detox-jest-spy

The problem

The detox framework allows us to orchestrate the application interaction, and to make assertions on the state of the interface.

As part of our testing strategy, we sometimes want to test certain critical side effects, such as analytics events being fired correctly as a result of interacting with the interface. Since detox is a greybox testing framework we're able to make some small changes to make this possible.

This library assumes jest as the test runner, but the same thing should work in the same manner for mocha.

Approach

We use Detox mocking to replace certain objects or functions with proxies that will track calls and inform the spy server. The spy server parleys this information into calls on jest Mocks, which can then be used for assertions within the Detox test context.

Install

npm install --save detox-jest-spy

Usage

The first thing we'll do is use Detox mocking to replace a file with the detox proxy implementation.

For example if you have a file such that

// analytics.js

export default {
  trackAction: () => {
    /* the usual implementation */
  },
  trackScreen: () => {
    /* the usual implementation */
  }
};

We'll add a file alongside it with the .e2e.js extension.

// analytics.e2e.js
import { getProxy } from "detox-jest-spy/dist/client";

export default getProxy("Analytics");

Ensure you add RN_SRC_EXT to your build commands as indicated in the docs so that the detox build will use the e2e.js file instead of the usual one.

Then you can write your detox tests:

// myfeature.smoke.spec.js

import { start, stop, getSpy } from "detox-jest-spy";
// detox overrides the global expect variable with it's own implementation, so we need to reimport it under another name
import default as jestExpect from 'expect';

beforeAll(() => {
  start(); // start the detox-jest-spy server
});

afterAll(() => {
  stop(); // stop the server after test run is complete
});

it("does somethin", async () => {
  // perform the usual detox UI orchetration
  await this.getElementById("myid").tap();

  // get the jest spy object
  const spy = getSpy("Analytics.trackAction");

  // use jest expect to perform assertions on the spy
  jestExpect(spy).toBeCalledWith({ id: 'myid' });
});