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Training - Redux Architecture

This is a multipart training workshop, aimed at teaching you the basic principles of Redux.

We'll be using the following technologies in this workshop:

  • Angular
  • Angular CLI
  • ngrx/store
  • ngrx/effects

Presentation

The presentation is provided along with the workshop. Simply open the /presentation folder and run the run.cmd file. The presentation should automatically open at http://localhost:8000.

Note that you must have Python installed to run the run.cmd command. Alternatively, you can probably just open index.html directly to view the presentation.

Part 1

  1. Clone this project: git clone https://github.com/landmarkhw/training-redux-arch.git
  2. Checkout the "part1" branch: git checkout part1

How to run

Run ng serve for a dev server. Navigate to http://localhost:4200/. The app will automatically reload if you change any of the source files.

No Redux, Yet

At this point, the application doesn't have ngrx (an Angular implementation of Redux) installed yet. It's just a simple Angular application.

Workshop Tasks

Let's get acquainted with the application.

Task 1: Look at movie-list.component.html:

Notice a few things:

  1. This file is pretty simple (although it's got a few Angular tricks).
  2. We're displaying both the movie details and the movie list in the same component.
  3. The app-movie-details component depends on data from the movie-list component. This means the movie-list component is responsible to track which movie has been selected.

Task 2: Look at movie-list.component.ts:

This file is a pretty standard Angular component:

  1. Calls the MovieService to retrieve search results.
    • NOTE: we don't call themoviedb.org API directly here. We properly separate the concerns by creating a MovieService class that's responsible for interacting with the API.
  2. Holds the search results in the searchResults$ observable variable (more on RxJS Observables in a later training).
  3. Holds the selected movie in the selectedMovie variable.
  4. Sets/clears the selectedMovie when a movie is clicked, or when the back button is clicked (respectively).

Task 3: Look at movie.service.ts:

  1. Calls themoviedb.org's API
  2. Uses an api_key specifically meant for this workshop.

Task 4: Look at movie-details-component.html:

Notice that there's business logic here: {{movie.vote_average / 2 }} We want to display the movie rating as a 5-star rating, whereas the data is based on a 1-10 rating. This business logic is currently happening directly in the view (hint: not good).

Part 2

In Part 2, ngrx/store has been added. Run git checkout part2 to see the changes to the code.

This part is all about Separation of Concerns - we begin using the Redux architecture to separate different responsibilties into different areas of the application.

Summary of Changes

  1. ngrx/store has been installed and added to app.module.ts
  2. ngrx/store-devtools has been installed. This allows you to use Redux DevTools to watch what's happening in Redux (actions & state).
  3. movie.actions.ts has been added, in a new folder - actions
  4. movie.reducers.ts has been added, in a new folder - reducers
  5. movie.service.ts now dispatches (e.g. publishes) the GotSearchResults action, which is reduced (saved) to state in the movieReducer.
  6. movie-list.component.ts now gets its search results by selecting (retrieving) it from the Redux store.

Workshop Tasks

Task 1: Look at movie.actions.ts

  1. This file is in a new folder - actions. This helps us organize the different parts of the application that have different responsibilities.
  2. This file defines the actions that are available for the movie area of the application (which is currently the whole app).
  3. The MovieActionTypes object is simply a collection of string constants that are used to uniqueqly identify each action.
  4. The GotSearchResultsAction is a TypeScript definition of the one action we have - GotSearchResults
  5. The MovieActions object is a collection of action creators that are used to create actions when they occur.
    • Note that actions in Redux are just plain old JavaScript objects. They have a type, which is a string constant that identifies what kind of action it is, and they have a payload, which is the data associated with the action.

