[webview_flutter_wkwebview] Add a listener for the canGoBack property change on the iOS platform. (flutter#8203)

Added a listener callback for the canGoBack property on iOS. Since it can only be effectively implemented on iOS, it is only exposed in the WKWebView layer.

Fixes flutter/flutter#158160

Author: StanleyCocos

packages/webview_flutter/webview_flutter_wkwebview/CHANGELOG.md

@@ -1,5 +1,7 @@
-## NEXT
+## 3.17.0
 
+* Adds a change listener for the `canGoBack` property. See
+  `WebKitWebViewController.setOnCanGoBackChange`.
 * Updates minimum supported SDK version to Flutter 3.22/Dart 3.4.
 
 ## 3.16.3

packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart

@@ -160,6 +160,14 @@ class WebKitWebViewController extends PlatformWebViewController {
       },
     );
 
+    _webView.addObserver(
+      _webView,
+      keyPath: 'canGoBack',
+      options: <NSKeyValueObservingOptions>{
+        NSKeyValueObservingOptions.newValue,
+      },
+    );
+
     final WeakReference<WebKitWebViewController> weakThis =
         WeakReference<WebKitWebViewController>(this);
     _uiDelegate = _webKitParams.webKitProxy.createUIDelegate(
@@ -299,6 +307,12 @@ class WebKitWebViewController extends PlatformWebViewController {
               final NSUrl? url = change[NSKeyValueChangeKey.newValue] as NSUrl?;
               urlChangeCallback(UrlChange(url: await url?.getAbsoluteString()));
             }
+          case 'canGoBack':
+            if (controller._onCanGoBackChangeCallback != null) {
+              final bool canGoBack =
+                  change[NSKeyValueChangeKey.newValue]! as bool;
+              controller._onCanGoBackChangeCallback!(canGoBack);
+            }
         }
       };
     }),
@@ -315,6 +329,7 @@ class WebKitWebViewController extends PlatformWebViewController {
   bool _zoomEnabled = true;
   WebKitNavigationDelegate? _currentNavigationDelegate;
 
+  void Function(bool)? _onCanGoBackChangeCallback;
   void Function(JavaScriptConsoleMessage)? _onConsoleMessageCallback;
   void Function(PlatformWebViewPermissionRequest)? _onPermissionRequestCallback;
 
@@ -602,6 +617,12 @@ class WebKitWebViewController extends PlatformWebViewController {
         .addUserScript(userScript);
   }
 
+  /// Sets the listener for canGoBack changes.
+  Future<void> setOnCanGoBackChange(
+      void Function(bool) onCanGoBackChangeCallback) async {
+    _onCanGoBackChangeCallback = onCanGoBackChangeCallback;
+  }
+
   /// Sets a callback that notifies the host application of any log messages
   /// written to the JavaScript console.
   ///

packages/webview_flutter/webview_flutter_wkwebview/pubspec.yaml

@@ -2,7 +2,7 @@ name: webview_flutter_wkwebview
 description: A Flutter plugin that provides a WebView widget based on Apple's WKWebView control.
 repository: https://github.com/flutter/packages/tree/main/packages/webview_flutter/webview_flutter_wkwebview
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+webview%22
-version: 3.16.3
+version: 3.17.0
 
 environment:
   sdk: ^3.5.0

packages/webview_flutter/webview_flutter_wkwebview/test/webkit_webview_controller_test.dart

@@ -1550,6 +1550,53 @@ window.addEventListener("error", function(e) {
       });
     });
 
+    test('setOnCanGoBackChange', () async {
+      final MockWKWebViewIOS mockWebView = MockWKWebViewIOS();
+
+      late final void Function(
+        String keyPath,
+        NSObject object,
+        Map<NSKeyValueChangeKey, Object?> change,
+      ) webViewObserveValue;
+
+      final WebKitWebViewController controller = createControllerWithMocks(
+        createMockWebView: (
+          _, {
+          void Function(
+            String keyPath,
+            NSObject object,
+            Map<NSKeyValueChangeKey, Object?> change,
+          )? observeValue,
+        }) {
+          webViewObserveValue = observeValue!;
+          return mockWebView;
+        },
+      );
+
+      verify(
+        mockWebView.addObserver(
+          mockWebView,
+          keyPath: 'canGoBack',
+          options: <NSKeyValueObservingOptions>{
+            NSKeyValueObservingOptions.newValue,
+          },
+        ),
+      );
+
+      late final bool callbackCanGoBack;
+
+      await controller.setOnCanGoBackChange(
+          (bool canGoBack) => callbackCanGoBack = canGoBack);
+
+      webViewObserveValue(
+        'canGoBack',
+        mockWebView,
+        <NSKeyValueChangeKey, Object?>{NSKeyValueChangeKey.newValue: true},
+      );
+
+      expect(callbackCanGoBack, true);
+    });
+
     test('setOnScrollPositionChange', () async {
       final WebKitWebViewController controller = createControllerWithMocks();