Reducer-assistant NPM

reducer-assistant

Library to manage side effects and async logic in applications using redux for state management. If you use redux but you don't like a complexity of sush solutions as redux-saga or redux-observable, you can manage your side effects with classes now.

Getting started

Assistants are managers of async effects for reducers. Inside an assistant you can fetch data, start timers or make any other async operations, you can subscribe to state changes and action dispatching. Also you can get a current state or dispatch an action during any async operation.

To start using assistants you should configure a redux store with an AssistantsMiddleware.

import { createStore, applyMiddleware } from 'redux';
import { createAssistantMiddleware } from 'reducer-assistant/redux';

const assistantMiddleware = createAssistantMiddleware();

const store = createStore(reducer, applyMiddleware(assistantMiddleware));

Now you can create an assistant. This is an example of an assistant which starts fetching data after the FETCH_START action, and dispatches the FETCH_SUCCESS or FETCH_ERROR actions when the request is complete.

import { Assistant } from 'reducer-assistant';

/**
 * To create an assistant you should create a class which extends a base Assistant class
 */
export class FetchDataAssistant extends Assistant {
	/**
	 * onInit runs when an assistant is applied to the store
	 *
	 * You can configure required listeners or dispatch some actions here
	 */
	onInit() {
		/** add 'FETCH_START' action listener */
		this.afterAction('FETCH_START', this.startFetch);
	}

	startFetch = async (action) => {
		try {
			const requestResult = await fetch(action.url, {
				/** you have access to the state from any method of assistant */
				...this.state.someData,
			});

			/** you can dispatch actions from any method of assistant */
			this.dispatch({ type: 'FETCH_SUCCESS', payload: requestResult });
		} catch (e) {
			this.dispatch({ type: 'FETCH_ERROR', payload: e });
		}
	};
}

Now you can connect the assistant to the store via a applyAssistants method of AssistantMiddleware.

import { FetchDataAssistant } from './fetchDataAssistant';

...

assistantMiddleware.applyAssistants([FetchDataAssistant]);

After that a new FetchDataAssistant instance will be created and it will start to listen to the FETCH_START actions to make a request. Now you can dispatch the FETCH_START action anywhere in your application to fetch data.

store.dispatch({ type: 'FETCH_START', url: 'https://github.com' });

Basic features (How to)

Perform some operation when an assistant starts to work

When an assistant instance is created, it is not completely ready to work yet. It should be connected to store first. So you must not use any methods of the base Assistant inside your assistant's constructor. Use them inside the onInit method or after it. It runs almost immediately after the constructor.

class MyAssistant extends Assistant {
	constructor() {
		super();
		/** Do not use any base Assistant methods here */
	}

	onInit() {
		/** Here you can perform any operations */
	}
}

Subscribe to actions

You can use beforeAction and afterAction methods to perform some operation when actions are dispatched.

class MyAssistant extends Assistant {
	onInit() {
		/** Subscribe to any action */
		this.afterAction((action) => {
			console.log('action', action);
		});

		/** Subscribe to an action of type 'MY_ACTION' */
		this.afterAction('MY_ACTION', (action) => {
			console.log('action', action);
		});
	}
}

You can use an action creator function instead of a string as action type. In this case the action creator must have a toString method or contain a type field which return type of action as string.

const createAction = () => ({ type: 'MY_ACTION' });

createAction.toString = () => 'MY_ACTION';
// or
createAction.type = 'MY_ACTION';

class MyAssistant extends Assistant {
	onInit() {
		this.afterAction(createAction, (action) => {
			console.log('action', action);
		});
	}
}

The beforeAction and afterAction methods return a function to unsubscribe these events.

Subscribe to state changes

You can use onChange method to perform some operation when state is changed.

class MyAssistant extends Assistant {
	onInit() {
		this.onChange((prevState) => {
			console.log('previous state', prevState);
			console.log('new state', this.state);
		});
	}
}

The onChange method returns a function to unsubscribe this event.

