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Learn how to manage a Redux application whose state is composed of multiple "slices" using the reducer composition pattern and the Redux Ducks file structure.

Strategies for Complex State

In the reducer composition pattern, the same steps are taken by the `rootReducer` for each slice reducer: 
1. Call the slice reducer with its slice of the `state` and the `action` as arguments.
2. Store the returned slice of state in a new object that is ultimately returned by the `rootReducer()`. 

```js
import { createStore } from 'redux';

// todosReducer and filterReducer omitted

const rootReducer = (state = {}, action) => {
  const nextState = {
    todos: todosReducer(state.todos, action),
    filter: filterReducer(state.filter, action)
  };
  return nextState;
};

const store = createStore(rootReducer);
```

The Redux package helps facilitate this pattern by providing a utility function called `combineReducers()`, which handles this boilerplate for us:

```js
import { createStore, combineReducers } from 'redux'

// todosReducer and filterReducer omitted.

const reducers = {
    todos: todosReducer,
    filter: filterReducer
};
const rootReducer = combineReducers(reducers);
const store = createStore(rootReducer);
```

Let's break this code down:
* The `reducers` object contains the slice reducers for the application. The keys of the object correspond to the name of the slice being managed by the reducer value.
* The `combineReducers()` function accepts this `reducers` object and returns a `rootReducer` function. 
* The returned `rootReducer` is passed to `createStore()` to create a `store` object.

Just as before, when an action is dispatched to the `store`, the `rootReducer()` is executed, which then calls each slice reducer, passing along the `action` and the appropriate slice of `state`.

The last 6 lines of this example can be rewritten inline:

```js
const store = createStore(combineReducers({
    todos: todosReducer,
    filter: filterReducer
}));
```

Take a look at **store.js** where you will find the slice reducers that you created in the previous exercise. Now, however, the `rootReducer()` is missing. Rather than creating this function by hand, you will use `combineReducers()`.

combineReducers

The `initialState` structure has been defined, and you know that the state of this application has 3 slices: `allRecipes`, `favoriteRecipes`, and `searchTerm`. Now, you can begin thinking about how the user will trigger changes to these slices of state through actions.

> Remember, actions in Redux are represented by plain JavaScript objects that have a `type` property and are dispatched to the store using the `store.dispatch()` method. 

When an application state has multiple slices, individual actions typically only change one slice at a time. Therefore, it is recommended that each action's `type` follows the pattern `'sliceName/actionDescriptor'`, to clarify which slice of state should be updated.

For example, in a todo application with a `state.todos` slice, the action type for adding a new todo might be `'todos/addTodo'`.

For the Recipes application, what do you think some of the action `type` strings might be? What user interactions might trigger them to be dispatched?

<details>

<summary>
Write some of your ideas down before revealing the actions you will be using:
</summary>
<p>

1. `'allRecipes/loadData'`: This action will be dispatched to fetch the needed data from an API right when the application starts.

2. `'favoriteRecipes/addRecipe'`: This action will be dispatched any time the user clicks on the ❤️  icon of a recipe from the full set of recipes.

3. `'favoriteRecipes/removeRecipe'`: This action will be dispatched any time the user clicks on the 💔 icon of a recipe from their list of favorites.
 
4. `'searchTerm/setSearchTerm'`: This action will be dispatched any time the user changes the text of the search input field to filter the full set of recipes.

5. `'searchTerm/clearSearchTerm'`: This action will be dispatched any time the user clicks on the "X" button next to the search input field.

</p>
</details>
<br>

It's also important to consider which of these actions will have a `payload` — additional data passed to the reducer in order to carry out the desired change-of-state. For example, consider the actions for the `searchTerm` slice:

```js
store.dispatch({ 
  type: 'searchTerm/setSearchTerm', 
  payload: 'Spaghetti' 
});
// The resulting state: { ..., searchTerm: 'Spaghetti' }

store.dispatch({ 
  type: 'searchTerm/clearSearchTerm' 
});
// The resulting state: { ..., searchTerm: '' }
```

* When the learner types in a search term, that data needs to be sent to the store so that the React components know which recipes to render and which to hide. 
* When the user clears the search field, no additional data needs to be sent because the store can simply set the search term to be an empty string again. 

