Running tasks in series, i.e. Task Dependency

By default, tasks run with maximum concurrency -- e.g. it launches all the tasks at once and waits for nothing. If you want to create a series where tasks run in a particular order, you need to do two things:

• give it a hint to tell it when the task is done,
• and give it a hint that a task depends on completion of another.

For these examples, let's presume you have two tasks, "one" and "two" that you specifically want to run in this order:

1. In task "one" you add a hint to tell it when the task is done. Either take in a callback and call it when you're done or return a promise or stream that the engine should wait to resolve or end respectively.

2. In task "two" you add a hint telling the engine that it depends on completion of the first task.

So this example would look like:

var gulp = require('gulp');

// takes in a callback so the engine knows when it'll be done
gulp.task('one', function (cb) {
    // do stuff -- async or otherwise
    fs.writeFile('filename', 'data', opts, function (err) {
        cb(err); // if err is not null and not undefined, the orchestration will stop, and 'two' will not run
    });
});

// identifies a dependent task must be complete before this one begins
gulp.task('two', ['one'], function() {
    // task 'one' is done now
});

gulp.task('default', ['one', 'two']);
// alternatively: gulp.task('default', ['two']);

Another example, which returns the stream instead of using a callback:

var gulp = require('gulp');
var del = require('del'); // rm -rf

gulp.task('clean', function() {
    return del(['output']);
});

gulp.task('templates', ['clean'], function() {
    var stream = gulp.src(['src/templates/*.hbs'])
        // do some concatenation, minification, etc.
        .pipe(gulp.dest('output/templates/'));
    return stream; // return the stream as the completion hint

});

gulp.task('styles', ['clean'], function() {
    var stream = gulp.src(['src/styles/app.less'])
        // do some hinting, minification, etc.
        .pipe(gulp.dest('output/css/app.css'));
    return stream;
});

gulp.task('build', ['templates', 'styles']);

// templates and styles will be processed in parallel.
// clean will be guaranteed to complete before either start.
// clean will not be run twice, even though it is called as a dependency twice.

gulp.task('default', ['build']);