CODEWITHSUNDEEP
javascriptnode.jschild-process
Yuriy
Yuriy

Reputation: 457

How to delay function call until its callback will be finished in other places

I'm using child_process to write commands to console, and then subscribe on 'data' event to get output from it. The problem is that sometime outputs are merged with each other.

let command = spawn('vlc', { shell: true });

writeCommand(cmd, callback) {
  process.stdin.write(`${cmd}\n`);
  this.isBusy = true;
  this.process.stdout.on('data', (d) => {
    callback(d);
  });
}

Function writeCommand is used in several places, how can I delay it from executing until output from previous command is finished?

My output can look like (for status command for example):

( audio volume: 230 ) ( state stopped ) >

Upvotes: 0

Views: 157

Answers (2)

jfriend00
jfriend00

Reputation: 707916

data events on a stream have zero guarantees that a whole "unit" of output will come together in a single data event. It could easily be broken up into multiple data events. So, this combined with the fact that you are providing multiple inputs which generate multiple outputs means that you need a way to parse both when you have a complete set of output and thus should call the callback with it and also how to delineate the boundaries between sets of output.

You don't show us what your output looks like so we can't offer any concrete suggestions on how to parse it in that way, but common delimiters are double line feeds of things like that. It would entirely depend upon what your output naturally does at the end or if you control the content the child process creates, what you can insert at the end of the output.

Another work-around for the merged output would be to not send the 2nd command until the 1st one is done (perhaps by using some sort of pending queue). But, you will still need a way to parse the output to know when you actually have the completion of the previous output.

Another problem:

In the code you show, every time you call writeCommand(), you will add yet another listener for the data event. So, when you call it twice to send different commands, you will now have two listeners both listening for the same data and you will be processing the same response twice instead of just once. 

let command = spawn('vlc', { shell: true });

writeCommand(cmd, callback) {
  process.stdin.write(`${cmd}\n`);
  this.isBusy = true;
  // every time writeCommand is called, it adds yet another listener
  this.process.stdout.on('data', (d) => {
    callback(d);
  });
}

If you really intend to call this multiple times and multiple commands could be "in flight" at the same time, then you really can't use this coding structure. You will probably need one permanent listener for the data event that is outside this function because you don't want to have more than one listener at the same time and since you've already found that the data from two commands can be merged, even if you separate them, you can't use this structure to capture the data appropriately for the second part of the merged output.

Upvotes: 1

Zaid Qureshi
Zaid Qureshi

Reputation: 1223

You can use a queuing mechanism to execute the next command after the first one is finished. You can also use a library like https://www.npmjs.com/package/p-limit to do it for you.

Upvotes: 1

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