[camera]add error handling in example, and update readme

Author: hellohuanlin

packages/camera/camera/README.md

@@ -80,6 +80,25 @@ void didChangeAppLifecycleState(AppLifecycleState state) {
 }
 ```
 
+### Handling camera access permissions
+
+Permission errors may be thrown when initializing the camera controller, and you are expected to handle them properly.
+
+Here is a list of all permission errors that can be thrown:
+
+- CameraAccessDenied
+Thrown when user denied the permission.
+
+- CameraAccessDeniedWithoutPrompt
+Thrown when user previously denied the permission.
+iOS only for now. iOS does not allow prompting alert dialog a second time. Users will have to go to Settings > Privacy in order to enable camera access.
+
+- CameraAccessRestricted
+Thrown when camera access is restricted and users cannot grant permission (parental control). iOS only for now.
+
+- cameraPermission
+A legacy error code for all kinds of camera permission errors. Currently used by Android and Web.
+
 ### Example
 
 Here is a small example flutter app displaying a full screen camera preview.
@@ -119,6 +138,15 @@ class _CameraAppState extends State<CameraApp> {
         return;
       }
       setState(() {});
+    }).catchError((e) {
+      switch (e.code) {
+        case 'CameraAccessDenied':
+          print('user denied camera access');
+          break;
+        default:
+          print('other errors');
+          break;
+      }
     });
   }
 

packages/camera/camera/example/lib/main.dart

@@ -685,7 +685,26 @@ class _CameraExampleHomeState extends State<CameraExampleHome>
             .then((double value) => _minAvailableZoom = value),
       ]);
     } on CameraException catch (e) {
-      _showCameraException(e);
+      switch (e.code) {
+        case 'CameraAccessDenied':
+          showInSnackBar('You have denied camera access.');
+          break;
+        case 'CameraAccessDeniedWithoutPrompt':
+          // iOS only
+          showInSnackBar('Please go to Settings app to enable camera access.');
+          break;
+        case 'CameraAccessRestricted':
+          // iOS only
+          showInSnackBar('Camera access is restricted.');
+          break;
+        case 'cameraPermission':
+          // Android & web only
+          showInSnackBar('Legacy error code for all kinds of permission errors');
+          break;
+        default:
+          _showCameraException(e);
+          break;
+      }
     }
 
     if (mounted) {