Once you have a clear idea of the types of actions that will be dispatched in your application, when they will be dispatched, and what `payload` data they will carry, the next step is to make action creators.

> Remember, action creators are functions that return a formatted action object.

Action creators enable Redux programmers to re-use action object structures without typing them out by hand, and they improve the readability of their code, particularly when dealing with bulky `payload`s.

Take a look at **store.js** where you will find that action creators for the two actions above have been defined for you. Your job is to create the remaining three: `loadData()`, `addRecipe()`, and `removeRecipe()`

Actions and Payloads For Complex State

At this point, you may have begun thinking that **store.js** is getting pretty long, and yet the Recipes app only has three slices! Imagine trying to fit the logic for an application with a dozen or more slices into one file. That would not be fun.

Instead, it is more common, and better practice, to break up a Redux application using the [Redux Ducks pattern](https://github.com/erikras/ducks-modular-redux), like so:

```
src/
|-- index.js
|-- app/
    |-- store.js
|-- features/
    |-- featureA/
        |-- featureASlice.js
    |-- featureB/
        |-- featureBSlice.js
```

As you can see in your coding workspace, this file structure has already been set up for you.

All of the Redux logic lives within the top-level directory called **src/**. It contains:
* The entry point for the entire application, **index.js** (we will return to this file in the next exercise). 
* The sub-directories **app/** and **features/**. 

The **src/app/** directory has only one file (for now), **store.js**. As before, the ultimate purpose of this file is to create the `rootReducer` and the Redux `store`. Now, however, you'll notice that the file is empty! So, where did the reducers and action creators go?!

The **src/features/** directory, and its own **src/features/featureX/** sub-directories, contain all of the code relating to each individual slice of the `store`'s state. For example, for the `state.favoriteRecipes` slice, its slice reducer and action creators can be found in the file called **src/features/favoriteRecipes/favoriteRecipesSlice.js**. 

Lucky for you, we took care of much of the tedious work involved in [refactoring](https://en.wikipedia.org/wiki/Code_refactoring) this code. In addition to creating new folders and files, and copying over the relevant code, this refactor involved exporting each of the slice reducers and action creators, so that they could be imported back into **store.js**.

And that's where you come in! 

A File Structure for Redux

Now that you have defined which changes can occur to your application's state, you need a reducer to execute those changes. 

> Remember, the `store`'s reducer function is called each time an action is dispatched. It is passed the `action` and the current `state` as arguments and returns the `store`'s next state. 

The [second rule of reducers](https://redux.js.org/tutorials/fundamentals/part-3-state-actions-reducers#rules-of-reducers) states that when the reducer is updating the `state`, it must make a copy and return the copy rather than directly mutating the incoming `state`. When the state is a mutable data type, like an array or object, this is typically done using the spread operator (`...`).

Below, the `todosReducer` for a todo app demonstrates this in action:

```js
const initialState = {
  filter: 'SHOW_INCOMPLETE',
  todos: [
    { id: 0, text: 'learn redux', completed: false },
    { id: 1, text: 'build a redux app', completed: true },
    { id: 2, text: 'do a dance', completed: false },
  ]
};

const todosReducer = (state = initialState, action) => {
  switch (action.type) {
    case 'filter/setFilter':
      return {
        ...state,
        filter: action.payload
      };
    case 'todos/addTodo': 
      return {
        ...state,
        todos: [...state.todos, action.payload]
      } ;
    case 'todos/toggleTodo':
      return {
        ...state,
        todos: state.todos.map(todo => {
          return (todo.id === action.payload.id) ? 
            { ...todo, completed: !todo.completed } : 
            todo;
        })
      }
    default:
      return state;
  }
};
```

* The `todosReducer` uses the `initialState` as the default `state` value.
* When a `'filter/setFilter'` action is received, it spreads the old `state`'s contents (`...state`) into a new object before updating the `filter` property with the new filter from `action.payload`.
* When a `'todos/addTodo'` action is received, it does the same except this time, since `state.todos` is a mutable array, its contents are also spread into a new array, with the new todo from `action.payload` added to the end.
* When a `'todos/toggleTodo` action is received, it uses the `.map()` method to create a copy of the `state.todos` array. Additionally, the todo being toggled is found using `action.payload.id`, and it is spread into a new object and updated.

