Add recording selector

As a proof of concept some widgets were seperated. The home screen now
links to this instead of the recording editor.

lib/selectors/recording.dart

@@ -0,0 +1,172 @@
+import 'package:flutter/material.dart';
+
+import '../backend.dart';
+import '../database.dart';
+import '../editors/recording.dart';
+import '../widgets/works_by_composer.dart';
+import '../widgets/performance_row.dart';
+
+class RecordingsSelector extends StatefulWidget {
+  @override
+  _RecordingsSelectorState createState() => _RecordingsSelectorState();
+}
+
+class _RecordingsSelectorState extends State<RecordingsSelector> {
+  Person composer;
+  Work work;
+
+  @override
+  Widget build(BuildContext context) {
+    final backend = Backend.of(context);
+
+    bool showBackButton;
+    String titleText;
+    Widget content;
+
+    if (composer != null) {
+      showBackButton = true;
+      if (work != null) {
+        titleText = work.title;
+        content = StreamBuilder<List<Recording>>(
+          stream: backend.db.recordingsByWork(work.id).watch(),
+          builder: (context, snapshot) {
+            if (snapshot.hasData) {
+              return ListView.builder(
+                itemCount: snapshot.data.length,
+                itemBuilder: (context, index) {
+                  final recording = snapshot.data[index];
+                  return ListTile(
+                    title: StreamBuilder<List<Performance>>(
+                      stream: backend.db
+                          .performancesByRecording(recording.id)
+                          .watch(),
+                      builder: (context, snapshot) {
+                        if (snapshot.hasData) {
+                          return Column(
+                            mainAxisSize: MainAxisSize.min,
+                            children: <Widget>[
+                              for (final performance in snapshot.data)
+                                PerformanceRow(performance),
+                            ],
+                          );
+                        } else {
+                          return Container();
+                        }
+                      },
+                    ),
+                    onTap: () {
+                      Navigator.pop(context, recording);
+                    },
+                  );
+                },
+              );
+            } else {
+              return Container();
+            }
+          },
+        );
+      } else {
+        titleText = '${composer.firstName} ${composer.lastName}';
+        content = WorksByComposer(
+          personId: composer.id,
+          onTap: (selectedWork) {
+            setState(() {
+              work = selectedWork;
+            });
+          },
+        );
+      }
+    } else {
+      showBackButton = false;
+      titleText = 'Composers';
+
+      content = StreamBuilder<List<Person>>(
+        stream: backend.db.allPersons().watch(),
+        builder: (context, snapshot) {
+          if (snapshot.hasData) {
+            return ListView.builder(
+              itemCount: snapshot.data.length,
+              itemBuilder: (context, index) {
+                final person = snapshot.data[index];
+                return ListTile(
+                  title: Text('${person.lastName}, ${person.firstName}'),
+                  onTap: () {
+                    setState(() {
+                      composer = person;
+                    });
+                  },
+                );
+              },
+            );
+          } else {
+            return Container();
+          }
+        },
+      );
+    }
+
+    return WillPopScope(
+      child: Scaffold(
+        appBar: AppBar(
+          leading: IconButton(
+            icon: const Icon(Icons.close),
+            onPressed: () => Navigator.pop(context),
+          ),
+          title: Text('Select recording'),
+        ),
+        body: Column(
+          children: <Widget>[
+            Material(
+              elevation: 2.0,
+              child: ListTile(
+                leading: IconButton(
+                  icon: const Icon(Icons.arrow_back),
+                  onPressed: showBackButton ? goBack : null,
+                ),
+                title: Text(titleText),
+              ),
+            ),
+            Expanded(
+              child: content,
+            ),
+          ],
+        ),
+        floatingActionButton: FloatingActionButton(
+          child: const Icon(Icons.add),
+          onPressed: () async {
+            final recording = await Navigator.push(
+              context,
+              MaterialPageRoute(
+                builder: (context) => RecordingEditor(),
+                fullscreenDialog: true,
+              ),
+            );
+
+            if (recording != null) {
+              Navigator.pop(context, recording);
+            }
+          },
+        ),
+      ),
+      onWillPop: () => Future.value(goBack()),
+    );
+  }
+
+  bool goBack() {
+    if (work != null) {
+      setState(() {
+        work = null;
+      });
+
+      return false;
+    } else if (composer != null) {
+      setState(() {
+        composer = null;
+      });
+
+      return false;
+    } else {
+      return true;
+    }
+  }
+}