Task 2: Look at movie.reducer.ts

  1. This file is in a new folder - 'reducers'. This helps us group all our reducers in one area.
  2. This file does 2 things:
    • Defines the shape of state for the movie area of the application (again, currently the whole app).
    • Defines a reducer, which is a function that, given the current state of the application and an action that's happening in the store, returns a new state.
  3. Reducers are commonly built with a switch statement - they only respond to the actions that affect this part of the application's state.
    • In this case, the reducer is reacting to the MovieActionTypes.GotSearchResults action, by saving the data from the API call into state.
  4. You'll also notice that the reducer defines the defaultState of that area of the application. This is what the state looks like before any changes are made.
  5. An important note: Reducers are pure functions.
    • Given a set of input data, the same output is returned.
    • Cannot produce side-effects

Being pure functions, reducers provide us many guarantees, along with being unit-testable.

Task 3: Look at movie.service.ts

1 change: instead of returning this.http.get(...) directly, we're subscribing to the results and dispatching an action to the store with the search results.

Task 4: Look at movie-list.component.ts

1 change: instead of getting searchResults$ directly from the movieService, we are instead retrieving the data from the store. Now the store is the "source of truth" for that data. One problem - we're manually navigating the state in the store to find the information we need. This can create some tightly-coupled situations where the view is tightly-coupled with the shape of state. In the next part, we'll look at how to address this problem.

Part 3

In Part 3, selectors have been added to the mix. Run git checkout part3 to see the changes to the code.

This part further separates our concerns - we address a couple of problems we noticed earlier:

  1. Business logic is found in the view
  2. Tight coupling of view to the shape of state

Summary of Changes

  1. movie.selectors.ts has been added, in a new folder - selectors
  2. movie-list.component.ts uses the new selector getSearchResultList
    • This decouples the view from the shape of the state. Now, if the shape of state changes, we can simply modify the getSearchResultList selector accordingly, and the view is unaffected.
  3. movie-details.component.ts and movie-details.component.html use the new selector get5StarRating
    • This moves business logic out of the view and into selectors.

Task 1: Look at movie.selectors.ts

New file - added 2 selectors * getSearchResultList - gets a list of movie search results from state * get5StarRating - gets the 5-star rating for a movie search result

Note that, like reducers, selectors are pure functions.

As with other pure functions, selectors can easily be unit tested when necessary.

Task 2: Look at movie-list.component.ts

1 change - uses the getSearchResultList selector. Note that it's passed directly to the select() function.

Task 3: Look at movie-details.component.ts and movie-details.component.html

2 changes - added get5StarRating() method and use it in the html template.

Part 4

In Part 4, effects have been added. Run git checkout part4 to see the changes to the code.

This part adds an important piece to Separation of Concerns with ngrx: Side-effect handling.

As mentioned before, reducers and selectors are both pure functions: They cannot produce side-effects.

Side-Effects

A few examples of side-effects are:

  1. Calling an API
  2. Mutating data
  3. Logging to console or file
  4. DOM manipulation
  5. Calling other functions that are not pure (e.g. Math.random())
    • new Date() - as it uses the current time, it's not pure

Best Practice

OK, so reducers and selectors can't have side-effects. What about our view -- it seems like that's the most logical place for these side-effects to go.

It turns out, the answer to this is both "yes", and "no":

  1. If the view has side-effects that are scoped directly to that view, then it's OK for it to contain its own side-effects.
    • Note that this rarely includes making API calls - those are best done elsewhere.
  2. Otherwise, side-effects should be contained elsewhere.

In this case, we move the movieService.getNowPlaying() call from the movie-list.component.ts file into an effect. As you've probably guessed, this helps us further separate our concerns.

@ngrx/effects

The @ngrx/effects library uses RxJS to address the question of "where should my side-effects go?"

Effects in @ngrx are RxJS Observables that watch actions pass through the Redux store.

New file defs.ts

I've added the defs.ts file, which has some helper interfaces and functions for creating and working with actions/action creators.

