MST Flow Pipe

This is a helper library for writing type-safe async code in MobX State Tree.

The Problem

MST recommends the use of flow for async code. This is great except one issue, you loose all type safety when you use yield:

import { types, onPatch, flow, IJsonPatch } from "mobx-state-tree";

const doSomethingAsync = (input: number) => Promise.resolve("this is the result");

const store = types.model({}).actions((self) => ({
  action1: flow(function* (input: number) {
    // Unfortunately "result" here is typed as "any" instead of "string"
    const result = yield doSomethingAsync(input);

    return "result: " + result;
  }),
}));

This makes it awkward to use as you have to manually type the return of every yield.

The Solution

This library attempts to solve the issue by converting the flow in a series of piped functions.

The above code now becomes:

import { types } from "mobx-state-tree";
import { flowPipe } from "mst-flow-pipe";

const doSomethingAsync = (input: number) => Promise.resolve("this is the result");

const store = types.model({}).actions((self) => ({
  // We type the input to the flow as number

  action1: flowPipe((input: number) => doSomethingAsync(input))
    // Result here is now correctly typed as "string"
    .then((result) => "result: " + result)

    // Note, we must also "end" the flow
    .end(),
}));

You can then chain together as many of these async steps as you wish. You can update the state mid-flow just fine:

import { types } from "mobx-state-tree";
import { flowPipe } from "mst-flow-pipe";

const loadUserName = (userId: string) => Promise.resolve("mike");

const loadUserAge = (userId: string) => Promise.resolve(36);

const User = types
  .model({
    name: types.string,
    age: types.number,
  })
  .actions((self) => ({
    action1: flowPipe(
      // We load the user name
      (userId: string) =>
        loadUserName(userId)
          // We also need the userId in the next step of the flow so we "map" the result of
          // this async step into the next step of the flow
          .then((name) => ({ name, userId }))
    )
      .then((result) => {
        // We can now safely set the name on the model because we are now in an "action"
        self.name = result.name;

        // We can then continue the flow with another async step
        return loadUserAge(result.userId);
      })
      .then((result) => {
        self.age = result;
      })

      // End the flow to return a valid action
      .end(),
  }));

If you want handle errors during the flow you can do that using the catch method.

import { types } from "mobx-state-tree";
import { flowPipe } from "mst-flow-pipe";

const loadUserName = (userId: string) => Promise.resolve("mike");

const loadUserAge = (userId: string) => Promise.resolve(36);

const reportErrorToServer = (err: Error) => Promise.resolve(err);

const User = types
  .model({
    name: types.string,
    age: types.number,
  })
  .actions((self) => ({
    // The type of this function is:
    // `(userId: string) => Promise<number | "error reported to server">`

    action1: flowPipe((userId: string) => loadUserName(userId).then((name) => ({ name, userId })))
      .then((result) => {
        self.name = result.name;
        return loadUserAge(result.userId);
      })
      .then((result: number) => {
        self.age = result;
        return result;
      })

      // If any of the above steps error then we can catch then and continue on
      .catch((error) => reportErrorToServer(new Error("could not load user, error: " + error)))

      // The type of result is now "number" or "Error"
      .then((result) => (result instanceof Error ? "error reported to server" : result))

      .end(),
  }));

More Examples

Checkout the tests for more examples of how to use this library

Contributors ✨

Thanks goes to these wonderful people (emoji key):

This project follows the all-contributors specification. Contributions of any kind welcome!