added new overlays and docs

packages/stream_chat_flutter/lib/src/overlays.dart

@@ -1,49 +1,66 @@
 import 'package:flutter/material.dart';
 
+/// Class to create an overlay anchored to a child widget
 class AnchoredOverlay extends StatelessWidget {
-  final bool showOverlay;
-  final Widget Function(BuildContext, Offset anchor) overlayBuilder;
-  final Widget child;
-
-  AnchoredOverlay({
+  /// Constructor for creating an [AnchoredOverlay]
+  const AnchoredOverlay({
+    Key? key,
     this.showOverlay = false,
     required this.overlayBuilder,
     required this.child,
-  });
+  }) : super(key: key);
+
+  /// Show or hide overlay
+  final bool showOverlay;
+
+  /// Builder for creating overlay widget
+  final Widget Function(BuildContext, Offset anchor) overlayBuilder;
+
+  /// Child to which overlay is anchored
+  final Widget child;
 
   @override
-  Widget build(BuildContext context) {
-    return new Container(
-      child: new LayoutBuilder(
-          builder: (BuildContext context, BoxConstraints constraints) {
-        return new OverlayBuilder(
-          showOverlay: showOverlay,
-          overlayBuilder: (BuildContext overlayContext) {
-            RenderBox box = context.findRenderObject() as RenderBox;
-            final center =
-                box.size.center(box.localToGlobal(const Offset(0.0, 0.0)));
-            return overlayBuilder(overlayContext, center);
-          },
-          child: child,
-        );
-      }),
-    );
-  }
+  Widget build(BuildContext context) => SizedBox(
+        child: LayoutBuilder(
+            builder: (BuildContext context, BoxConstraints constraints) =>
+                OverlayBuilder(
+                  showOverlay: showOverlay,
+                  overlayBuilder: (BuildContext overlayContext) {
+                    final box = context.findRenderObject() as RenderBox?;
+                    if (box != null) {
+                      final center = box.size
+                          .center(box.localToGlobal(const Offset(0, 0)));
+                      return overlayBuilder(overlayContext, center);
+                    } else {
+                      return const SizedBox();
+                    }
+                  },
+                  child: child,
+                )),
+      );
 }
 
+/// Class for creating a declarative-ish overlay
 class OverlayBuilder extends StatefulWidget {
-  final bool showOverlay;
-  final Widget Function(BuildContext) overlayBuilder;
-  final Widget child;
-
-  OverlayBuilder({
+  /// Constructor for creating an [OverlayBuilder]
+  const OverlayBuilder({
+    Key? key,
     this.showOverlay = false,
     required this.overlayBuilder,
     required this.child,
-  });
+  }) : super(key: key);
+
+  /// Show or hide overlay
+  final bool showOverlay;
+
+  /// Builder for creating overlay widget
+  final Widget Function(BuildContext) overlayBuilder;
+
+  /// Child to which overlay is anchored
+  final Widget child;
 
   @override
-  _OverlayBuilderState createState() => new _OverlayBuilderState();
+  _OverlayBuilderState createState() => _OverlayBuilderState();
 }
 
 class _OverlayBuilderState extends State<OverlayBuilder> {
@@ -112,31 +129,35 @@ class _OverlayBuilderState extends State<OverlayBuilder> {
   }
 
   @override
-  Widget build(BuildContext context) {
-    return widget.child;
-  }
+  Widget build(BuildContext context) => widget.child;
 }
 
+/// Class to position Overlay
 class CenterAbout extends StatelessWidget {
-  final Offset position;
-  final Widget child;
-
-  CenterAbout({
+  /// Constructor for creating an [CenterAbout]
+  const CenterAbout({
+    Key? key,
     required this.position,
     required this.child,
-  });
+  }) : super(key: key);
+
+  /// Position of child
+  final Offset position;
+
+  /// Child widget to which overlay is attached
+  final Widget child;
 
   @override
   Widget build(BuildContext context) {
-    var size = MediaQuery.of(context).size;
+    final size = MediaQuery.of(context).size;
 
     return Positioned(
       bottom: size.height - position.dy,
       left: position.dx,
       child: FractionalTranslation(
-        translation: const Offset(-0.5, 0.0),
+        translation: const Offset(-0.5, 0),
         child: Padding(
-          padding: const EdgeInsets.only(bottom: 32.0),
+          padding: const EdgeInsets.only(bottom: 32),
           child: child,
         ),
       ),