Task 1: look at movie.actions.ts

  1. Removed the GotSearchResults action
  2. Added new Search action, an async action
    • We did this since we have multiple actions that we need to represent now:
      • 1st action - call the API
      • 2nd action - receive results from the API
      • 3rd action - handle any failures from the API
    • Async actions do this for us. They are simply a collection of normal actions:
      • BEGIN - the action is beginning, usually triggers some action or API call
      • SUCCESS - the action has succeeded
      • FAILURE - the action has failed
      • UPDATE - triggered when a long-running action publishes a status update
  3. Updated MovieActions - see line 18.
    • NowPlayingSearchOptions - the data passed to the BEGIN action.
    • SearchResults - the data passed to the SUCCESS action.
    • MovieActionTypes.Search - the type of action being created.

Task 2: look at movie-list.component.ts

  1. Instead of calling the MovieService directly, we now dispatch an action

Task 3: look at movie.effects.ts

  1. The MovieService API is called here, when we see the MovieActionTypes.Search.BEGIN action occur.
  2. If the MovieService succeeds, it dispatches the MovieActionTypes.Search.SUCCESS action.
  3. If the MovieService fails, it dispatches the MovieActionTypes.Search.FAILURE action.

Part 5

There are still some concerns remaining that should be separated, that haven't been mentioned yet:

  1. The selectedMovie is tracked from the movie-list component, but this information is useful for other parts of the application.
  2. The movie-list component currently contains the movie-details component, and is responsible to provide all data to it. This dependency should be eliminated.

This final part of the workshop will involve (finally), you doing some coding. Since this is a brand new concept, and for many (most) of you, Angular is a new technology, I'll walk you through it step-by-step.

Task 1: Move the selectedMovie into Redux state.

  1. In movie.actions.ts,
    • Add Select: Action("movie/select"), to the MovieActionTypes object.
    • Add select: createAction<SearchResult>(MovieActionTypes.Select), to the MovieActions object.
  2. In movie.reducers.ts,
    • Add selectedMovie: SearchResult; to the MovieState interface.
    • Add selectedMovie: null, to the defaultState object.
  3. In movie.reducers.ts,
    • Create a case statement for the MovieActionTypes.Select action type.
    • Return a new state using Object.assign() that sets the selectedMovie.
  4. In movie.selectors.ts,
    • Create a new selector getSelectedMovie that returns the selectedMovie from the movie state.
  5. In movie-list.component.ts,
    • Remove the selectedMovie field.
    • Add a new selectedMovie$ field. Note that the $ suffix is called "Finnish Notation", and indicates that the variable is Observable.
    • Set the selectedMovie$ field like we set searchResults$ - this.selectedMovie$ = this.store.select(getSelectedMovie);
    • Change handleClick to instead dispatch the MovieActions.select action: this.store.dispatch(MovieActions.select(movie));
    • Change handleClose to instead dispatch the MovieActions.select action: this.store.dispatch(MovieActions.select(null));
  6. In movie-list.component.html,
    • Anywhere selectedMovie is found, replace it with (selectedMovie$ | async).

This should take care of moving that data into Redux state. I know, it's a lot of steps, but now this data is agnostic, and can be accessed and worked with from anywhere in the application. It can also be used in selectors to gather multiple pieces of state together and work with them.

More on that in another training...

You can see the results so far by checking out the part5-step1 branch

Task 2: Move the movie-details component out of the movie-list component.

  1. In movie-list.component.html,
    • Move the node into app.component.html.
  2. In movie-list.component.ts,
    • Move the selectedMovie$ field to app.component.ts.
    • Move the handleClose() function to app.component.ts.
  3. In movie-list.component.css,
    • Move the app-movie-details CSS rules to app.component.css.
    • Change the rule .movie-container.active { to :host.active .movie-container {
      • This is because we're moving the responsibility of telling us which component is active to the app component.
  4. In movie-list.component.html,
    • Change the line starting with [class] line to read class="movie-container flex-row"

That's it! You've so completely decoupled the selectedMovie from the movie-list state, that you can select (or deselect) a movie from either the movie-list component or the movie-details component.

You can see the final results by checking out the part5-step2 branch

The End

I hope you found this training enlightening and valuable, but I'd love to hear your feedback. If this isn't the kind of information you need, the kind of training you'd like, please let me know -- I can't get better if I don't get feedback!

Regards, Doug