Docs: Add advanced section for creation of custom registries (#2479)

Co-authored-by: Blaine Bublitz <blaine.bublitz@gmail.com>

docs/advanced/creating-custom-registries.md

@@ -0,0 +1,199 @@
+<!-- front-matter
+id: creating-custom-registries
+title: Creating Custom Registries
+hide_title: true
+sidebar_label: Creating Custom Registries
+-->
+
+# Creating Custom Registries
+
+Allows custom registries to be plugged into the task system, which can provide shared tasks or augmented functionality. Registries are registered using [`registry()`](registry).
+
+## Structure
+
+In order to be accepted by gulp, custom registries must follow a specific format.
+
+```js
+// as a function
+function TestRegistry() {}
+
+TestRegistry.prototype.init = function (gulpInst) {}
+TestRegistry.prototype.get = function (name) {}
+TestRegistry.prototype.set = function (name, fn) {}
+TestRegistry.prototype.tasks = function () {}
+
+// as a class
+class TestRegistry {
+  init(gulpInst) {}
+
+  get(name) {}
+
+  set(name, fn) {}
+
+  tasks() {}
+}
+```
+
+If a registry instance passed to `registry()` doesn't have all four methods, an error will be thrown.
+
+## Registration
+
+If we want to register our example registry from above, we will need to pass an instance of it to `registry()`.
+
+```js
+const { registry } = require('gulp');
+
+// ... TestRegistry setup code
+
+// good!
+registry(new TestRegistry())
+
+// bad!
+registry(TestRegistry())
+// This will trigger an error: 'Custom registries must be instantiated, but it looks like you passed a constructor'
+```
+
+## Methods
+
+### `init(gulpInst)`
+
+The `init()` method of a registry is called at the very end of the `registry()` function. The gulp instance passed as the only argument (`gulpInst`) can be used to pre-define tasks using
+`gulpInst.task(taskName, fn)`.
+
+#### Parameters
+
+| parameter | type | note |
+|:---------:|:----:|------|
+| gulpInst | object | Instance of gulp. |
+
+### `get(name)`
+
+The `get()` method receives a task `name` for the custom registry to resolve and return, or `undefined` if no task with that name exists.
+
+#### Parameters
+
+| parameter | type | note |
+|:---------:|:----:|------|
+| name | string | Name of the task to be retrieved. |
+
+### `set(name, fn)`
+
+The `set()` method receives a task `name` and `fn`. This is called internally by `task()` to provide user-registered tasks to custom registries.
+
+#### Parameters
+
+| parameter | type | note |
+|:---------:|:----:|------|
+| name | string | Name of the task to be set. |
+| fn | function | Task function to be set. |
+
+### `tasks()`
+
+Must return an object listing all tasks in the registry.
+
+## Use Cases
+
+### Sharing Tasks
+
+To share common tasks with all your projects, you can expose an `init` method on the registry and it will receive the an instance of gulp as the only argument. You can then use `gulpInst.task(name, fn)` to register pre-defined tasks.
+
+For example, you might want to share a `clean` task:
+
+```js
+const fs = require('fs');
+const util = require('util');
+
+const DefaultRegistry = require('undertaker-registry');
+const del = require('del');
+
+function CommonRegistry(opts){
+  DefaultRegistry.call(this);
+
+  opts = opts || {};
+
+  this.buildDir = opts.buildDir || './build';
+}
+
+util.inherits(CommonRegistry, DefaultRegistry);
+
+CommonRegistry.prototype.init = function(gulpInst) {
+  const buildDir = this.buildDir;
+  const exists = fs.existsSync(buildDir);
+
+  if(exists){
+    throw new Error('Cannot initialize common tasks. ' + buildDir + ' directory exists.');
+  }
+
+  gulpInst.task('clean', function(){
+    return del([buildDir]);
+  });
+}
+
+module.exports = CommonRegistry;
+```
+
+Then to use it in a project:
+
+```js
+const { registry, series, task } = require('gulp');
+const CommonRegistry = require('myorg-common-tasks');
+
+registry(new CommonRegistry({ buildDir: '/dist' }));
+
+task('build', series('clean', function build(cb) {
+  // do things
+  cb();
+}));
+```
+
+### Sharing Functionality
+
+By controlling how tasks are added to the registry, you can decorate them.
+
+For example, if you wanted all tasks to share some data, you can use a custom registry to bind them to that data. Be sure to return the altered task, as per the description of registry methods above:
+
+```js
+const { registry, series, task } = require('gulp');
+const util = require('util');
+const DefaultRegistry = require('undertaker-registry');
+
+// Some task defined somewhere else
+const BuildRegistry = require('./build.js');
+const ServeRegistry = require('./serve.js');
+
+function ConfigRegistry(config){
+  DefaultRegistry.call(this);
+  this.config = config;
+}
+
+util.inherits(ConfigRegistry, DefaultRegistry);
+
+ConfigRegistry.prototype.set = function set(name, fn) {
+  // The `DefaultRegistry` uses `this._tasks` for storage.
+  var task = this._tasks[name] = fn.bind(this.config);
+  return task;
+};
+
+registry(new BuildRegistry());
+registry(new ServeRegistry());
+
+// `registry` will reset each task in the registry with
+// `ConfigRegistry.prototype.set` which will bind them to the config object.
+registry(new ConfigRegistry({
+  src: './src',
+  build: './build',
+  bindTo: '0.0.0.0:8888'
+}));
+
+task('default', series('clean', 'build', 'serve', function(cb) {
+  console.log('Server bind to ' + this.bindTo);
+  console.log('Serving' + this.build);
+  cb();
+}));
+```
+
+## Examples
+
+* [`undertaker-registry`](https://github.com/gulpjs/undertaker-registry): The Gulp 4 default registry.
+* [`undertaker-common-tasks`](https://github.com/gulpjs/undertaker-common-tasks): Proof-of-concept custom registry that pre-defines tasks.
+* [` undertaker-task-metadata`](https://github.com/gulpjs/undertaker-task-metadata): Proof-of-concept custom registry that attaches metadata to each task.

docs/api/registry.md

@@ -60,4 +60,4 @@ When a registry without an `init` method is passed as `registryInstance`, throws
 
 When a registry without a `tasks` method is passed as `registryInstance`, throws an error with the message, "Custom registry must have `tasks` function".
 
-[creating-custom-registries]: ../documentation-missing.md
+[creating-custom-registries]: ../advanced/creating-custom-registries.md