Access to state

You have access to current state inside any method of assistant via the state getter. You can not use it before the onInit method call.

class MyAssistant extends Assistant {
	onInit() {
		console.log('current state', this.state);
	}
}

Dispatch actions

You can dispatch any actions inside any method of assistant via the dispatch method. You can not use it before the onInit method call.

class MyAssistant extends Assistant {
	onInit() {
		this.dispatch({ type: 'MY_ACTION' });
	}
}

Connect assistants to a store

An assistant should be connected to a store to start to work. You can do it via applyAssistants method of an AssistantMiddleware. The applyAssistants method receives an array of assistant constructors or assistant configs (see Assistant config).

import { createStore, applyMiddleware } from 'redux';
import { createAssistantMiddleware } from 'reducer-assistant/redux';
import { Assistant } from 'reducer-assistant';

const assistantMiddleware = createAssistantMiddleware();

const store = createStore(reducer, applyMiddleware(assistantMiddleware));

class MyAssistant extends Assistant {
	/* ...*/
}

assistantMiddleware.applyAssistants([MyAssistant]);

You can invoke the applyAssistants method many times. All previous assistants will be destroyed.

Connect assistants to a store inside other assistants

You can start some assistant inside another assistant via the createAssistant method.

class AnotherAssistant extends Assistant {
	/* ... */
}

class MyAssistant extends Assistant {
	onInit() {
		const anotherAssistantInstance = this.createAssistant(AnotherAssistant);
	}
}

Pass parameters to assistants

Assistant configs passed to applyAssistants or createAssistant methods must not require any parameters. If they need some parameters, they should be configured before applyAssistants or createAssistant calls.

Consider some assistant requires the outer url parameter.

You can create a function which returns a new assistant class.

function createAssistant(url) {
	return class FetchAssistant extends Assistant {
		onInit() {
			fetch(url).then(/* ... */);
		}
	};
}

middleware.applyAssistants([createAssistant(url)]);

Or you can create an assistant with a parameter in constructor and a function which returns a AssistantConfig (see Assistant config) configured with an url.

class FetchAssistant extends Assistant {
	constructor(url) {
		super();
		this.url = url;
	}

	/** ... */
}

function getAssistantConfig(url) {
	return {
		create: () => new FetchAssistant(url);
	}
}

middleware.applyAssistants([getAssistantConfig(url)]);

Connect assistants to a part of a store

Usually reducers are responsible only for a part of a page state. If you write an assistant for such reducer, you would like the assistant to be responsible for the same part of the state.

Consider you have a timer reducer which returns a number. You can create an assistant which expect its state to be also a number.

const timerReducer = (state: number, action): number => {
	/* ... */
};

class TimerAssistant extends Assistant<number> {}

Then you can set the timerReducer to work with a timer field of the page state.

const rootReducer = combineReducers({
	/* ... */
	timer: timerReducer,
});

And you can set the TimerAssistant to work with a timer field of the page state to via the ofStatePart function. See Assistant config for details.

import { ofStatePart } from 'reducer-assistant';

middleware.applyAssistants([ofStatePart('timer', TimerAssistant)]);

Stop assistants and remove any subscription

If you need to stop an assistant for some reason you can use the destroy method of assistants.

class MyAssistant extends Assistant {
	childAssistant = null;

	onInit() {
		this.childAssistant = this.createAssistant(ChildAssistant);

		this.afterAction('DESTROY', () => {
			/** destroy the current assistant instance */
			this.destroy();
		});

		this.afterAction('DESTROY_CHILD', () => {
			this.childAssistant.destroy();
		});
	}
}

When your assistant is being destroyed there is no need to unsubscribe to any base assistant events such as afterAction, onChange or destroy child assistants. But if you have subscribed to any extenral resource you should unsubscribe to them before your assistant is destroyed. You can do it inside the onDestroy method.

Assistant API

To create assistant you should create new class which extends base Assistant class.

import { Assistant } from 'reducer-assistant';

class MyAssistant extends Assistant {}

Assistant's constructor has no parameters. If you need any parameter you can create them.

import { Assistant } from 'reducer-assistant';

class MyAssistant extends Assistant {
	constructor(params) {
		super();

		this.someValue = params.someValue;
	}
}

To find out how to pass parameters to assistant's constructor see Assistant parameters section.