> It should be clarified that the `state.todos.map()` method only makes a "shallow" copy of the array, meaning the objects inside share the same references as the originals. Therefore, mutations to the objects within the copy will affect the objects within the original. For now, we can make do with this solution, but you will learn how to bypass this issue in a later lesson on the [Redux Toolkit](https://redux-toolkit.js.org/).

Immutable Updates & Complex State

The file structure that you helped implement in the last exercise works nicely when we add in React components. Take a look at the **src** folder in your workspace, and you will find the following file structure (new files have a `(+)` next to their name):

```
src/
|-- index.js
|-- app/
    |-- App.js (+)
    |-- store.js
|-- components/
    |-- FavoriteButton.js (+)
    |-- Recipe.js (+)
|-- features/
    |-- allRecipes/
        |-- AllRecipes.js (+)
        |-- allRecipesSlice.js
    |-- favoriteRecipes/
        |-- FavoriteRecipes.js (+)
        |-- favoriteRecipesSlice.js
    |-- searchTerm/
        |-- SearchTerm.js (+)
        |-- searchTermSlice.js
```

> If you look at the actual file structure in your code editor, you may notice a few unfamiliar files/directories not mentioned in the structure above. The **test/** directory and **index.compiled.js** file are used to test your code on Codecademy. You can ignore them. 

The new components are:
* `<App />`: The top-level component for the entire application.
* `<AllRecipes />`: The component for rendering the recipes loaded from the "database".
* `<FavoriteRecipes />`: The component for rendering the recipes favorited by the user.
* `<SearchTerm />`: The component for rendering the search bar that filters the visible recipes.
* `<Recipe />` and `<FavoriteButton />`: Generic components used by `<AllRecipes />` and `<FavoriteRecipes />`

Aside from the generic components, each feature-related React component file is located in the same directory as the slice file that manages the data rendered by that component. For example, **FavoriteRecipes.js** and **favoriteRecipesSlice.js** are both in the **src/features/favoriteRecipes/** directory.

Open the **src/app/App.js** file where the top-level component, `<App />`, is stored. As in most React applications, this top-level component will render each feature-component and pass any data needed by those components as prop values. In Redux applications, the data passed to each feature-component includes:
1. The slice of the `store`'s state to be rendered. For example, the `state.searchTerm` slice is passed to the `<SearchTerm />` component.
2. The `store.dispatch` method to trigger state changes through user interactions within the component. For example, the `<SearchTerm />` component will need to dispatch `setSearchTerm()` actions.

This distribution of the `store.dispatch` method and the slices of state to all of the feature-components, via the `<App />` component, begins in the **index.js** file. Open up the **src/index.js** file where you will see some standard React code for rendering the top-level `<App />` component. You'll notice that the `store` is missing, and the `<App />` component isn't receiving any props! 

Passing Store Data Through the Top-Level React Component

Redux is best suited for complex applications with many features that each have some state-related data to be managed. In these cases, objects are the go-to data type to represent the entire store's state.

For example, consider a todo app that allows a user to: 
* Add to a list of todos.
* Mark individual todos as complete or incomplete.
* Apply a filter to show only the completed todos, only the incomplete todos, or all of the todos.

After adding a few todos and setting the filter to show incomplete todos, the state might look like this:

```js
state = {
  todos: [
    {
      id: 0, 
      text: 'Complete the Learn Redux course', 
      isCompleted: false
    },
    {
      id: 1, 
      text: 'Build a counter app', 
      isCompleted: true
    },
  ],
  visibilityFilter: 'SHOW_INCOMPLETE'
};
```

In a Redux application, the top-level `state` properties, `state.todos` and `state.visibilityFilter`, are known as _slices_. Each slice typically represents a different feature of the entire application. Notice that a slice can be any data value, like an array of objects (`state.todos`) or just a string (`state.visibilityFilter`).

