Move reusable code from mobile to common

This will be useful for a future desktop application.

common/lib/src/editors/instrument.dart

@@ -0,0 +1,97 @@
+import 'package:flutter/material.dart';
+import 'package:musicus_database/musicus_database.dart';
+
+import '../backend.dart';
+
+class InstrumentEditor extends StatefulWidget {
+  final Instrument instrument;
+
+  InstrumentEditor({
+    this.instrument,
+  });
+
+  @override
+  _InstrumentEditorState createState() => _InstrumentEditorState();
+}
+
+class _InstrumentEditorState extends State<InstrumentEditor> {
+  final nameController = TextEditingController();
+
+  bool uploading = false;
+
+  @override
+  void initState() {
+    super.initState();
+
+    if (widget.instrument != null) {
+      nameController.text = widget.instrument.name;
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final backend = MusicusBackend.of(context);
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Instrument/Role'),
+        actions: <Widget>[
+          uploading
+              ? Padding(
+                  padding: const EdgeInsets.all(16.0),
+                  child: Center(
+                    child: SizedBox(
+                      width: 24.0,
+                      height: 24.0,
+                      child: CircularProgressIndicator(
+                        strokeWidth: 2.0,
+                      ),
+                    ),
+                  ),
+                )
+              : FlatButton(
+                  child: Text('DONE'),
+                  onPressed: () async {
+                    setState(() {
+                      uploading = true;
+                    });
+
+                    final instrument = Instrument(
+                      id: widget.instrument?.id ?? generateId(),
+                      name: nameController.text,
+                    );
+
+                    final success =
+                        await backend.client.putInstrument(instrument);
+
+                    setState(() {
+                      uploading = false;
+                    });
+
+                    if (success) {
+                      Navigator.pop(context, instrument);
+                    } else {
+                      Scaffold.of(context).showSnackBar(SnackBar(
+                        content: Text('Failed to upload'),
+                      ));
+                    }
+                  },
+                ),
+        ],
+      ),
+      body: ListView(
+        children: <Widget>[
+          Padding(
+            padding: const EdgeInsets.all(16.0),
+            child: TextField(
+              controller: nameController,
+              decoration: InputDecoration(
+                labelText: 'Name',
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}