21-Displaying_Loading_Indicators.md

21. Displaying Loading Indicators

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When the data is fetched asynchronously, we want to display some kind of visual cue to the user. We'll add a condition to the render function that says that if we're fetching data, and we have no to-dos to show, we'll return a loading indicator from the render function of VisibleTodoList.

We will grab todos and isFetching from props. Since todos is the only extra prop we need to pass to the list, instead of using the spread operator, we will just pass the todos directly.

Our mapStateToProps function already calculates visibleTodos and includes the todos in the props. We need to do something similar for isFetching. getIsFetching accepts the current state of the app, and the filter for the todos being fetched. It is declared alongside other top level selectors in the top level reducer file.

Inside VisibleTodoList.js

render() {
    const { isFetching, toggleTodo, todos } = this.props;
    if (isFetching && !todos.length) {
      return <p>Loading...</p>;
    }

    return (
      <TodoList
        todos={todos}
        onTodoClick={toggleTodo}
      />
    );
  }

  /// PropType Declarations...

  const mapStateToProps = (state, { params }) => {
    const filter = params.filter || 'all';
    return {
      isFetching: getIsFetching(state, filter),
      todos: getVisibleTodos(state, filter),
      filter,
    };
  };

Updating the Root Reducer

Switching to our root reducer file (src/reducers/index.js) we will add another exported named selector function for getIsFetching. It accepts the state and the filter as arguments, and it delegates to another selector to find if the list is currently being fetched.

We will pass in the state of this list from state.listByFilter, but we haven't written getIsFetching yet.

export const getIsFetching = (state, filter) =>
  fromList.getIsFetching(state.listByFilter[filter]);

Updating createList.js

Before creating our new getIsFetching selector, we need to modify the state shape of the list. Rather than assume state is an array of ids, we will assume it is an object that contains this array as a property.

Now we can add another selector called getIsFetching that reads state.isFetching.

export const getIds = (state) => state.ids;
export const getIsFetching = state => state.isFetching;

We want our reducer to keep track of both of these fields, so rather than complicate the existing createList reducer, we will rename it to ids, because it manages just the ids.

Creating isFetching

First, at the top of our file we need to add an import for the combineReducers utility from Redux:

import { combineReducers } from 'redux';

Now we can create our isFetching reducer that will manage just the state's isFetching flag.

Its initial state is false, and it looks just like any other reducer. We switch on the action type, and if it is 'REQUEST_TODOS', we'll return true because we started fetching.

If it's 'RECEIVE_TODOS', we'll return false because the operation has finished. For any unknown action, we'll return the current state.

We'll include the same condition as our ids reducer that ignores any actions with a filter that does not match the filter argument to createList.

Inside createList.js

const isFetching = (state = false, action) => {
    if (filter !== action.filter) {
      return state;
    }
    switch (action.type) {
      case 'REQUEST_TODOS':
        return true;
      case 'RECEIVE_TODOS':
        return false;
      default:
        return state;
    }
  };

Notice that we handle the REQUEST_TODOS action, but we are not dispatching it anywhere.

Adding the 'REQUEST_TODOS' Action Creator

In our action creators file (src/actions/index.js), we will add a new exported function called requestTodos that takes the filter and returns an action object describing the 'REQUEST_TODOS' action with the corresponding filter.

export const requestTodos = (filter) => ({
  type: 'REQUEST_TODOS',
  filter,
});

Every exported action creator will be available on the props of the VisibleToDoList component.

Updating fetchData inside VisibleToDoList

We can destructure requestTodos from the props, and call it right before starting the asynchronous fetchToDos operation.

fetchData() {
  const { filter, fetchTodos, requestTodos } = this.props;
  requestTodos(filter);
  fetchTodos(filter);
}

Recap at 3:51 in video

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