As a best practice, most Redux applications begin with an `initialState` that allows the programmer to do two key things:

1. Plan out the general structure of the state
2. Provide an initial state value to the reducer function

For the todo app, this may look like this: 

```javascript
const initialState = {
  todos: [],
  visibilityFilter: 'SHOW_ALL'
};
const todosReducer = (state = initialState, action) => {
  // rest of todosReducer logic omitted
};
```

The Recipes application will have the following three slices:
1. `allRecipes`: an array of all recipe objects
2. `favoriteRecipes`: an array of recipe objects chosen by the user from `state.allRecipes`
3. `searchTerm`: a string that filters which recipes are displayed

An example of the store's state may look like this:

```js
state = {
  allRecipes: [
    {id: 0, name: 'Jjampong', img: 'img/jjampong.png' },
    {id: 2, name: 'Cheeseburger', img: 'img/cheeseburger.png' },
    //… more recipes omitted
  ],
  favoriteRecipes: [
    {id: 1, name: 'Doro Wat', img: 'img/doro-wat.png' },
  ],
  searchTerm: 'Doro'
};
```

Notice that each recipe is represented as an object with an `id`, `name,` and `img` property.

Now that you know what the state structure looks like, the first step is to create an `initialState` object.

Slices

In the previous exercise, you saw how a single reducer was able to handle the logic for updating every slice of the `store`'s state. Though this approach does work for these relatively small examples, as the application state becomes increasingly more complex, managing it all with a single reducer will become impractical.  

The solution is to follow a pattern called _reducer composition_. In this pattern, individual _slice reducers_ are responsible for updating only one slice of the application's state, and their results are recombined by a `rootReducer` to form a single state object.

```js
// Handles only `state.todos`.
const initialTodos = [
  { id: 0, text: 'learn redux', completed: false },
  { id: 1, text: 'build a redux app', completed: true },
  { id: 2, text: 'do a dance', completed: false },
];
const todosReducer = (todos = initialTodos, action) => {
  switch (action.type) {
    case 'todos/addTodo': 
      return [...todos, action.payload]
    case 'todos/toggleTodo':
      return todos.map(todo => {
        return (todo.id === action.payload.id) ? 
          { ...todo, completed: !todo.completed } : 
          {...todo};
      });
    default:
      return todos;
  }
};

// Handles only `state.filter`
const initialFilter = 'SHOW_INCOMPLETE',
const filterReducer = (filter = initialFilter, action) => {
  switch (action.type) {
    case 'filter/setFilter':
      return action.payload;
    default:
      return filter;
};

const rootReducer = (state = {}, action) => {
  const nextState = {
    todos: todosReducer(state.todos, action),
    filter: filterReducer(state.filter, action)
  };
  return nextState;
};

const store = createStore(rootReducer);
```

In the reducer composition pattern, when an `action` is dispatched to the `store`:
* The `rootReducer` calls each slice reducer, regardless of the `action.type`, with the incoming `action` and the appropriate slice of the state as arguments. 
* The slice reducers each determine if they need to update their slice of state, or simply return their slice of state unchanged. 
* The `rootReducer` reassembles the updated slice values in a new state object.

One major advantage of this approach is that each slice reducer only receives its slice of the entire application's state. Therefore, each slice reducer only needs to immutably update its own slice and doesn't care about the others. This removes the problem of copying potentially deeply nested state objects.

Take a look at **store.js**, where you will find that the reducer for the Recipe app that you wrote in the last exercise (which can be found in **reducer-old.js**) has been partially rewritten to follow the reducer composition pattern:
* The `initialState` object has been replaced by individual `initialSliceName` variables which are used as default values for each slice reducer's slice of state.  This is another common feature of the reducer composition pattern.
* The `allRecipesReducer` and `searchTermReducer` slice reducers have been created for you. Notice that they each have a `default` case.
* Both slice reducers are called within the `rootReducer` to update their respective slices of state.

All that's left is to complete the `favoriteRecipesReducer()` and include it in the `rootReducer()`!

Reducer Composition

Congratulations! You've learned how to build and organize a React Redux application with multiple slices of state. 

