Asynchronous tasks in grunt.registerTask - gruntjs

Question

I need to call two functions within grunt.registerTask, but the second function has to be called after the first function is done.
So I was wondering if we can use callbacks or promises or other asynchronous mechanisms within grunt.registerTask.
(More specifically, I need to launch karma in my first function call, and run karma in the second function call (to execute the initial unit tests). But in order to run karma, I need to launch it first. And that's what I'm missing.)

Answer 1

I had this:
grunt.registerTask("name_of_task", ["task_a", "task_b", "task_c"]);
And "task_b" had to be executed after "task_a" was done. The problem is that "task_a" is asynchronous and returns right away, so I needed a way to give "task_a" a few seconds to execute.
The solution:
grunt.registerTask("name_of_task", ["task_a", "task_b:proxy", "task_c"]);
grunt.registerTask("task_b:proxy", "task_b description", function () {
var done = this.async();
setTimeout(function () {
grunt.task.run("task_b");
done();
}, 2000);
});
};

Answer 2

From http://gruntjs.com/creating-tasks:
Tasks can be asynchronous.
grunt.registerTask('asyncfoo', 'My "asyncfoo" task.', function() {
// Force task into async mode and grab a handle to the "done" function.
var done = this.async();
// Run some sync stuff.
grunt.log.writeln('Processing task...');
// And some async stuff.
setTimeout(function() {
grunt.log.writeln('All done!');
done();
}, 1000);
});

Answer 3

For the sake of simplicity... having this Grunfile.js
grunt.registerTask('a', function () {
let done = this.async();
setTimeout(function () {
console.log("a");
done();
}, 3000);
});
grunt.registerTask('b', function () {
let done = this.async();
console.log("b1");
setTimeout(function () {
console.log("b2");
done();
}, 3000);
console.log("b3");
});
grunt.registerTask('c', function () {
console.log("c");
});
grunt.registerTask("run", ["a", "b", "c"]);
and then running run task, will produce the following output
Running "a" task
a
Running "b" task
b1
b3
b2 <-- take a look here
Running "c" task
c
Done.
The command is executed in this order
wait 3000 ms
console.log("a")
console.log("b1")
console.log("b3")
wait 3000 ms
console.log("b2")
console.log("c")