Build a simple component

Build a simple component in isolation

We’ll build our UI following a Component-Driven Development (CDD) methodology. It’s a process that builds UIs from the “bottom up” starting with components and ending with screens. CDD helps you scale the amount of complexity you’re faced with as you build out the UI.

Task

Task component in three states

TaskComponent is the core component in our app. Each task displays slightly differently depending on exactly what state it’s in. We display a checked (or unchecked) checkbox, some information about the task, and a “pin” button, allowing us to move tasks up and down the list. Putting this together, we’ll need these props:

  • title – a string describing the task
  • state - which list is the task currently in and is it checked off?

As we start to build TaskComponent, we first write our test states that correspond to the different types of tasks sketch above. Then we use Storybook to build the component in isolation using mocked data. We’ll “visual test” the component’s appearance given each state as we go.

This process is similar to Test-driven development (TDD) that we can call “Visual TDD”.

Get setup

First, let’s create the task component and its accompanying story file: src/app/components/task.component.ts and src/app/components/task.stories.ts.

We’ll begin with the baseline implementation of the TaskComponent, simply taking in the inputs we know we’ll need and the two actions you can take on a task (to move it between lists):

// src/app/components/task.component.ts
import { Component, OnInit, Input, Output, EventEmitter } from '@angular/core';

@Component({
  selector: 'app-task',
  template: `
    <div class="list-item">
      <input type="text" [value]="task.title" readonly="true" />
    </div>
  `,
})
export class TaskComponent implements OnInit {
  title: string;
  @Input() task: any;

  // tslint:disable-next-line: no-output-on-prefix
  @Output() onPinTask: EventEmitter<any> = new EventEmitter();

  // tslint:disable-next-line: no-output-on-prefix
  @Output() onArchiveTask: EventEmitter<any> = new EventEmitter();

  constructor() {}

  ngOnInit() {}
}

Above, we render straightforward markup for TaskComponent based on the existing HTML structure of the Todos app.

Below we build out Task’s three test states in the story file:

// src/app/components/task.stories.ts
import { action } from '@storybook/addon-actions';
import { TaskComponent } from './task.component';
export default {
  title: 'Task',
  excludeStories: /.*Data$/,
};

export const actionsData = {
  onPinTask: action('onPinTask'),
  onArchiveTask: action('onArchiveTask'),
};

export const taskData = {
  id: '1',
  title: 'Test Task',
  state: 'Task_INBOX',
  updated_at: new Date(2019, 0, 1, 9, 0),
};
export const Default = () => ({
  component: TaskComponent,
  props: {
    task: taskData,
    onPinTask: actionsData.onPinTask,
    onArchiveTask: actionsData.onArchiveTask,
  },
});
// pinned task state
export const Pinned = () => ({
  component: TaskComponent,
  props: {
    task: {
      ...taskData,
      state: 'TASK_PINNED',
    },
    onPinTask: actionsData.onPinTask,
    onArchiveTask: actionsData.onArchiveTask,
  },
});
// archived task state
export const Archived = () => ({
  component: TaskComponent,
  props: {
    task: {
      ...taskData,
      state: 'TASK_ARCHIVED',
    },
    onPinTask: actionsData.onPinTask,
    onArchiveTask: actionsData.onArchiveTask,
  },
});

There are two basic levels of organization in Storybook: the component and its child stories. Think of each story as a permutation of a component. You can have as many stories per component as you need.

  • Component

    • Story
    • Story
    • Story

To tell Storybook about the component we are documenting, we create a default export that contains:

  • component -- the component itself,
  • title -- how to refer to the component in the sidebar of the Storybook app,
  • excludeStories -- information required by the story, but should not be rendered by the Storybook app.

To define our stories, we export a function for each of our test states to generate a story. The story is a function that returns a rendered element (i.e. a component class with a set of props) in a given state---exactly like a Stateless Functional Component.

action() allows us to create a callback that appears in the actions panel of the Storybook UI when clicked. So when we build a pin button, we’ll be able to determine in the test UI if a button click is successful.

As we need to pass the same set of actions to all permutations of our component, it is convenient to bundle them up into a single actionsData variable and use pass them into our story definition each time.

Another nice thing about bundling the actionsData that a component needs, is that you can export them and use them in stories for components that reuse this component, as we'll see later.

When creating a story we use a base task (taskData) to build out the shape of the task the component expects. This is typically modelled from what the true data looks like. Again, export-ing this shape will enable us to reuse it in later stories, as we'll see.

