add threadheader

example/lib/custom_theme.dart

@@ -104,6 +104,9 @@ class ThreadPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: ThreadHeader(
+        parent: parent,
+      ),
       body: Column(
         children: <Widget>[
           Expanded(

example/lib/threads.dart

@@ -88,6 +88,9 @@ class ThreadPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      appBar: ThreadHeader(
+        parent: parent,
+      ),
       body: Column(
         children: <Widget>[
           Expanded(

lib/src/thread_header.dart

@@ -0,0 +1,87 @@
+import 'package:flutter/material.dart';
+import 'package:stream_chat/stream_chat.dart';
+import 'package:stream_chat_flutter/src/stream_chat_theme.dart';
+
+import 'stream_channel.dart';
+
+class ThreadHeader extends StatelessWidget implements PreferredSizeWidget {
+  final bool showBackButton;
+  final VoidCallback onBackPressed;
+  final Message parent;
+
+  ThreadHeader({
+    Key key,
+    @required this.parent,
+    this.showBackButton = true,
+    this.onBackPressed,
+  })  : preferredSize = Size.fromHeight(kToolbarHeight),
+        super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    final channel = StreamChannel.of(context).channel;
+    return AppBar(
+      automaticallyImplyLeading: false,
+      elevation: 1,
+      backgroundColor:
+          StreamChatTheme.of(context).channelTheme.channelHeaderTheme.color,
+      actions: <Widget>[
+        Container(
+          child: showBackButton ? _buildBackButton(context) : Container(),
+        ),
+      ],
+      centerTitle: false,
+      title: Text.rich(
+        TextSpan(
+          text: 'Thread',
+          children: [
+            TextSpan(
+              text:
+                  '   ${parent.replyCount} ${parent.replyCount == 1 ? 'reply' : 'replies'}',
+              style: StreamChatTheme.of(context)
+                  .channelTheme
+                  .channelHeaderTheme
+                  .lastMessageAt,
+            ),
+          ],
+        ),
+        style:
+            StreamChatTheme.of(context).channelTheme.channelHeaderTheme.title,
+      ),
+    );
+  }
+
+  Padding _buildBackButton(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsets.all(14.0),
+      child: AspectRatio(
+        aspectRatio: 1,
+        child: RawMaterialButton(
+          shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(4)),
+          elevation: 0,
+          highlightElevation: 0,
+          focusElevation: 0,
+          disabledElevation: 0,
+          hoverElevation: 0,
+          onPressed: () {
+            if (onBackPressed != null) {
+              onBackPressed();
+            } else {
+              Navigator.of(context).pop();
+            }
+          },
+          fillColor: Colors.black.withOpacity(.1),
+          padding: EdgeInsets.all(4),
+          child: Icon(
+            Icons.close,
+            size: 15,
+            color: Colors.black,
+          ),
+        ),
+      ),
+    );
+  }
+
+  @override
+  final Size preferredSize;
+}

lib/stream_chat_flutter.dart

@@ -11,3 +11,4 @@ export 'src/message_widget.dart';
 export 'src/stream_channel.dart';
 export 'src/stream_chat.dart';
 export 'src/stream_chat_theme.dart';
+export 'src/thread_header.dart';