You CAN'T use any inherited methods or properties of base class in constructor. Use them inside onInit or after it.

The base Assistant class has the following method and properties:

state

The state getter returns a current state.

dispatch

The dispatch dispatches an action.

onInit

You can override this method to add listeners, dispatch action, start fetching data, start timers, create child assistants etc.

class MyAssistant extends Assistant {
	onInit() {
		this.afterAction('SOME_ACTION', (action) => {
			console.log(action);
			console.log(state);
		});
	}
}

onChange

Listeners passed to onChange will be invoked when the state has been changed after any action. An assistant can watch to changes of the whole state or some part of state. To see how to watch to a part of state see Assistant config section. A function passed to onChange receives a previous state as a parameter.

class MyAssistant extends Assistant {
	onInit() {
		this.onChange((prevState) => {
			console.log('previous state', prevState);
			console.log('new state', this.state);
		});
	}
}

The onChange method return a function to unsubscribe events.

class MyAssistant extends Assistant {
	onInit() {
		/** add onChange listener */
		const unsubscribe = this.onChange(() => {
			console.log('new state action', this.state);
		});

		/** remove listener  */
		this.afterAction('UNSUBSCRIBE', () => {
			unsubscribe();
		});
	}
}

afterAction

The afterAction listeners will be invoked when an action has been dispatched and the state has been changed. You can add listener to some specific action or to any action.

class MyAssistant extends Assistant {
	onInit() {
		/** runs after any action */
		this.afterAction((action) => {
			/*...*/
		});
		/** runs after the 'ACTION1' action */
		this.afterAction('ACTION1', (action) => {
			/*...*/
		});
	}
}

An afterAction listener receives an action as a parameter.

The afterAction method return a function to unsubscribe events.

beforeAction

The beforeAction method works as the afterAction. But its listeners will be invoked before state changes.

createAssistant

You can run assistants dynamically from other assistants in any time inside and after onInit and before onDestroy. The createAssistant method receives an AssistantConfig and returns a created assistant instance.

class MyAssistant extends Assistant {
	onInit() {
		this.createAssistant(ChildAssistant);
	}
}

applyAssistant

applyAssistant method receives an existing instance of assistant and connects it to the store. You can connect any assistant to a store only once.

import { SecondAssistant } from './second';

class MyAssistant extends Assistant {
	onInit() {
		const secondAssistant = new SecondAssistant();
		this.createAssistant(secondAssistant);
	}
}

If you need to connect the assistant to a part of the store you can use the aecond argument of the applyAssistant method.

type User = {
	/*...*/
};

// this assistant can be connected to a state of 'User' type
class UserAssistant extends Assistant<User> {}

type PageState = {
	user: User;
};

class PageAssistant extends Assistant<PageState> {
	onInit() {
		const userAssistant = new UserAssistant();
		// you should connect userAssistant to the 'user' field of the page state
		this.createAssistant(userAssistant, 'user');
		// or
		this.createAssistant(userAssistant, (pageState) => pageState.user);
	}
}

destroy

Via the destroy method you can stop an current assistant or a child assistant.

class MyAssistant extends Assistant {
	childAssistant = null;

	onInit() {
		this.childAssistant = this.createAssistant(ChildAssistant);

		this.afterAction('DESTROY', () => {
			this.destroy();
		});

		this.afterAction('DESTROY_CHILD', () => {
			this.childAssistant.destroy();
		});
	}
}

onDestroy

The onDestroy function is invoked before an assistant is destroyed. For example, if you add listeners to any events of DOM objects you can remove them in this method.

class ClickAssistant extends Assistant {
	onInit() {
		document
			.getElementById('myDIV')
			.addEventListener('click', this.onClick);
	}

	onDestroy() {
		document
			.getElementById('myDIV')
			.removeEventListener('click', this.onClick);
	}

	onClick = () => {
		this.dispatch({ type: 'CLICK' });
	};
}

There is no need to remove listeners of the base assistant class events such as onChange or afterAction or destroy child assistants. They will be removed automatically.

