Facade service pattern in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to inject the facade service into the component constructor.

Angular

constructor(private [1]: UserFacade) {}

Drag options to blanks, or click blank then click option'

AuserFacade

BUserFacade

CfacadeUser

Duser_service

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of a variable name.

Using underscores in variable names.

Capitalizing the first letter of the variable.

In Angular, services are injected by specifying a private variable name in the constructor. The variable name should be camelCase, like userFacade.

2fill in blank

medium

Complete the code to call the facade's method to load users inside ngOnInit.

Angular

ngOnInit() {
  this.userFacade.[1]();
}

Drag options to blanks, or click blank then click option'

AfetchUsers

BloadUsers

CgetUsers

DretrieveUsers

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names that are not defined in the facade.

Using verbs like 'get' or 'fetch' which might not be the facade's method.

The facade service typically provides a method named loadUsers to trigger loading users from the backend.

3fill in blank

hard

Fix the error in the facade service method to return the users observable.

Angular

getUsers(): Observable<User[]> {
  return this.store.[1](selectUsers);
}

Drag options to blanks, or click blank then click option'

Apipe

Bdispatch

Cselect

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using dispatch which is for sending actions, not selecting data.

Using subscribe inside the facade which breaks observable chaining.

To get data from the store, the facade uses select with a selector function.

4fill in blank

hard

Fill both blanks to create a facade method that dispatches an action to add a user.

Angular

addUser(user: User) {
  this.store.[1](new [2](user));
}

Drag options to blanks, or click blank then click option'

Adispatch

Bselect

CAddUserAction

DLoadUsersAction

Attempts:

3 left

💡 Hint

Common Mistakes

Using select instead of dispatch.

Using the wrong action name that does not add a user.

The facade dispatches an action to the store using dispatch. The action to add a user is typically named AddUserAction.

5fill in blank

hard

Fill all three blanks to create a facade observable for users with error handling.

Angular

users$ = this.store.[1](selectUsers).[2](catchError(() => of([])));

constructor(private store: Store) {
  this.users$ = this.users$.[3]();
}

Drag options to blanks, or click blank then click option'

Aselect

Bpipe

CshareReplay

Ddispatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using dispatch instead of select.

Not using pipe to chain operators.

Forgetting to share the observable with shareReplay.

The facade selects users from the store using select, then uses pipe to add error handling with catchError. Finally, shareReplay is used to share the observable result efficiently.

Practice

1.

What is the main purpose of using a Facade Service in Angular?

easy

A. To directly manipulate the DOM from services

B. To replace Angular modules with a single service

C. To simplify component code by hiding complex service logic behind simple methods

D. To create multiple instances of services for each component

Solution

1. Step 1: Understand the role of Facade Service

   A Facade Service acts as a simple interface hiding complex logic from components.

2. Step 2: Identify the benefit in Angular components

   This pattern keeps components clean and easier to maintain by centralizing service calls.

3. Final Answer:

   To simplify component code by hiding complex service logic behind simple methods -> Option C

4. Quick Check:

   Facade Service purpose = Simplify logic [OK]

Hint: Facade hides complexity behind simple methods [OK]

Common Mistakes:

• Thinking Facade replaces Angular modules
• Believing Facade manipulates DOM directly
• Assuming Facade creates multiple service instances

2.

Which of the following is the correct way to inject a facade service MyFacadeService into an Angular component constructor?

easy

A. constructor(private myFacadeService: MyFacadeService) {}

B. constructor(public MyFacadeService) {}

C. constructor(myFacadeService: new MyFacadeService()) {}

D. constructor(private myFacadeService = MyFacadeService) {}

Solution

1. Step 1: Recall Angular dependency injection syntax

   Angular injects services via constructor parameters with access modifiers and type annotations.

2. Step 2: Match correct syntax

   constructor(private myFacadeService: MyFacadeService) {} uses private and type MyFacadeService, which is correct.

3. Final Answer:

   constructor(private myFacadeService: MyFacadeService) {} -> Option A

4. Quick Check:

   Inject service with private + type [OK]

Hint: Use private and type in constructor for injection [OK]

Common Mistakes:

• Omitting access modifier (private/public)
• Using new keyword inside constructor parameters
• Assigning service without type annotation

3.

Given this facade service method:

getUserName(): Observable<string> {
  return this.userService.getUser().pipe(
    map(user => user.name)
  );
}

What will the component receive when subscribing to getUserName()?

medium

A. A synchronous string value of the user's name

B. An observable emitting the entire user object

C. A promise resolving to the user's name

D. An observable emitting the user's name as a string

Solution

1. Step 1: Analyze the facade method return type

   The method returns an Observable<string> by mapping the user object to user.name.

2. Step 2: Understand what subscribing receives

   Subscribing to this Observable emits the user's name string asynchronously.

3. Final Answer:

   An observable emitting the user's name as a string -> Option D

4. Quick Check:

   Facade returns Observable of user name [OK]

Hint: Facade returns Observable mapped to user name [OK]

Common Mistakes:

• Confusing Observable with Promise
• Expecting synchronous value instead of Observable
• Assuming entire user object is emitted

4.

Identify the error in this facade service method:

fetchData() {
  this.apiService.getData().subscribe(data => {
    this.data = data;
  });
  return this.data;
}

medium

A. Using arrow function inside subscribe is invalid syntax

B. Returns data before subscription completes, causing undefined result

C. Subscription should be inside component, not service

D. Missing return type annotation causes compile error

Solution

1. Step 1: Understand asynchronous subscription

   The subscribe callback runs later, so this.data is not set immediately.

2. Step 2: Identify return timing issue

   The method returns this.data immediately, likely undefined before data arrives.

3. Final Answer:

   Returns data before subscription completes, causing undefined result -> Option B

4. Quick Check:

   Async subscribe returns undefined early [OK]

Hint: Return inside subscribe or use Observable return [OK]

Common Mistakes:

• Returning data before async call finishes
• Thinking arrow functions are invalid in subscribe
• Believing subscription must be in component only

5.

You want to create a facade service that combines data from UserService and SettingsService and exposes a single observable userSettings$. Which approach correctly implements this?

class UserSettingsFacade {
  userSettings$: Observable<UserSettings>;

  constructor(private userService: UserService, private settingsService: SettingsService) {
    // Fill in here
  }
}

hard

A. this.userSettings$ = combineLatest([this.userService.getUser(), this.settingsService.getSettings()]).pipe(map(([user, settings]) => ({ user, settings })));

B. this.userSettings$ = this.userService.getUser().pipe(map(user => this.settingsService.getSettings()));

C. this.userSettings$ = this.userService.getUser() + this.settingsService.getSettings();

D. this.userSettings$ = forkJoin(this.userService.getUser(), this.settingsService.getSettings());

Solution

1. Step 1: Combine multiple observables correctly

   Use combineLatest to emit latest values from both observables together.

2. Step 2: Map combined values into single object

   Use map operator to create an object with user and settings properties.

3. Final Answer:

   this.userSettings$ = combineLatest([this.userService.getUser(), this.settingsService.getSettings()]).pipe(map(([user, settings]) => ({ user, settings }))); -> Option A

4. Quick Check:

   Use combineLatest + map to merge observables [OK]

Hint: Use combineLatest and map to merge observables [OK]

Common Mistakes:

• Using + operator to add observables
• Mapping one observable to another observable instead of values
• Using forkJoin which waits for all to complete once