The method getClassesAndSubjects initiates an asynchronous process, meaning that when you call getClasses, the operation is still ongoing. It is crucial to return an observable or a promise from getClassesAndSubjects:
public getClassesAndSubjects(school: number, whenData: string): Observable<your-type> {
const observable = this.classService.GetClassesAndSubjects(school, whenDate);
observable.subscribe(data => {
if (!data.hasOwnProperty('errors')) {
this.classesSubjects = data;
}
}, error => {
console.log("ERROR loading GetClassesAndSubjects: " + error);
});
return observable;
}
Now:
a.getClassesAndSubjects(1,'2018-01-01').subscribe(value => {
a.getClasses();
});
If a function involves asynchronous tasks, any subsequent actions relying on the outcomes of said operations must be patient and wait for them to complete.
You may also opt for using async/await. Here, the asynchronous function needs to return a Promise:
public async getClassesAndSubjects(school: number, whenData: string): Promise<your-type> {
const observable = this.classService.GetClassesAndSubjects(school, whenDate);
observable.subscribe(data => {
if (!data.hasOwnProperty('errors')) {
this.classesSubjects = data;
}
}, error => {
console.log("ERROR loading GetClassesAndSubjects: " + error);
});
return observable.toPromise();
}
Then, wherever it's needed:
async function whatever() {
// ...
await a.getClasseandSubjects(1, '2018-01-01');
a.getClasses();
}
By employing this approach, the execution of the function whatever pauses until the promise derived from a.getClassesAndSubjects either resolves or rejects, permitting the availability of data when executing a.getClasses. Although the function appears 'suspended,' the application remains operational as the async function is internally segmented and processed within a Promise.then structure by the compiler, delivering a refined solution.
It's important to note that any function leveraging await should be marked with async.