Improved back button handling when note has been modified

ReactNativeClient/lib/services/back-button.js

@@ -0,0 +1,43 @@
+import { BackHandler } from 'react-native';
+
+class BackButtonService {
+
+	static initialize(defaultHandler) {
+		this.defaultHandler_ = defaultHandler;
+
+		BackHandler.addEventListener('hardwareBackPress', async () => {
+			return this.back();
+		});
+	}
+
+	static async back() {
+		if (this.handlers_.length) {
+			let r = await this.handlers_[this.handlers_.length - 1]();
+			if (r) return r;
+		}
+
+		return await this.defaultHandler_();
+	}
+
+	static addHandler(handler) {
+		for (let i = this.handlers_.length - 1; i >= 0; i--) {
+			const h = this.handlers_[i];
+			if (h === handler) return;
+		}
+
+		return this.handlers_.push(handler);
+	}
+
+	static removeHandler(hanlder) {
+		for (let i = this.handlers_.length - 1; i >= 0; i--) {
+			const h = this.handlers_[i];
+			if (h === hanlder) this.handlers_.splice(i, 1);
+		}
+	}
+
+}
+
+BackButtonService.defaultHandler_ = null;
+BackButtonService.handlers_ = [];
+
+export { BackButtonService };