joplin/packages/app-mobile/services/BackButtonService.ts

import { BackHandler } from 'react-native';

type BackButtonHandler = ()=> boolean|Promise<boolean>;

export default class BackButtonService {
	private static handlers_: BackButtonHandler[] = [];
	private static defaultHandler_: BackButtonHandler;

	public static initialize(defaultHandler: BackButtonHandler) {
		this.defaultHandler_ = defaultHandler;

		BackHandler.addEventListener('hardwareBackPress', () => {
			void this.back();
			return true;
		});
	}

	public static async back() {
		if (this.handlers_.length) {
			const r = await this.handlers_[this.handlers_.length - 1]();
			if (r) return r;
		}

		return await this.defaultHandler_();
	}

	public static addHandler(handler: BackButtonHandler) {
		for (let i = this.handlers_.length - 1; i >= 0; i--) {
			const h = this.handlers_[i];
			if (h === handler) return false;
		}

		this.handlers_.push(handler);
		return true;
	}

	public static removeHandler(handler: BackButtonHandler) {
		this.handlers_ = this.handlers_.filter(h => h !== handler);
	}
}