    public async Task<Result> GetHttpResponseAsync(string uri) {

as well as store the status code and update the return statement from return content; to:

      var statusCode = httpResponseMessage.StatusCode;

      return new Result()
      {
        statusCode = (int)statusCode,
        content = content,
      };

But wait, we've only discussed the success path, what happens if GetHttpResponse doesn't succeed? We don't handle any exceptions in this example. If an exception is thrown, how do we marshal an error back to JavaScript? That is actually taken care of for you by the framework: any exception in the task will be marshaled to the JavaScript side as a JavaScript exception.

That's it! If you want to see the complete SimpleHttpModule, see AsyncMethodExamples.cs.

`SimpleHttpModule` in C++/WinRT

Let's start with the asynchronous native method which performs the HTTP request:

static winrt::Windows::Foundation::IAsyncAction GetHttpResponseAsync(std::wstring uri) noexcept
{
  // Create an HttpClient object
  auto httpClient = winrt::Windows::Web::Http::HttpClient();

  // Send the GET request asynchronously
  auto httpResponseMessage = co_await httpClient.GetAsync(winrt::Windows::Foundation::Uri(uri));

  // Parse response
  auto statusCode = httpResponseMessage.StatusCode();
  auto content = co_await httpResponseMessage.Content().ReadAsStringAsync();

  // TODO: How to return the result?
}

The GetHttpResponseAsync method is pretty straight-forward at this point, it takes a wstring URI and "returns" an IAsyncAction (which is to say, the method is asynchronous and doesn't actually return a value when it's done).

If you're not familiar with writing asynchronous C++/WinRT code, see Concurrency and asynchronous operations with C++/WinRT.

Inside GetHttpResponseAsync, we see it:

Creates a HttpClient.
Asynchronous calls the GetAsync method to make an HTTP request for the URI.
Parses the status code out of the returned HttpResponseMessage object.
Asynchronously parses the content out of the returned HttpResponseMessage object.

Now we have statusCode and content, but what do we do with it? How do we call this method from JS, and how do we get the result back to the JS?

Let's pause this and start building our native module:

namespace NativeModuleSample
{
  REACT_MODULE(SimpleHttpModule);
  struct SimpleHttpModule
  {
    REACT_METHOD(GetHttpResponse);
    void GetHttpResponse(std::wstring uri,
        winrt::Microsoft::ReactNative::ReactPromise<winrt::Microsoft::ReactNative::JSValueObject> promise) noexcept
    {
    }
  };
}

Here we simply define SimpleHttpModule with an empty GetHttpResponse method.

Notice the method itself is void and that the last parameter in the signature is of type ReactPromise<JSValueObject>. This indicates to React Native Windows that we want a promise-based method in JS, and that the expected return value of a success is of type JSValueObject.

All method parameters before this final promise are the input parameters we expect to be marshaled in from the JS. In this case, we want a single string for the URI to request.

The promise object is our interface for handling the promise and marshaling a result to the JS. To do so we simply call promise.Resolve() with the result object (if the operation was a success) or promise.Reject() with an error (if the operation failed).

Now that we know how to return results, let's prep GetHttpResponseAsync to take in a ReactPromise<JSValueObject> parameter and use it:

static winrt::Windows::Foundation::IAsyncAction GetHttpResponseAsync(std::wstring uri,
  winrt::Microsoft::ReactNative::ReactPromise<winrt::Microsoft::ReactNative::JSValueObject> promise) noexcept
{
  auto capturedPromise = promise;
  
  // Create an HttpClient object
  auto httpClient = winrt::Windows::Web::Http::HttpClient();

  // Send the GET request asynchronously
  auto httpResponseMessage = co_await httpClient.GetAsync(winrt::Windows::Foundation::Uri(uri));

  // Parse response
  auto statusCode = httpResponseMessage.StatusCode();
  auto content = co_await httpResponseMessage.Content().ReadAsStringAsync();

  // Build result object
  auto resultObject = winrt::Microsoft::ReactNative::JSValueObject();

  resultObject["statusCode"] = static_cast<int>(statusCode);
  resultObject["content"] = winrt::to_string(content);

  capturedPromise.Resolve(resultObject);
}

What have we done here? First off, we've "captured" the promise locally within the asynchronous method by copying it into capturedPromise. We do this because this is an asynchronous method calling other asynchronous methods, and otherwise we risk the ReactPromise object getting deleted prematurely by React Native Windows.

