Add task with the HTTP client

Note

Learn how to add a task to a list using the HTTP client in an Angular application.

The TaskService

We just communicated with our API server to retrieve a list of tasks. Now we want to update the API server list by adding a new task.

Based on the HTTP protocol, we will use the post function to add a task to the list: http.post(). This function expects 2 parameters:

1. The URL of the API server
2. The task to send to the server

addTask(task: TaskForm) {
  return this.http.post<TaskForm>('http://localhost:3000/tasks', task);
}

As JSON-server is able to generate an id for us, we won’t use uuid to generate an id for the task.

This new addTask function defines our contract with the API server:

• it makes a POST request
• it sends a Task object as a request body
• it communicates with the API server at http://localhost:3000/tasks

🎓 Instructions

1. Open the src/app/task.service.ts file.

2. Add the following code to the file:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Task, TaskForm } from './task.model';

@Injectable({
  providedIn: 'root'
})
export class TaskService {
  constructor(private http: HttpClient) { }

  addTask(task: TaskForm) {
    return this.http.post<TaskForm>('http://localhost:3000/tasks', task);
  }
}

Update the TaskFormComponent

In the previous step, we explained Observables need to be subscribed to in order to execute the request. We wera able to use the async pipe in the HTML template to subscribe to the observable.

But in the current situation, the request does happen in the component.ts file.

We need to subscribe to the observable by using the subscribe function.

🎓 Instructions

1. Open the src/app/task-form/task-form.component.ts file.

2. Add the following code to the file:

import { Component, OnInit } from '@angular/core';
import { TaskService } from '../task.service';
import { TaskForm } from '../task.model';

@Component({
  selector: 'app-task-form',
  templateUrl: './task-form.component.html',
  styleUrls: ['./task-form.component.css']
})
export class TaskFormComponent implements OnInit {
  task: TaskForm = {
    title: '',
    description: ''
  };

  constructor(private taskService: TaskService) { }

  ngOnInit() {}

  submit(task: TaskForm) {
    this.taskService.addTask(task).subscribe();
    this.router.navigate(['/tasks']);
  }
}

It now works, but not exactly as we should expect…

Asynchronous programming

Remember the newsletter example from the previous chapter? Given that’s a physical newsletter, it takes time to be delivered to your mailbox. Some time happen between the moment you subscribe and the moment you receive the newsletter.

That’s the same in our situation. By sending a request with the subscribe() function, we’re asking the server to add a task to the list. But the navigation to the /tasks route will happen immediately after the request is sent, not after the request is completed.

Let’s review the code we just changed in the submit function of the TaskFormComponent:

submit(task: TaskForm) {
  this.taskService.addTask(task).subscribe();
  this.router.navigate(['/tasks']);
}

1. We call the addTask function from the TaskService to add a task to the API server.
2. We navigate to the /tasks route.

Danger

This code won’t run in the expected order

API communication is asynchronous, meaning that the addTask function will not be executed immediately. Our server is served locally, so the request will be fast, but it’s still asynchronous.

A request to the server can take a long time if the server or the network is slow for any reason. In this situation, While the request is still being processed, the router.navigate function will be executed.

That’s not what we want as by navigating, we want to display the updated task list.

We need to wait for the response from the server before navigating to the /tasks route.

The Subscription

The subscribe function accepts a callback function as a parameter. This callback function will be executed when the observable emits a value.

We’ll use this callback function to navigate to the /tasks route.

Open the src/app/task-form/task-form.component.ts file.

Update the following code to the file:

import { Component, OnInit } from '@angular/core';
import { Router } from '@angular/router';
import { TaskService } from '../task.service';
import { TaskForm } from '../task.model';

@Component({
  selector: 'app-task-form',
  templateUrl: './task-form.component.html',
  styleUrls: ['./task-form.component.css']
})
export class TaskFormComponent implements OnInit {
  task: TaskForm = {
    title: '',
    description: ''
  };

  constructor(private taskService: TaskService, private router: Router) { }

  ngOnInit() {}

  submit(task: TaskForm) {
    this.taskService.addTask(task).subscribe(() => {
      this.router.navigate(['/tasks']);
    });
  }
}

✔️ What you learned

In this chapter, you learned how to add a task to a list using the HTTP client in an Angular application.