Actions help you verify interactions when building UI components in isolation. Oftentimes you won't have access to the functions and state you have in context of the app. Use action() to stub them in.

Config

We also need to make one small change to the Storybook configuration so it notices our recently created stories. Change your Storybook configuration file (.storybook/main.js) to the following:

module.exports = {
  stories: ['../src/app/components/**/*.stories.ts'],
  addons: ['@storybook/addon-actions', '@storybook/addon-links', '@storybook/addon-notes'],
};

Once we’ve done this, restarting the Storybook server should yield test cases for the three states of TaskComponent:

Specify data requirements

It’s best practice to specify the shape of data that a component expects. Not only is it self documenting, it also helps catch problems early. Here, we'll use Typescript and create a interface for the Task model.

Create a new folder called models inside app folder and inside a new file called task.model.ts with the following content:

// src/app/models/task.model.ts

export interface Task {
  id: string;
  title: string;
  state: string;
}

Build out the states

Now we have Storybook setup, styles imported, and test cases built out, we can quickly start the work of implementing the HTML of the component to match the design.

Our component is still rather rudimentary at the moment. We're going to make some changes so that it matches the intended design without going into too much detail:

// src/app/components/task.component.ts
import { Component, OnInit, Input, Output, EventEmitter } from '@angular/core';
import { Task } from '../models/task.model';
@Component({
  selector: 'app-task',
  template: `
    <div class="list-item {{ task?.state }}">
      <label class="checkbox">
        <input
          type="checkbox"
          [defaultChecked]="task?.state === 'TASK_ARCHIVED'"
          disabled="true"
          name="checked"
        />
        <span class="checkbox-custom" (click)="onArchive(task.id)"></span>
      </label>
      <div class="title">
        <input type="text" [value]="task?.title" readonly="true" placeholder="Input title" />
      </div>
      <div class="actions">
        <a *ngIf="task?.state !== 'TASK_ARCHIVED'" (click)="onPin(task.id)">
          <span class="icon-star"></span>
        </a>
      </div>
    </div>
  `,
})
export class TaskComponent implements OnInit {
  title: string;
  @Input() task: Task;

  // tslint:disable-next-line: no-output-on-prefix
  @Output() onPinTask: EventEmitter<any> = new EventEmitter();

  // tslint:disable-next-line: no-output-on-prefix
  @Output() onArchiveTask: EventEmitter<any> = new EventEmitter();

  constructor() {}

  ngOnInit() {}

  onPin(id: any) {
    this.onPinTask.emit(id);
  }
  onArchive(id: any) {
    this.onArchiveTask.emit(id);
  }
}

The additional markup from above combined with the CSS we imported earlier yields the following UI:

Component built!

We’ve now successfully built out a component without needing a server or running the entire frontend application. The next step is to build out the remaining Taskbox components one by one in a similar fashion.

As you can see, getting started building components in isolation is easy and fast. We can expect to produce a higher-quality UI with fewer bugs and more polish because it’s possible to dig in and test every possible state.

Automated Testing

Storybook gave us a great way to visually test our application during construction. The ‘stories’ will help ensure we don’t break our Task visually as we continue to develop the app. However, it is a completely manual process at this stage, and someone has to go to the effort of clicking through each test state and ensuring it renders well and without errors or warnings. Can’t we do that automatically?

Snapshot testing

Snapshot testing refers to the practice of recording the “known good” output of a component for a given input and then flagging the component whenever the output changes in future. This complements Storybook, because it’s a quick way to view the new version of a component and check out the changes.

Make sure your components render data that doesn't change, so that your snapshot tests won't fail each time. Watch out for things like dates or randomly generated values.

With the Storyshots addon a snapshot test is created for each of the stories. Use it by adding the following development dependency:

npm install -D @storybook/addon-storyshots

Then create the src/storybook.test.js file with the following in it:

// src/storybook.test.js
import initStoryshots from '@storybook/addon-storyshots';

initStoryshots();

Finally we need to make a small change in the jest key in our package.json:

{
  ....
   "transform": {
      "^.+\\.(ts|html)$": "ts-jest",
      "^.+\\.js$": "babel-jest",
      "^.+\\.stories\\.[jt]sx?$": "@storybook/addon-storyshots/injectFileName"

    },
}

Once the above is done, we can run npm run jest and see the following output:

Task test runner

We now have a snapshot test for each of our TaskComponent stories. If we change the implementation of TaskComponent, we’ll be prompted to verify the changes.

Additionally, jest will also run the test for app.component.ts.

Next Chapter
Composite component
Assemble a composite component out of simpler components