feat: Initial clean release of CHM

packages/app-mobile/services/BackButtonService.ts

@@ -0,0 +1,45 @@
+import { BackHandler, Platform } from 'react-native';
+
+export 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;
+
+		// On web, `BackHandler.addEventListener` fails with warning "BackHandler is not supported on web and should not be used."
+		if (Platform.OS !== 'web') {
+			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);
+	}
+}
+
+