Observable.forkJoin with a for loop

Question

I am trying to populate an array in my component called processes which is an array of process. Each process also has a list of tasks.

So currently, I am working with two api calls which are:

/processes and /process/{processId}/tasks

I use /processes to get all the processes and initially populate the processes array. Then I use the process id of each process to call the second API to get the tasks of that process.

Currently, my code looks something like this:

this.processes.forEach((process, index) => {
    myService.getTasks().subscribe((tasks) => {
        process.tasks = tasks;
    })
})

I understand that I can create an array of observables, and use Observable.forkJoin() to wait for all these async calls to finish but I want to be able to define the subscribe callback function for each of the calls since I need a reference to the process. Any ideas on how I can go about approaching this issue?

Answer 1

Using the for loop to make multiple HTTP requests, and then subscribe to all of them separately should be avoided in order not to have many Observable connections opened.

As @Juan Mendes mentioned, Observable.forkJoin will return an array of tasks that match the index of each process in your processes array. You can also assign tasks to each process as they arrive as follows:

getTasksForEachProcess(): Observable<any> {

    let tasksObservables = this.processes.map(process, processIdx) => {
        return myService.getTasks(process)
            .map(tasks => {
                this.processes[processIdx].tasks = tasks; // assign tasks to each process as they arrive
                return tasks;
             })
            .catch((error: any) => {
                console.error('Error loading tasks for process: ' + process, 'Error: ', error);
                return Observable.of(null); // In case error occurs, we need to return Observable, so the stream can continue
            });
    });

    return Observable.forkJoin(tasksObservables);
};

this.getTasksForEachProcess().subscribe(
    tasksArray => {
        console.log(tasksArray); // [[Task], [Task], [Task]];
        // In case error occurred e.g. for the process at position 1,
        // Output will be: [[Task], null, [Task]];

        // If you want to assign tasks to each process after all calls are finished:
        tasksArray.forEach((tasks, i) => this.processes[i].tasks = tasksArray[i]);
    }
);

Please also take a look at this post: Send multiple asynchronous HTTP GET requests