Let's summarize what you've learned in this lesson:
* The `action.payload` property is used to hold additional data that the reducer might need to carry out a given action. The name `payload` is simply a convention, and its value can be anything!
* The spread syntax (`...`) and array methods such as `.map()`, `.slice()`, and `.filter()` can be used to immutably update the state of a complex app.
* _Reducer composition_ is a design pattern for managing a Redux store with multiple slices. 
* The _root reducer_ delegates actions to _slice reducers_ that are responsible for updating only their assigned slice of the store's state. The root reducer then reassembles the slices into a new state object.
* `combineReducers()` is a method provided by the `redux` library that accepts a collection of reducer functions and returns a `rootReducer` that implements the reducer composition pattern.
* In a Redux application, slice reducers are often written in separate files. This pattern is known as [Redux Ducks](https://github.com/erikras/ducks-modular-redux).

In the Recipes application you completed in the final exercise, the `store` is passed from the entry point (**index.js**) through the main `<App />` component as a prop. The `<App />` component can then pass the slices of the store's state to its sub-components. 

This approach is called "prop drilling" or "prop threading" because the props are "threaded" through the top-level component in order to get them to the presentational components. This isn't ideal, considering that the top-level component doesn't make use of those props. In the next lesson, you'll learn how you can use Redux Toolkit to avoid "prop threading" and more tricks for building robust React Redux applications!

Review

In the previous lesson, you built a simple counter app whose store state was just a single number. Though the counter app illustrates how Redux can manage the state of an application, it isn't a great example of an application that needs Redux. 

Redux really shines when used in applications with many features and a lot of data, where having a centralized store to keep it all organized is advantageous. In this lesson, you will learn strategies for managing an application with a more complex store state and, in the process, you will begin to build an app that will grow throughout the rest of this course.

In the browser, you can see the final product. This application, which we will refer to as the "Recipes App", does the following:
* displays a set of recipes that are pulled from a database.
* allows the user to add/remove their favorite recipes to/from a separate list.
* allows the user to enter a search term to filter the visible recipes.

Now, imagine you are working for the software development company whose main product is this Recipes application. The product manager has determined the desired features and functionality, the graphic designer has defined its style, and the React engineer has created its components. Now, it is up to you, the Redux Engineer, to design the state management system that can put it all together! 

> In reality, the Front-End Engineer would implement both React and Redux.

Before continuing on, make sure that you are familiar with the following terms and concepts relating to React and Redux:
* React
  * How to create components
  * How to render components using `ReactDOM.render()`
  * How to nest components and pass data through props
* Redux
  * One-way data flow model: State → View → Actions → State → View … 
  * How to create a reducer function: `(state, action) => nextState`
  * How to write action objects and action creators
  * How to create a `store` using `createStore()`
  * How to use the `store` methods `getState()`, `dispatch()`, and `subscribe()`

> Note to learners: The slightly wordy nature of Redux means that the examples in this course can be quite large. It is recommended that you expand the "Learn" section while reading through this lesson's materials. You can do so by clicking and dragging the dividing line that separates the "Learn" section from the code editor.

Introduction to Strategies for Complex State

At the end of the last exercise, you were able to pass the current state of the `store` and its `store.dispatch` method to the top-level component, `<App />`. This allowed the `<App />` component to distribute the `dispatch` method and the slices of the `store`'s state to each feature-component.

So it looks like you're done, right? Not quite. Try adding a favorite recipe and you'll see that it just disappears! Take a closer look at **App.js** and you'll notice that the `<FavoriteRecipes />` component is missing. Then, open up **FavoriteRecipes.js** and you'll see that it is also incomplete. Let's fix that.

Plugging in a feature-component to a Redux application involves the following steps:
* Import the React feature-components into the top-level **App.js** file.
* Render each feature-component and pass along the slice of `state` and the `dispatch` method as props.
* Within each feature-component:
  * Extract the slice of `state` and `dispatch` from `props`. 
  * Render the component using data from the slice of state.
  * Import any action creators from the associated slice file.
  * Dispatch actions in response to user inputs within the component.

This process is not different from how you implemented a React + Redux application in the past. Now, however, you must consider that the slices of the `store`'s state and the `dispatch` method must be passed through props.