Important: Our only input parameter in this example is a wstring, but if your method uses JSValue, JSValueArray, or JSValueObject parameter types, you'll need to "capture" those with a copy too. Example:
static winrt::Windows::Foundation::IAsyncAction MethodAsync(winrt::Microsoft::ReactNative::JSValueObject options) noexcept
{
  auto captureOptions = options.Copy();
  ...
}

At the bottom of the method, we simply build the result object to be returned to JS, and pass it to capturedPromise.Resolve(). That's it for GetHttpResponseAsync - if the method execution gets to the end without any problems, it will resolve the promise, which marshals the result back to the JS.

Now that GetHttpResponseAsync is taken care of, let's bridge the gap between it and our new GetHttpResponse native module method.

REACT_METHOD(GetHttpResponse);
void GetHttpResponse(std::wstring uri,
    winrt::Microsoft::ReactNative::ReactPromise<winrt::Microsoft::ReactNative::JSValueObject> promise) noexcept
{
  auto asyncOp = GetHttpResponseAsync(uri, promise);
}

Looks simple enough, right? We call GetHttpResponseAsync with the uri and promise parameters, and get back an IAsyncAction object which we store in asyncOp. When this executes, GetHttpResponseAsync will return control when it hits its first co_await, which in turn will return control for the JS code to continue running. When everything in GetHttpResponseAsync succeeds, it itself is responsible for resolving the promise with the result.

But wait, what happens if GetHttpResponseAsync doesn't succeed? We don't handle any exceptions in this example, so if an exception is thrown, how do we marshal an error back to the JS? We have one more thing to do, and that's to check for unhandled exceptions:

REACT_METHOD(GetHttpResponse);
void GetHttpResponse(std::wstring uri,
    winrt::Microsoft::ReactNative::ReactPromise<winrt::Microsoft::ReactNative::JSValueObject> promise) noexcept
{
  auto asyncOp = GetHttpResponseAsync(uri, promise);
  asyncOp.Completed([promise](auto action, auto status)
  {
    if (status == winrt::Windows::Foundation::AsyncStatus::Error)
    {
      std::stringstream errorCode;
      errorCode << "0x" << std::hex << action.ErrorCode() << std::endl;

      auto error = winrt::Microsoft::ReactNative::ReactError();
      error.Message = "HRESULT " + errorCode.str() + ": " + std::system_category().message(action.ErrorCode());
      promise.Reject(error);
    }
  });
}

We've defined an AsyncActionCompletedHandler lambda and set it to be run when asyncOp completes. Here we check if the action failed (i.e. status == AsyncStatus::Error) and if so, we build a ReactError object where the message contains both the error code (a Windows HRESULT) and the error message for that code. Then we pass that error to promise.Reject(), thereby marshaling the error back to the JS.

Important: This example shows the minimum case, where you don't handle any errors within GetHttpResponseAsync, but you're not limited to this. You're free to detect error conditions within your code and call capturedPromise.Reject() yourself with (more useful) error messages at any time. However you should always include this final handler, to catch any unexpected and unhandled exceptions that may occur, especially when calling Windows APIs. Just be sure that you only call Reject() once and that nothing executes afterwards.

That's it! If you want to see the complete SimpleHttpModule, see AsyncMethodExamples.h.

Writing Native Modules that call Asynchronous Windows APIs

SimpleHttpModule in C#

SimpleHttpModule in C++/WinRT

`SimpleHttpModule` in C#

`SimpleHttpModule` in C++/WinRT