[camera_web] Add buildPreview implementation (#4190)

Author: bselwe

packages/camera/camera_web/example/integration_test/camera_test.dart

@@ -507,6 +507,24 @@ void main() {
       });
     });
 
+    group('getViewType', () {
+      testWidgets('returns a correct view type', (tester) async {
+        const textureId = 1;
+
+        final camera = Camera(
+          textureId: textureId,
+          window: window,
+        );
+
+        await camera.initialize();
+
+        expect(
+          camera.getViewType(),
+          equals('plugins.flutter.io/camera_$textureId'),
+        );
+      });
+    });
+
     group('dispose', () {
       testWidgets('resets the video element\'s source', (tester) async {
         final camera = Camera(

packages/camera/camera_web/example/integration_test/camera_web_test.dart

@@ -11,6 +11,7 @@ import 'package:camera_web/camera_web.dart';
 import 'package:camera_web/src/camera.dart';
 import 'package:camera_web/src/camera_settings.dart';
 import 'package:camera_web/src/types/types.dart';
+import 'package:flutter/widgets.dart' as widgets;
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:integration_test/integration_test.dart';
@@ -634,10 +635,24 @@ void main() {
       );
     });
 
-    testWidgets('buildPreview throws UnimplementedError', (tester) async {
+    testWidgets(
+        'buildPreview returns an HtmlElementView '
+        'with an appropriate view type', (tester) async {
+      final camera = Camera(
+        textureId: cameraId,
+        window: window,
+      );
+
+      // Save the camera in the camera plugin.
+      (CameraPlatform.instance as CameraPlugin).cameras[cameraId] = camera;
+
       expect(
-        () => CameraPlatform.instance.buildPreview(cameraId),
-        throwsUnimplementedError,
+        CameraPlatform.instance.buildPreview(cameraId),
+        isA<widgets.HtmlElementView>().having(
+          (view) => view.viewType,
+          'viewType',
+          camera.getViewType(),
+        ),
       );
     });
 

packages/camera/camera_web/lib/src/camera.dart

@@ -196,6 +196,9 @@ class Camera {
     }
   }
 
+  /// Returns the registered view type of the camera.
+  String getViewType() => _getViewType(textureId);
+
   /// Disposes the camera by stopping the camera stream
   /// and reloading the camera source.
   void dispose() {

packages/camera/camera_web/lib/src/camera_web.dart

@@ -361,7 +361,9 @@ class CameraPlugin extends CameraPlatform {
 
   @override
   Widget buildPreview(int cameraId) {
-    throw UnimplementedError('buildPreview() is not implemented.');
+    return HtmlElementView(
+      viewType: getCamera(cameraId).getViewType(),
+    );
   }
 
   @override