Assistant config

To create assistants you can use the applyAssistants method of a middleware or the createAssistant method of an assistant. They receives AssistantConfig values.

The simplified AssistantConfig type has the following form:

type AssistantConfig =
	| { new (): Assistant }
	| {
			Constructor: { new (): Assistant };
			select?: (fullstate: any) => any;
	  }
	| {
			create: () => Assistant;
			select?: (fullstate: any) => any;
	  };

Constructor

The simplest version of AssistantConfigs is an assistant's constructor.

class MyAssistant extends Assustant {}

middleware.applyAssistants([MyAssistant]);

Constructor with select

Sometimes you may need to create an assistant which manages some part of state. Such assistants can be reusable and independent of any other part of state.

type TimerState = {
	value: number;
};

class TimerAssistant extends Assistant<TimerState> {
	private intervalId;

	onInit() {
		this.intervalId = setInterval(() => {
			this.dispatch({ type: 'INCREMENT', payload: this.state.value + 1 });
		});
	}

	onDestroy() {
		clearInterval(this.intervalId);
	}
}

The TimerAssistant class expects the state property returns a value of type {value: number}. But state of a page reducer can be different and can contain the timer's value in any field;

type PageState = {
	timer: {value: number};
	....
}

When you create an assistant you can specify a part of state which will be managed by the assistant via select function of AssistantConfig.

middleware.applyAssistants([
	{
		Constructor: TimerAssistant,
		/** select part of the PageState for TimerAssistant */
		select: (fullstate) => fullstate.timer,
	},
]); // instead of middleware.applyAssistants([TimerAssistant])

Now the state property of a TimerAssistant's instance will return the timer field value of the page state. And listeners of the onChange event will be invoked only after the timer field changed.

select in child assistant config

When you create an assistant inside another assistant, the child assistant receives the parent state by default. You can pass an AssistantConfig with select function to the createAssistant method if you want the child assistant to manage some part of the parent state.

class PageAssistant extends Assistant<PageState> {
	onInit() {
		this.createAssistant({
			Constructor: TimerAssistant,
			select: (fullstate) => fullstate.timer,
		}); // instead of this.createAssistant(TimerAssistant)
	}
}

ofStatePart

To simplify creation of AssistantConfig with select there is the ofStatePart function.

import { ofStatePart } from 'reducer-assistant';

/** all these calls are equal */

middleware.applyAssistants([
	{
		Constructor: TimerAssistant,
		/** select part of the PageState for TimerAssistant */
		select: (fullstate) => fullstate.timer,
	},
]);

middleware.applyAssistants([
	ofStatePart((fullstate) => fullstate.timer, TimerAssistant),
]);

middleware.applyAssistants([ofStatePart('timer', TimerAssistant)]);

The first parameter of the ofStatePart is a select function or a field name of a whole state. The second parameter is an AssistantConfig.

There is another version of the ofStatePart. You can pass an array of configs to it. Then the ofStatePart returns an array too.

import { ofStatePart } from 'reducer-assistant';

middleware.applyAssistants(
	ofStatePart(
		(fullstate) => fullstate.timer,
		[Assistant1, Assistant2, ...]
	),
);

// or

middleware.applyAssistants(
	ofStatePart(
		'timer',
		[Assistant1, Assistant2, ...]
	)
);

The ofStatePart can be invoked many times.

ofStatePart(
	'field1',
	ofStatePart(
		'field2',
		ofStatePart(
			'timer',
			[Assistant1, Assistant2, ...]
		)
	)
)

Assistant config with `create` function

Another form of AssistantConfig is the object with a create method instead of Constructor. A create method should return a new instance of Assistant. The following examples are equal.

middleware.applyAssistants([
	{
		Constructor: TimerAssistant,
		select: (fullstate) => fullstate.timer,
	},
]);

middleware.applyAssistants([
	{
		create: () => new TimerAssistant(),
		select: (fullstate) => fullstate.timer,
	},
]);

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