# Vue

For more complex frontend features, we recommend using Vue.js. It shares
some ideas with React.js as well as Angular.

To get started with Vue, read through [their documentation][vue-docs].

## When to use Vue.js

We recommend using Vue for more complex features. Here are some guidelines for when to use Vue.js:

- If you are starting a new feature or refactoring an old one that highly interacts with the DOM;
- For real time data updates;
- If you are creating a component that will be reused elsewhere;

## When not to use Vue.js

We don't want to refactor all GitLab frontend code into Vue.js, here are some guidelines for
when not to use Vue.js:

- Adding or changing static information;
- Features that highly depend on jQuery will be hard to work with Vue.js;
- Features without reactive data;

As always, the Frontend Architectural Experts are available to help with any Vue or JavaScript questions.

## Vue architecture

All new features built with Vue.js must follow a [Flux architecture][flux].
The main goal we are trying to achieve is to have only one data flow and only one data entry.
In order to achieve this goal, each Vue bundle needs a Store - where we keep all the data -,
a Service - that we use to communicate with the server - and a main Vue component.

Think of the Main Vue Component as the entry point of your application. This is the only smart
component that should exist in each Vue feature.
This component is responsible for:
1. Calling the Service to get data from the server
1. Calling the Store to store the data received
1. Mounting all the other components

  ![Vue Architecture](img/vue_arch.png)

You can also read about this architecture in vue docs about [state management][state-management]
and about [one way data flow][one-way-data-flow].

### Components, Stores and Services

In some features implemented with Vue.js, like the [issue board][issue-boards]
or [environments table][environments-table]
you can find a clear separation of concerns:

```
new_feature
├── components
│   └── component.js.es6
│   └── ...
├── store
│  └── new_feature_store.js.es6
├── service
│  └── new_feature_service.js.es6
├── new_feature_bundle.js.es6
```
_For consistency purposes, we recommend you to follow the same structure._

Let's look into each of them:

### A `*_bundle.js` file

This is the index file of your new feature. This is where the root Vue instance
of the new feature should be.

The Store and the Service should be imported and initialized in this file and
provided as a prop to the main component.

Don't forget to follow [these steps.][page_specific_javascript]

### A folder for Components

This folder holds all components that are specific of this new feature.
If you need to use or create a component that will probably be used somewhere
else, please refer to `vue_shared/components`.

A good thumb rule to know when you should create a component is to think if
it will be reusable elsewhere.

For example, tables are used in a quite amount of places across GitLab, a table
would be a good fit for a component. On the other hand, a table cell used only
in one table would not be a good use of this pattern.

You can read more about components in Vue.js site, [Component System][component-system]

### A folder for the Store

The Store is a class that allows us to manage the state in a single
source of truth. It is not aware of the service or the components.

The concept we are trying to follow is better explained by Vue documentation
itself, please read this guide: [State Management][state-management]

### A folder for the Service

The Service is a class used only to communicate with the server.
It does not store or manipulate any data. It is not aware of the store or the components.
We use [vue-resource][vue-resource-repo] to communicate with the server.

