Add basic files selector

This required to add a platform channel with a method to get the
toplevel storage devices. The app will need to request the
WRITE_EXTERNAL_STORAGE permission later. Also the dependency on "path"
was introduced.

lib/selectors/files.dart

@@ -0,0 +1,181 @@
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:path/path.dart' as path;
+
+class FilesSelector extends StatefulWidget {
+  @override
+  _FilesSelectorState createState() => _FilesSelectorState();
+}
+
+class _FilesSelectorState extends State<FilesSelector> {
+  static const platform = MethodChannel('de.johrpan.musicus/platform');
+
+  List<Directory> storageRoots = [];
+  List<Directory> directories = [];
+  List<FileSystemEntity> contents = [];
+  Set<String> selectedPaths = {};
+
+  @override
+  void initState() {
+    super.initState();
+
+    platform.invokeListMethod<String>('getStorageRoots').then((sr) {
+      setState(() {
+        storageRoots = sr.map((path) => Directory(path)).toList();
+      });
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    Widget body;
+
+    if (directories.isEmpty) {
+      if (storageRoots != null) {
+        body = ListView(
+          children: storageRoots
+              .map((dir) => ListTile(
+                    leading: const Icon(Icons.storage),
+                    title: Text(dir.path),
+                    onTap: () {
+                      setState(() {
+                        directories.add(dir);
+                      });
+
+                      openDirectory(dir);
+                    },
+                  ))
+              .toList(),
+        );
+      } else {
+        body = Container();
+      }
+    } else {
+      if (contents != null) {
+        body = ListView(
+          children: contents.map((fse) {
+            Widget result;
+
+            if (fse is Directory) {
+              result = ListTile(
+                leading: const Icon(Icons.folder),
+                title: Text(path.basename(fse.path)),
+                onTap: () {
+                  setState(() {
+                    directories.add(fse);
+                  });
+
+                  openDirectory(fse);
+                },
+              );
+            } else if (fse is File) {
+              result = CheckboxListTile(
+                value: selectedPaths.contains(fse.path),
+                secondary: Icon(Icons.insert_drive_file),
+                title: Text(path.basename(fse.path)),
+                onChanged: (selected) {
+                  setState(() {
+                    if (selected) {
+                      selectedPaths.add(fse.path);
+                    } else {
+                      selectedPaths.remove(fse.path);
+                    }
+                  });
+                },
+              );
+            }
+
+            return result;
+          }).toList(),
+        );
+      } else {
+        body = Container();
+      }
+    }
+
+    return WillPopScope(
+      child: Scaffold(
+        appBar: AppBar(
+          title: Text('Choose files'),
+          actions: <Widget>[
+            FlatButton(
+              child: Text('DONE'),
+              onPressed: () {
+                Navigator.pop(context, selectedPaths);
+              },
+            ),
+          ],
+        ),
+        body: Column(
+          children: <Widget>[
+            Material(
+              elevation: 2.0,
+              child: ListTile(
+                leading: directories.isNotEmpty ? IconButton(
+                  icon: const Icon(Icons.arrow_upward),
+                  onPressed: up,
+                ) : null,
+                title: Text(directories.isEmpty
+                    ? 'Storage devices'
+                    : directories.last.path),
+              ),
+            ),
+            Expanded(
+              child: body,
+            ),
+          ],
+        ),
+      ),
+      onWillPop: () {
+        if (directories.isNotEmpty) {
+          up();
+          return Future.value(false);
+        } else {
+          return Future.value(true);
+        }
+      },
+    );
+  }
+
+  Future<void> openDirectory(Directory directory) async {
+    setState(() {
+      contents.clear();
+    });
+
+    final fses = await directory.list().toList();
+    fses.sort((fse1, fse2) {
+      int compareBasenames() =>
+          path.basename(fse1.path).compareTo(path.basename(fse2.path));
+
+      if (fse1 is Directory) {
+        if (fse2 is Directory) {
+          return compareBasenames();
+        } else {
+          return -1;
+        }
+      } else if (fse2 is Directory) {
+        return 1;
+      } else {
+        return compareBasenames();
+      }
+    });
+
+    setState(() {
+      contents = fses;
+    });
+  }
+
+  void up() {
+    if (directories.isNotEmpty) {
+      setState(() {
+        directories.removeLast();
+      });
+
+      if (directories.isNotEmpty) {
+        openDirectory(directories.last);
+      }
+    }
+  }
+}