[Async Attachment Upload] Initial implementation

Signed-off-by: Sahil Kumar <[email protected]>

packages/stream_chat_flutter/lib/src/attachment/attachment_widget.dart

@@ -0,0 +1,77 @@
+import 'package:flutter/material.dart';
+import 'package:stream_chat_flutter_core/stream_chat_flutter_core.dart';
+
+import '../stream_chat_theme.dart';
+
+enum AttachmentSource {
+  local,
+  network,
+}
+
+extension AttachmentSourceX on AttachmentSource {
+  /// The [when] method is the equivalent to pattern matching.
+  /// Its prototype depends on the AttachmentSource defined.
+  T when<T>({
+    @required T Function() local,
+    @required T Function() network,
+  }) {
+    assert(() {
+      if (local == null || network == null) {
+        throw 'check for all possible cases';
+      }
+      return true;
+    }());
+    switch (this) {
+      case AttachmentSource.local:
+        return local();
+      case AttachmentSource.network:
+        return network();
+    }
+  }
+}
+
+abstract class AttachmentWidget extends StatelessWidget {
+  final Size size;
+  final Message message;
+  final Attachment attachment;
+  final AttachmentSource _source;
+
+  AttachmentSource get source => _source ?? attachment.file != null
+      ? AttachmentSource.local
+      : AttachmentSource.network;
+
+  const AttachmentWidget({
+    Key key,
+    @required this.message,
+    @required this.attachment,
+    this.size,
+    AttachmentSource source,
+  })  : _source = source,
+        super(key: key);
+}
+
+class AttachmentError extends StatelessWidget {
+  final Size size;
+
+  const AttachmentError({
+    Key key,
+    this.size,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Container(
+        width: size?.width,
+        height: size?.height,
+        color: StreamChatTheme.of(context).colorTheme.accentRed.withOpacity(.1),
+        child: Center(
+          child: Icon(
+            Icons.error_outline,
+            color: StreamChatTheme.of(context).colorTheme.black,
+          ),
+        ),
+      ),
+    );
+  }
+}