### End Result

The following example shows an  application:

```javascript
// store.js
export default class Store {

  /**  
   * This is where we will iniatialize the state of our data.
   * Usually in a small SPA you don't need any options when starting the store. In the case you do
   * need guarantee it's an Object and it's documented.
   *    
   * @param  {Object} options   
   */   
  constructor(options) {
    this.options = options;

    // Create a state object to handle all our data in the same place
    this.todos = []:
  }

  setTodos(todos = []) {
    this.todos = todos;
  }

  addTodo(todo) {
    this.todos.push(todo);
  }

  removeTodo(todoID) {
    const state = this.todos;

    const newState = state.filter((element) => {element.id !== todoID});

    this.todos = newState;
  }
}

// service.js
import Vue from 'vue';
import VueResource from 'vue-resource';
import 'vue_shared/vue_resource_interceptor';

Vue.use(VueResource);

export default class Service {
  constructor(options) {
    this.todos = Vue.resource(endpoint.todosEndpoint);
  }

  getTodos() {
    return this.todos.get();
  }

  addTodo(todo) {
    return this.todos.put(todo);
  }
}
// todo_component.vue
<script>
export default {
  props: {
    data: {
      type: Object,
      required: true,
    },
  }
}
</script>
<template>
  <div>
    <h1>
      Title: {{data.title}}
    </h1>
    <p>
      {{data.text}}
    </p>
  </div>
</template>

// todos_main_component.vue
<script>
import Store from 'store';
import Service from 'service';
import TodoComponent from 'todoComponent';
export default {
  /**  
   * Although most data belongs in the store, each component it's own state.
   * We want to show a loading spinner while we are fetching the todos, this state belong
   * in the component.
   *
   * We need to access the store methods through all methods of our component.
   * We need to access the state of our store.
   */   
  data() {
    const store = new Store();

    return {
      isLoading: false,
      store: store,
      todos: store.todos,
    };
  },

  components: {
    todo: TodoComponent,
  },

  created() {
    this.service = new Service('todos');

    this.getTodos();
  },

  methods: {
    getTodos() {
      this.isLoading = true;

      this.service.getTodos()
        .then(response => response.json())
        .then((response) => {
          this.store.setTodos(response);
          this.isLoading = false;
        })
        .catch(() => {
          this.isLoading = false;
          // Show an error
        });
    },

    addTodo(todo) {
      this.service.addTodo(todo)
      then(response => response.json())
      .then((response) => {
        this.store.addTodo(response);
      })
      .catch(() => {
        // Show an error
      });
    }
  }
}
</script>
<template>
  <div class="container">
    <div v-if="isLoading">
      <i
        class="fa fa-spin fa-spinner"
        aria-hidden="true" />
    </div>

    <div
      v-if="!isLoading"
      class="js-todo-list">
      <template v-for='todo in todos'>
        <todo :data="todo" />
      </template>

      <button
        @click="addTodo"
        class="js-add-todo">
        Add Todo
      </button>
    </div>
  <div>
</template>

// bundle.js
import todoComponent from 'todos_main_component.vue';

new Vue({
  el: '.js-todo-app',
  components: {
    todoComponent,
  },
  render: createElement => createElement('todo-component' {
    props: {
      someProp: [],
    }
  }),
});

```

The [issue boards service][issue-boards-service] is a good example of this pattern.

## Information hiding

To make components more reusable, readable, and performant, aim to only pass props that 
the child component needs. 

Instead of this:

```
const ParentComponent = {
    data: bigGiantObject,
    template: `<child-component :big-giant-object='bigGiantObject'/>`,
}

const ChildComponent = {
    props: {
       bigGiantObject: { type: Object } 
    },
    template: `
      <div> 
         {{ bigGiantObject.littlePropertyOne }} 
         {{ bigGiantObject.littlePropertyTwo }} 
         {{ bigGiantObject.littlePropertyThree }}
     </div>` 
};
```

Component props should be passed explicitly, like this:

```
const BetterParentComponent = {
    data: bigGiantObject,
    template: `
      <child-component 
        :little-property-one='bigGiantObject.littlePropertyOne'
        :little-property-two='bigGiantObject.littlePropertyTwo'
        :little-property-three='bigGiantObject.littlePropertyThree'/>
    `,
};

const BetterChildComponent = {
    props: {
       littlePropertyOne: { type: String },
       littlePropertyTwo: { type: String },
    },
    template: `
      <div> 
         {{ littlePropertyOne }} {{ littlePropertyTwo }}
     </div>` 
};
```

For deeper models, still try to encapsulate the data conservatively. An example 
would look like this:

```
const DeepModelParentComponent = {
    data: bigGiantObject,
    template: `
      <child-component 
        :tier-one='bigGiantObject.tierOne.tierTwo'
        :tier-two='bigGiantObject.tierThree'/>
    `,
};

const DeepModelChildComponent = {
    props: {
        tierTwo: { type: Object },
        tierThree: { type: Object },
    },
    template: `
      <div> 
         {{ tierTwo.littlePropertyOne }} {{ tierTwo: littlePropertyTwo }}
     </div>
    <div> 
        {{ tierThree.littlePropertyOne }} {{ tierThree: littlePropertyTwo }}
    </div>
     ` 
};
```

If the number of props you need to pass is very large or you need to pass a group of 
properties down many layers, it may be simpler and more maintainable to just pass 
a single object. If you're not sure, ask a Frontend Architecture expert.


