Using the value from a BehaviorSubject as a parameter for a function: A step-by-step guide

Question

Using the value from a BehaviorSubject as a parameter for a function: A step-by-step guide

Using a BehaviorSubject to store and share a variable (username) among components has been working well for displaying the username on templates. However, there is a desire to utilize the value of the BehaviorSubject in a service where it can be used as a parameter for a method like myMethod(username) { do stuff }.

The issue appears to be that the value of the BehaviorSubject isn't readily available when the method is called. Attempts have been made to call the method in both the constructor and ngOnInit lifecycle hook, but logging it to the console reveals that initially, it is undefined and only later does it log with the correct username value. Is it feasible to use a BehaviorSubject to retrieve a value passed as an argument to a method?

In this scenario, there is an auth.service containing the BehaviorSubject (with login/auth details omitted as they are not pertinent here). This BehaviorSubject is then subscribed to in newform.component.ts. While the value can be displayed on the newform template, the aim is to subscribe to the BehaviorSubject's value in the newform component and pass it to a method that invokes a service returning an array of locations based on the username. The goal is to populate an input on the newform template with these locations before the user begins filling out the form.

Below is some code:

auth.service.ts

...

newform.component.ts

...

locationslist.service.ts

...

While the locationslist service functions appropriately if the "accountname" is hardcoded into the getUserLocations(accountname) argument, using the BehaviorSubject value results in an initial 500 HTTP response error in the console followed by successful location retrieval. It appears that the BehaviorSubject variable is delayed in appearing. Is there a way to effectively pass the BehaviorSubject value as an argument to a method, or perhaps a better approach to achieve the desired outcome?

angular typescript behaviorsubject

Answer 1

Answer №1

Discover the power of Angular 12 - now with added support for null or undefined values!

subjectUser:BehaviorSubject<User> = new BehaviorSubject<User>(null!);

Answer 2

Discover the power of Angular 12 - now with added support for null or undefined values!

subjectUser:BehaviorSubject<User> = new BehaviorSubject<User>(null!);

Answer 3

Answer №2

Of course, all you need is a proactive approach within the function.

name$: BehaviorSubject<string> = new BehaviorSubject("");

function doSomething() {
  this.name$.subscribe(console.log); // Get the name here
  // You can filter it like (.pipe(filter(name => !!name)) if you don't want undefined value
}

// Your location function will be like this:
function getLocations() {
  this.name$.pipe(switchMap(name => {  // Every time name subject changes, switch to another getUserLocations
    return this.locations.getUserLocations(name)
  }))
  .subscribe(foundset => {
    this.locations = foundset;
   });
}

Eliminate undefined values in the function above for the getLocations method too.

Answer 4

Of course, all you need is a proactive approach within the function.

name$: BehaviorSubject<string> = new BehaviorSubject("");

function doSomething() {
  this.name$.subscribe(console.log); // Get the name here
  // You can filter it like (.pipe(filter(name => !!name)) if you don't want undefined value
}

// Your location function will be like this:
function getLocations() {
  this.name$.pipe(switchMap(name => {  // Every time name subject changes, switch to another getUserLocations
    return this.locations.getUserLocations(name)
  }))
  .subscribe(foundset => {
    this.locations = foundset;
   });
}

Eliminate undefined values in the function above for the getLocations method too.

Answer 5

Answer №3

If you're facing a similar situation, consider leveraging Resolve Guard. Its purpose is to fetch data before your components load. In this scenario, it seems like you need the username in a component beforehand for the rest of the functionality to work smoothly.

To implement a resolve guard, follow these basic steps:

Develop a service that retrieves the username
Create a resolve guard that utilizes the resolve method to return an observable by invoking your service
Include the reference to your resolve guard in the route configuration specific to that component
In your component, subscribe to the component's data parameter and retrieve the username

Keep in mind, a component in the route configuration can have multiple resolve guards attached to it.

An excellent example demonstrating this concept can be found at

Answer 6

If you're facing a similar situation, consider leveraging Resolve Guard. Its purpose is to fetch data before your components load. In this scenario, it seems like you need the username in a component beforehand for the rest of the functionality to work smoothly.

To implement a resolve guard, follow these basic steps:

Develop a service that retrieves the username
Create a resolve guard that utilizes the resolve method to return an observable by invoking your service
Include the reference to your resolve guard in the route configuration specific to that component
In your component, subscribe to the component's data parameter and retrieve the username

Keep in mind, a component in the route configuration can have multiple resolve guards attached to it.

An excellent example demonstrating this concept can be found at

Answer 7

Answer №4

To ensure the second method waits for the completion of the first one, you need to connect these two methods.

One way to achieve this is by utilizing the map operator. Simply place the second function inside the map function.

 integrateMethods() {
    this.authService.accountInfoCurrent.pipe(map(() => {
                this.locations.getUserLocations(this.accountName)  
                      .subscribe(data => {
                        this.locations = data;
                      });
    })).subscribe(accountInfo => {
      this.accountName = accountInfo.AccountName;
    } );
}

Answer 8

To ensure the second method waits for the completion of the first one, you need to connect these two methods.

One way to achieve this is by utilizing the map operator. Simply place the second function inside the map function.

 integrateMethods() {
    this.authService.accountInfoCurrent.pipe(map(() => {
                this.locations.getUserLocations(this.accountName)  
                      .subscribe(data => {
                        this.locations = data;
                      });
    })).subscribe(accountInfo => {
      this.accountName = accountInfo.AccountName;
    } );
}

Answer 9

Answer №5

Appreciate all the support and suggestions from everyone. I opted to utilize route params to transfer the data between components, which turned out to be a more straightforward approach and likely the conventional method for passing parameters to another component (hence the name route params!). Unlike using BehaviorSubject observables, this method is synchronous and does not require waiting time. The data can be loaded directly OnInIt.

Hopefully, this explanation proves helpful!

Answer 10

Appreciate all the support and suggestions from everyone. I opted to utilize route params to transfer the data between components, which turned out to be a more straightforward approach and likely the conventional method for passing parameters to another component (hence the name route params!). Unlike using BehaviorSubject observables, this method is synchronous and does not require waiting time. The data can be loaded directly OnInIt.

Hopefully, this explanation proves helpful!

Using the value from a BehaviorSubject as a parameter for a function: A step-by-step guide

auth.service.ts

newform.component.ts

locationslist.service.ts

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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