npm package: @kesplus/tapable
Description: Just a little module for plugins.
Author: Tobias Koppers @sokra
Latest version: 2.2.0

# Tapable

The tapable package expose many Hook classes, which can be used to create hooks for plugins.

``` javascript
const {
	SyncHook,
	SyncBailHook,
	SyncWaterfallHook,
	SyncLoopHook,
	AsyncParallelHook,
	AsyncParallelBailHook,
	AsyncSeriesHook,
	AsyncSeriesBailHook,
	AsyncSeriesWaterfallHook
 } = require("tapable");
```

## Installation

``` shell
npm install --save tapable
```

## Usage

All Hook constructors take one optional argument, which is a list of argument names as strings.

``` js
const hook = new SyncHook(["arg1", "arg2", "arg3"]);
```

The best practice is to expose all hooks of a class in a `hooks` property:

``` js
class Car {
	constructor() {
		this.hooks = {
			accelerate: new SyncHook(["newSpeed"]),
			brake: new SyncHook(),
			calculateRoutes: new AsyncParallelHook(["source", "target", "routesList"])
		};
	}

	/* ... */
}
```

Other people can now use these hooks:

``` js
const myCar = new Car();

// Use the tap method to add a consument
myCar.hooks.brake.tap("WarningLampPlugin", () => warningLamp.on());
```

It's required to pass a name to identify the plugin/reason.

You may receive arguments:

``` js
myCar.hooks.accelerate.tap("LoggerPlugin", newSpeed => console.log(`Accelerating to ${newSpeed}`));
```

For sync hooks, `tap` is the only valid method to add a plugin. Async hooks also support async plugins:

``` js
myCar.hooks.calculateRoutes.tapPromise("GoogleMapsPlugin", (source, target, routesList) => {
	// return a promise
	return google.maps.findRoute(source, target).then(route => {
		routesList.add(route);
	});
});
myCar.hooks.calculateRoutes.tapAsync("BingMapsPlugin", (source, target, routesList, callback) => {
	bing.findRoute(source, target, (err, route) => {
		if(err) return callback(err);
		routesList.add(route);
		// call the callback
		callback();
	});
});

// You can still use sync plugins
myCar.hooks.calculateRoutes.tap("CachedRoutesPlugin", (source, target, routesList) => {
	const cachedRoute = cache.get(source, target);
	if(cachedRoute)
		routesList.add(cachedRoute);
})
```
The class declaring these hooks need to call them:

``` js
class Car {
	/**
	  * You won't get returned value from SyncHook or AsyncParallelHook,
	  * to do that, use SyncWaterfallHook and AsyncSeriesWaterfallHook respectively
	 **/

	setSpeed(newSpeed) {
		// following call returns undefined even when you returned values
		this.hooks.accelerate.call(newSpeed);
	}

	useNavigationSystemPromise(source, target) {
		const routesList = new List();
		return this.hooks.calculateRoutes.promise(source, target, routesList).then((res) => {
			// res is undefined for AsyncParallelHook
			return routesList.getRoutes();
		});
	}

	useNavigationSystemAsync(source, target, callback) {
		const routesList = new List();
		this.hooks.calculateRoutes.callAsync(source, target, routesList, err => {
			if(err) return callback(err);
			callback(null, routesList.getRoutes());
		});
	}
}
```

The Hook will compile a method with the most efficient way of running your plugins. It generates code depending on:
* The number of registered plugins (none, one, many)
* The kind of registered plugins (sync, async, promise)
* The used call method (sync, async, promise)
* The number of arguments
* Whether interception is used

This ensures fastest possible execution.

## Hook types

Each hook can be tapped with one or several functions. How they are executed depends on the hook type:

* Basic hook (without “Waterfall”, “Bail” or “Loop” in its name). This hook simply calls every function it tapped in a row.

* __Waterfall__. A waterfall hook also calls each tapped function in a row. Unlike the basic hook, it passes a return value from each function to the next function.

* __Bail__. A bail hook allows exiting early. When any of the tapped function returns anything, the bail hook will stop executing the remaining ones.

* __Loop__. When a plugin in a loop hook returns a non-undefined value the hook will restart from the first plugin. It will loop until all plugins return undefined.

Additionally, hooks can be synchronous or asynchronous. To reflect this, there’re “Sync”, “AsyncSeries”, and “AsyncParallel” hook classes:

* __Sync__. A sync hook can only be tapped with synchronous functions (using `myHook.tap()`).

* __AsyncSeries__. An async-series hook can be tapped with synchronous, callback-based and promise-based functions (using `myHook.tap()`, `myHook.tapAsync()` and `myHook.tapPromise()`). They call each async method in a row.

