screen sharing for desktop. (#135)

* screen sharing for desktop (WIP).

* use git repo for testing.

* update.

* Add sourceId/frameRate to ScreenShareCaptureOptions.

* Compatible with flutter web.

* Rendering of camera and screenshare video tracks at same time.

* Remote screen sharing is shown first, and fixed sub/unsub button for screenshare widget.

* resolved conflict.

* modify flutter-webrtc to pub version.

* remove unused import.

* chore: Common params for Camera and ScreenShare.

lib/src/track/options.dart

@@ -1,6 +1,5 @@
-import 'dart:io';
-
 import 'package:flutter/foundation.dart' show kIsWeb;
+import 'package:flutter_webrtc/flutter_webrtc.dart';
 
 import '../track/local/audio.dart';
 import '../track/local/video.dart';
@@ -23,26 +22,22 @@ extension CameraPositionExt on CameraPosition {
 
 /// Options used when creating a [LocalVideoTrack] that captures the camera.
 class CameraCaptureOptions extends VideoCaptureOptions {
-  /// The deviceId of the capture device to use.
-  /// Available deviceIds can be obtained through `flutter_webrtc`:
-  /// <pre>
-  /// import 'package:flutter_webrtc/flutter_webrtc.dart' as rtc;
-  ///
-  /// List<MediaDeviceInfo> devices = await rtc.navigator.mediaDevices.enumerateDevices();
-  /// </pre>
-  final String? deviceId;
   final CameraPosition cameraPosition;
 
   const CameraCaptureOptions({
-    this.deviceId,
     this.cameraPosition = CameraPosition.front,
+    String? deviceId,
+    double? maxFrameRate,
     VideoParameters params = VideoParametersPresets.h540_169,
-  }) : super(params: params);
+  }) : super(params: params, deviceId: deviceId, maxFrameRate: maxFrameRate);
 
   CameraCaptureOptions.from({required VideoCaptureOptions captureOptions})
-      : deviceId = null,
-        cameraPosition = CameraPosition.front,
-        super(params: captureOptions.params);
+      : cameraPosition = CameraPosition.front,
+        super(
+          params: captureOptions.params,
+          deviceId: captureOptions.deviceId,
+          maxFrameRate: captureOptions.maxFrameRate,
+        );
 
   @override
   Map<String, dynamic> toMediaConstraintsMap() {
@@ -60,6 +55,9 @@ class CameraCaptureOptions extends VideoCaptureOptions {
         ];
       }
     }
+    if (maxFrameRate != null) {
+      constraints['frameRate'] = {'max': maxFrameRate};
+    }
     return constraints;
   }
 
@@ -67,21 +65,26 @@ class CameraCaptureOptions extends VideoCaptureOptions {
   CameraCaptureOptions copyWith({
     VideoParameters? params,
     CameraPosition? cameraPosition,
+    String? deviceId,
+    double? maxFrameRate,
   }) =>
       CameraCaptureOptions(
         params: params ?? this.params,
         cameraPosition: cameraPosition ?? this.cameraPosition,
+        deviceId: deviceId ?? this.deviceId,
+        maxFrameRate: maxFrameRate ?? this.maxFrameRate,
       );
 }
 
 /// Options used when creating a [LocalVideoTrack] that captures the screen.
 class ScreenShareCaptureOptions extends VideoCaptureOptions {
   final bool useiOSBroadcastExtension;
-
   const ScreenShareCaptureOptions({
     this.useiOSBroadcastExtension = false,
+    String? sourceId,
+    double? maxFrameRate,
     VideoParameters params = VideoParametersPresets.screenShareH720FPS15,
-  }) : super(params: params);
+  }) : super(params: params, deviceId: sourceId, maxFrameRate: maxFrameRate);
 
   ScreenShareCaptureOptions.from(
       {this.useiOSBroadcastExtension = false,
@@ -91,9 +94,17 @@ class ScreenShareCaptureOptions extends VideoCaptureOptions {
   @override
   Map<String, dynamic> toMediaConstraintsMap() {
     var constraints = super.toMediaConstraintsMap();
-    if (useiOSBroadcastExtension && Platform.isIOS) {
+    if (useiOSBroadcastExtension && WebRTC.platformIsIOS) {
       constraints['deviceId'] = 'broadcast';
     }
+    if (WebRTC.platformIsDesktop) {
+      if (deviceId != null) {
+        constraints['deviceId'] = {'exact': deviceId};
+      }
+      if (maxFrameRate != 0.0) {
+        constraints['mandatory'] = {'frameRate': maxFrameRate};
+      }
+    }
     return constraints;
   }
 }
@@ -111,8 +122,24 @@ abstract class VideoCaptureOptions extends LocalTrackOptions {
   // final LocalVideoTrackType type;
   final VideoParameters params;
 
+  /// The deviceId of the capture device to use.
+  /// Available deviceIds can be obtained through `flutter_webrtc`:
+  /// <pre>
+  /// import 'package:flutter_webrtc/flutter_webrtc.dart' as rtc;
+  ///
+  /// List<MediaDeviceInfo> devices = await rtc.navigator.mediaDevices.enumerateDevices();
+  /// // or
+  /// List<DesktopCapturerSource> desktopSources = await rtc.desktopCapturer.getSources(types: [rtc.SourceType.Screen, rtc.SourceType.Window]);
+  /// </pre>
+  final String? deviceId;
+
+  // Limit the maximum frameRate of the capture device.
+  final double? maxFrameRate;
+
   const VideoCaptureOptions({
     this.params = VideoParametersPresets.h540_169,
+    this.deviceId,
+    this.maxFrameRate,
   });
 
   @override