Class and async functions, wait for result of previous calls

Question

I have a class with multiple methods, each method either call async functions, or manipulating data created by other methods.

How can I make sure that calling class methods would wait for previous method finish before proceeding?

Here is a simple example that calls random methods of the class, each method takes random number of seconds to execute, I need to make sure that each called method waited for result of previous method:

class Test
{
  constructor()
  {
    //ignore the constructor, it's simplified for this example and does not play any role in the question
    const list = [...Array(~~(Math.random()*3)+3)].map(e=>~~(Math.random()*3+1));
    console.log("expected: " + list);
    list.forEach(f => this["method_" + f]());
  }
  method_1()
  {
    return new Promise(resolve => setTimeout(() => 
    {
      console.log(1);
      resolve("res 1");
    }, Math.random() * 1000 + 100));
  }
  method_2()
  {
    return new Promise(resolve => setTimeout(() => 
    {
      console.log(2);
      resolve("res 2");
    }, Math.random() * 1000 + 100));
  }
  method_3()
  {
    func3();
  }
}

new Test();

function func3()
{
  new Promise(resolve => setTimeout(() => 
  {
    console.log(3);
    resolve("res 3");
  }, Math.random() * 1000 + 100));
}

I can't use then method, because each method doesn't know which method was executed before it...or can I?

I was thinking about adding a sort of queue, where called method would be placed when called, and executed when previous promise was resolved...not sure if that would be a best approach.

yes, you can use .then ... like this let p = Promise.resolve(); list.forEach(f => p = p.then(() => this["method_" + f]())); - but you get "neater" code using reduce - list.reduce((p, f) => p.then(() => this["method_" + f]()), Promise.resolve()); — Jaromanda X
– Jaromanda X, Commented Aug 25, 2022 at 1:32
You really really should just use for (const f of list) { await this["method_" + f](); } instead of forEach. Yes, you could implement such a queue as part of your instance state, but it'll become unwieldy since all your class methods become asynchronous with that. — Bergi
– Bergi, Commented Aug 25, 2022 at 1:36
"not all methods are returning a promise, they might only execute an async function" - don't do that. An method doing something asynchronous should always return a promise. — Bergi
– Bergi, Commented Aug 25, 2022 at 1:46

vanowm · Accepted Answer · 2022-08-25 10:43:29Z

0

Since essentially I needed make this class synchronous, using queue suggested by @Bergi works well:

class Test
{
  constructor()
  {
    this.queueList = Promise.resolve();

    //ignore the rest of the constructor, it's simplified for this example and does not play any role in the question
    const list = [...Array(~~(Math.random()*3)+3)].map(e=>~~(Math.random()*3+1));
    console.log("expected: " + list);
    list.forEach(f => this["method_" + f]());
  }
  queue(func)
  {
    return (this.queueList = this.queueList.then(func).catch(console.error));
  }
  method_1()
  {
    return this.queue(() => new Promise(resolve => setTimeout(() => 
    {
      console.log(1);
      resolve("res 1");
    }, Math.random() * 1000 + 100)));
  }
  method_2()
  {
    return this.queue(() => new Promise(resolve => setTimeout(() => 
    {
      console.log(2);
      resolve("res 2");
    }, Math.random() * 1000 + 100)));
  }
  method_3()
  {
    return this.queue(func3);
  }
}

new Test();

function func3()
{
    return new Promise(resolve => setTimeout(() => 
    {
      console.log(3);
      resolve("res 3");
    }, Math.random() * 1000 + 100));
}

edited Aug 25, 2022 at 10:43

answered Aug 25, 2022 at 2:36

vanowm

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1 Comment

Bergi Over a year ago

Yes, but don't forget the error handling!

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