javascript - Wait for a child process in CasperJS

Question

I have a CasperJS process that loads some page and then it needs to call a go-process to analyze the page and to decide what page should be loaded next. go-process takes a while to execute. My problem is that CasperJS doesn't wait for the go-process to complete and exits.

casper.then(function(){
  var p = cp.execFile('/path/parse', [], {}, function(error, stdout, stderr) {
    console.log(stdout);
  });

});

How can I wait for my child process to complete?

Answer 1

All then* and wait* functions are scheduling steps to be executed. When CasperJS runs out of steps to execute and no function is passed into casper.run(), then it will automatically exit.

The easy fix would be to always pass an empty function into casper.run() and schedule new steps from inside the function:

casper.then(function(){
  var p = cp.execFile('/path/parse', [], {}, function(error, stdout, stderr) {
    console.log(stdout);
    casper.thenOpen(parsedUrl).then(function(){
      // do something on page
    });
  });
});
casper.run(function(){});

A more clean approach would be to write your own step function that wraps the execFile function:

casper.waitForFileExec = function(process, callback, onTimeout, timeout){
    this.then(function(){
        var cp = require('child_process'),
            finished = false,
            self = this;
        timeout = timeout === null || this.options.stepTimeout;
        cp.execFile(process, [], {}, function(error, stdout, stderr) {
            finished = true;
            callback.call(self, error, stdout, stderr);
        });
        this.waitFor(function check(){
            return finished;
        }, null, onTimeout, timeout);
    });
    return this; // for builder/promise pattern
};

...
casper.waitForFileExec('/path/parse', function(error, stdout, stderr) {
    this.echo(stdout);
    this.thenOpen(parsedUrl).then(function(){
        // do something on page
    });
}).run();