## Style guide

Please refer to the Vue section of our [style guide](style_guide_js.md#vuejs)
for best practices while writing your Vue components and templates.

## Testing Vue Components

Each Vue component has a unique output. This output is always present in the render function.

Although we can test each method of a Vue component individually, our goal must be to test the output
of the render/template function, which represents the state at all times.

Make use of Vue Resource Interceptors to mock data returned by the service.

Here's how we would test the Todo App above:

```javascript
import component from 'todos_main_component';

describe('Todos App', () => {
  it('should render the loading state while the request is being made', () => {
    const Component = Vue.extend(component);

    const vm = new Component().$mount();

    expect(vm.$el.querySelector('i.fa-spin')).toBeDefined();
  });

  describe('with data', () => {
    // Mock the service to return data
    const interceptor = (request, next) => {
      next(request.respondWith(JSON.stringify([{
        title: 'This is a todo',
        body: 'This is the text'
      }]), {
        status: 200,
      }));
    };

    let vm;

    beforeEach(() => {
      Vue.http.interceptors.push(interceptor);

      const Component = Vue.extend(component);

      vm = new Component().$mount();
    });

    afterEach(() => {
      Vue.http.interceptors = _.without(Vue.http.interceptors, interceptor);
    });


    it('should render todos', (done) => {
      setTimeout(() => {
        expect(vm.$el.querySelectorAll('.js-todo-list div').length).toBe(1);
        done();
      }, 0);
    });
  });

  describe('add todo', () => {
    let vm;
    beforeEach(() => {
      const Component = Vue.extend(component);
      vm = new Component().$mount();
    });
    it('should add a todos', (done) => {
      setTimeout(() => {
        vm.$el.querySelector('.js-add-todo').click();

        // Add a new interceptor to mock the add Todo request
        Vue.nextTick(() => {
          expect(vm.$el.querySelectorAll('.js-todo-list div').length).toBe(2);
        });
      }, 0);
    });
  });
});
```

### Stubbing API responses
[Vue Resource Interceptors][vue-resource-interceptor] allow us to add a interceptor with
the response we need:

```javascript
  // Mock the service to return data
  const interceptor = (request, next) => {
    next(request.respondWith(JSON.stringify([{
      title: 'This is a todo',
      body: 'This is the text'
    }]), {
      status: 200,
    }));
  };

  beforeEach(() => {
    Vue.http.interceptors.push(interceptor);
  });

  afterEach(() => {
    Vue.http.interceptors = _.without(Vue.http.interceptors, interceptor);
  });

  it('should do something', (done) => {
    setTimeout(() => {
      // Test received data
      done();
    }, 0);
  });
```


[vue-docs]: http://vuejs.org/guide/index.html
[issue-boards]: https://gitlab.com/gitlab-org/gitlab-ce/tree/master/app/assets/javascripts/boards
[environments-table]: https://gitlab.com/gitlab-org/gitlab-ce/tree/master/app/assets/javascripts/environments
[page_specific_javascript]: https://docs.gitlab.com/ce/development/frontend.html#page-specific-javascript
[component-system]: https://vuejs.org/v2/guide/#Composing-with-Components
[state-management]: https://vuejs.org/v2/guide/state-management.html#Simple-State-Management-from-Scratch
[one-way-data-flow]: https://vuejs.org/v2/guide/components.html#One-Way-Data-Flow
[vue-resource-repo]: https://github.com/pagekit/vue-resource
[vue-resource-interceptor]: https://github.com/pagekit/vue-resource/blob/develop/docs/http.md#interceptors
[issue-boards-service]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/boards/services/board_service.js.es6
[flux]: https://facebook.github.io/flux