* __AsyncParallel__. An async-parallel hook can also be tapped with synchronous, callback-based and promise-based functions (using `myHook.tap()`, `myHook.tapAsync()` and `myHook.tapPromise()`). However, they run each async method in parallel.

The hook type is reflected in its class name. E.g., `AsyncSeriesWaterfallHook` allows asynchronous functions and runs them in series, passing each function’s return value into the next function.


## Interception

All Hooks offer an additional interception API:

``` js
myCar.hooks.calculateRoutes.intercept({
	call: (source, target, routesList) => {
		console.log("Starting to calculate routes");
	},
	register: (tapInfo) => {
		// tapInfo = { type: "promise", name: "GoogleMapsPlugin", fn: ... }
		console.log(`${tapInfo.name} is doing its job`);
		return tapInfo; // may return a new tapInfo object
	}
})
```

**call**: `(...args) => void` Adding `call` to your interceptor will trigger when hooks are triggered. You have access to the hooks arguments.

**tap**: `(tap: Tap) => void` Adding `tap` to your interceptor will trigger when a plugin taps into a hook. Provided is the `Tap` object. `Tap` object can't be changed.

**loop**: `(...args) => void` Adding `loop` to your interceptor will trigger for each loop of a looping hook.

**register**: `(tap: Tap) => Tap | undefined` Adding `register` to your interceptor will trigger for each added `Tap` and allows to modify it.

## Context

Plugins and interceptors can opt-in to access an optional `context` object, which can be used to pass arbitrary values to subsequent plugins and interceptors.

``` js
myCar.hooks.accelerate.intercept({
	context: true,
	tap: (context, tapInfo) => {
		// tapInfo = { type: "sync", name: "NoisePlugin", fn: ... }
		console.log(`${tapInfo.name} is doing it's job`);

		// `context` starts as an empty object if at least one plugin uses `context: true`.
		// If no plugins use `context: true`, then `context` is undefined.
		if (context) {
			// Arbitrary properties can be added to `context`, which plugins can then access.
			context.hasMuffler = true;
		}
	}
});

myCar.hooks.accelerate.tap({
	name: "NoisePlugin",
	context: true
}, (context, newSpeed) => {
	if (context && context.hasMuffler) {
		console.log("Silence...");
	} else {
		console.log("Vroom!");
	}
});
```

## HookMap

A HookMap is a helper class for a Map with Hooks

``` js
const keyedHook = new HookMap(key => new SyncHook(["arg"]))
```

``` js
keyedHook.for("some-key").tap("MyPlugin", (arg) => { /* ... */ });
keyedHook.for("some-key").tapAsync("MyPlugin", (arg, callback) => { /* ... */ });
keyedHook.for("some-key").tapPromise("MyPlugin", (arg) => { /* ... */ });
```

``` js
const hook = keyedHook.get("some-key");
if(hook !== undefined) {
	hook.callAsync("arg", err => { /* ... */ });
}
```

## Hook/HookMap interface

Public:

``` ts
interface Hook {
	tap: (name: string | Tap, fn: (context?, ...args) => Result) => void,
	tapAsync: (name: string | Tap, fn: (context?, ...args, callback: (err, result: Result) => void) => void) => void,
	tapPromise: (name: string | Tap, fn: (context?, ...args) => Promise<Result>) => void,
	intercept: (interceptor: HookInterceptor) => void
}

interface HookInterceptor {
	call: (context?, ...args) => void,
	loop: (context?, ...args) => void,
	tap: (context?, tap: Tap) => void,
	register: (tap: Tap) => Tap,
	context: boolean
}

interface HookMap {
	for: (key: any) => Hook,
	intercept: (interceptor: HookMapInterceptor) => void
}

interface HookMapInterceptor {
	factory: (key: any, hook: Hook) => Hook
}

interface Tap {
	name: string,
	type: string
	fn: Function,
	stage: number,
	context: boolean,
	before?: string | Array
}
```

Protected (only for the class containing the hook):

``` ts
interface Hook {
	isUsed: () => boolean,
	call: (...args) => Result,
	promise: (...args) => Promise<Result>,
	callAsync: (...args, callback: (err, result: Result) => void) => void,
}

interface HookMap {
	get: (key: any) => Hook | undefined,
	for: (key: any) => Hook
}
```

## MultiHook

A helper Hook-like class to redirect taps to multiple other hooks:

``` js
const { MultiHook } = require("tapable");

this.hooks.allHooks = new MultiHook([this.hooks.hookA, this.hooks.hookB]);
```