# Recipes

 * [Prevent pipe breaking on errors](#prevent-pipe-breaking-on-errors)
 * [Starting tasks on events](#starting-tasks-on-events)
 * [Filtering custom events](#filtering-custom-events)
 * [Incremental build](#incremental-build)

### Prevent pipe breaking on errors

> Until gulp 4.0 is released this is actual information

When you pipe one Stream to another and do not attaching `on('error')` handler they will `unpipe` on every error. This is frustrating, when you have watcher and something like `coffeescript` builder. It is pretty easy to put typo in file and breake pipeline forever. To avoid this, you can use [`gulp-plumber`](https://github.com/floatdrop/gulp-plumber):

```js
var gulp = require('gulp');
var watch = require('gulp-watch');
var plumber = require('gulp-plumber');
var sass = require('gulp-ruby-sass');

gulp.task('styles', function () {
    return gulp.src('scss/*.scss')
        .pipe(watch('scss/*.scss'))
        .pipe(plumber())
        .pipe(sass())
        .pipe(gulp.dest('dist'));
});
```

### Starting tasks on events

Often you want just to launch some tasks (like `build`) when something happened to watched files. You can pass plain callback, that will be called on every event or wrap it in `gulp-batch` to run it once:

```js
var gulp = require('gulp');
var watch = require('gulp-watch');
var batch = require('gulp-batch');

gulp.task('build', function () { console.log('Working!'); });

gulp.task('watch', function () {
    watch('**/*.js', batch(function (events, done) {
        gulp.start('build', done);
    }));
});
```

### Filtering custom events

When you want to make actions only on specific events, you can use [`gulp-filter`](https://github.com/sindresorhus/gulp-filter) and the `event` attribute, which is added to all files that were `add`, `change` or `unlink` (per [`chokidar`'s documentation](https://github.com/paulmillr/chokidar#events)):

```js
var filter = require('gulp-filter');

function isAdded(file) {
    return file.event === 'add';
}

var filterAdded = filter(isAdded);

gulp.task('default', function () {
    return gulp.src('**/*.js')
        .pipe(watch('**/*.js'))
        .pipe(filterAdded)
        .pipe(gulp.dest('newfiles'))
        .pipe(filterAdded.restore())
        .pipe(gulp.dest('oldfiles'));
});
```

**Notice:** `event` property is not added to files that were emitted by `emitOnGlob` and `emit: 'all'` options, only to files that actually caused the event.

### Incremental build

One of the nice features, that can be achieved with `gulp-watch` - is incremental build.
When you want to build all files at start and then get only changed files - you can use these snippets:

```js
gulp.task('default', function () {
    return gulp.src('**/*.js')
        .pipe(watch('**/*.js'))
        .pipe(gulp.dest('build'));
});
```