Better use of async functions for API

- Use local_preferences for storing data

lib/api.dart

@@ -8,6 +8,8 @@ import 'package:http/http.dart' as http;
 
 class InvenTreeAPI {
 
+  static const _URL_GET_TOKEN = "user/token/";
+
   // Ensure we only ever create a single instance of the API class
   static final InvenTreeAPI _api = new InvenTreeAPI._internal();
 
@@ -56,6 +58,11 @@ class InvenTreeAPI {
 
   // Construct an API URL
   String _makeUrl(String url) {
+
+    if (url.startsWith('/')) {
+      url = url.substring(1);
+    }
+
     return path.join(_base_url, url);
   }
 
@@ -65,50 +72,113 @@ class InvenTreeAPI {
 
   // Request the raw /api/ endpoing to see if there is an InvenTree server listening
   void _tryConnection() async {
-    final response = get("");
+
+    print("Testing connection to server");
+
+    final response = await get("").then((http.Response response) {
+      print("response!");
+      print(response.body);
+    }).catchError((error) {
+      print("Error trying connection");
+    });
+
+    // TODO - Add timeout handler
   }
 
   // Request an API token from the server.
   // A valid username/password combination must be provided
   void _secureToken() async {
-
     if (_token.isNotEmpty) {
       print("Discarding old token - " + _token);
     }
     _token = "";
 
-    var _url = _makeUrl("user/token/");
-    final response = await http.post(_url,
+    var response = post(_URL_GET_TOKEN,
         body: {
           "username": _username,
-          "password": _password
-      });
+          "password": _password,
+        });
 
-    if (response.statusCode != 200) {
-      print("Invalid status code:" + String.fromCharCode(response.statusCode));
-    } else {
-      var _json = json.decode(response.body);
+    response.then((http.Response response) {
+      if (response.statusCode != 200) {
+        print("Invalid status code: " + response.statusCode.toString());
+      } else {
+        var _json = json.decode(response.body);
 
-      if (_json["token"] != null) {
-        _token = _json["token"];
-        print("Received token: " + _token);
+        if (_json["token"] != null) {
+          _token = _json["token"];
+          print("Received token: " + _token);
+        }
       }
-    }
+    }).catchError((error) {
+      print("Error retrieving token");
+    });
   }
 
-  Future<http.Response> get(String url) async {
-    var _url = _makeUrl(url);
-    final response = await http.get(_url,
-      headers: {
-          HttpHeaders.authorizationHeader: "Token: " + _token
-        }
-    );
+  // Perform a PATCH request
+  Future<http.Response> patch(String url, {Map<String, String> body}) async {
 
-    print("Making request to " + _url);
-    print(response.statusCode);
-    print(response.body);
+    var _url = _makeUrl(url);
+    var _headers = _defaultHeaders();
+    var _body = Map<String, String>();
+
+    // Copy across provided data
+    body.forEach((K, V) => _body[K] = V);
+
+    print("PATCH: " + _url);
+
+    final response = await http.patch(_url,
+      headers: _headers,
+      body: _body,
+    );
 
     return response;
   }
 
+  // Perform a POST request
+  Future<http.Response> post(String url, {Map<String, String> body}) async {
+
+    var _url = _makeUrl(url);
+    var _headers = _defaultHeaders();
+    var _body = Map<String, String>();
+
+    // Copy across provided data
+    body.forEach((K, V) => _body[K] = V);
+
+    print("POST: " + _url);
+
+    final response = await http.post(_url,
+      headers: _headers,
+      body: _body,
+    );
+
+    return response;
+  }
+
+  // Perform a GET request
+  Future<http.Response> get(String url) async {
+
+    var _url = _makeUrl(url);
+    var _headers = _defaultHeaders();
+
+    print("GET: " + _url);
+
+    final response = await http.get(_url,
+      headers: _headers,
+    );
+
+    return response;
+  }
+
+  Map<String, String> _defaultHeaders() {
+
+    var headers = Map<String, String>();
+
+    if (_token.isNotEmpty) {
+      headers[HttpHeaders.authorizationHeader] = "Token " + _token;
+    }
+
+    return headers;
+  }
+
 }