[RFR] Add an alert preventing data loss when a user closes the app while in optimistic mode

packages/ra-core/src/sideEffect/admin.js

@@ -9,6 +9,7 @@ import redirection from './redirection';
 import accumulate from './accumulate';
 import refresh from './refresh';
 import undo from './undo';
+import unload from './unload';
 
 /**
  * @param {Object} dataProvider A Data Provider function
@@ -26,5 +27,6 @@ export default (dataProvider, authProvider, i18nProvider) =>
             refresh(),
             notification(),
             callback(),
+            unload(),
         ]);
     };

packages/ra-core/src/sideEffect/index.js

@@ -9,6 +9,7 @@ import accumulateSaga from './accumulate';
 import refreshSaga from './refresh';
 import i18nSaga from './i18n';
 import undoSaga from './undo';
+import unloadSaga from './unload';
 
 export {
     adminSaga,
@@ -22,4 +23,5 @@ export {
     refreshSaga,
     i18nSaga,
     undoSaga,
+    unloadSaga,
 };

packages/ra-core/src/sideEffect/unload.ts

@@ -0,0 +1,44 @@
+import { takeEvery } from 'redux-saga/effects';
+
+import {
+    START_OPTIMISTIC_MODE,
+    STOP_OPTIMISTIC_MODE,
+} from '../actions/undoActions';
+
+/**
+ * Unload saga
+ *
+ * When a user closes the browser window while an optimistic update/delete
+ * hasn't been sent to the dataProvider yet, warn them that their edits
+ * may be lost.
+ *
+ * To achieve that, this saga registers a window event handler on the
+ * 'beforeunload' event when entering the optimistic mode, and removes
+ * the event when quitting the optimistic mode.
+ */
+export default function* watchUnload() {
+    yield takeEvery(START_OPTIMISTIC_MODE, handleStartOptimistic);
+    yield takeEvery(STOP_OPTIMISTIC_MODE, handleStopOptimisticMode);
+}
+
+const eventListener = event => {
+    event.preventDefault(); // standard
+    event.returnValue = ''; // Chrome
+    return 'Your latest modifications are not yet sent to the server. Are you sure?'; // Old IE
+};
+
+export function* handleStartOptimistic() {
+    // SSR escape hatch
+    if (!window) {
+        return;
+    }
+    window.addEventListener('beforeunload', eventListener);
+}
+
+export function* handleStopOptimisticMode() {
+    // SSR escape hatch
+    if (!window) {
+        return;
+    }
+    window.removeEventListener('beforeunload', eventListener);
+}