Add simple python method QgsTask.fromFunction for creation of tasks

from a function without having to create a QgsTask subclass

python/core/__init__.py

@@ -183,3 +183,56 @@ def __exit__(self, ex_type, ex_value, traceback):
         else:
             self.layer.rollBack()
             return False
+
+
+class QgsTaskException(Exception):
+
+    def __init__(self, msg):
+        self.msg = msg
+
+    def __str__(self):
+        return self.msg
+
+
+class QgsTaskResult(Exception):
+
+    def __init__(self, r):
+        self.r = r
+
+    def result(self):
+        return self.r
+
+
+class QgsTaskWrapper(QgsTask):
+
+    def __init__(self, description, function, *extraArgs):
+        QgsTask.__init__(self, description, QgsTask.ProgressReport)
+        self.extraArgs = extraArgs
+        self.function = function
+        self.task_result = None
+        self.task_error = None
+
+    def run(self):
+        try:
+            for status in self.function(*self.extraArgs):
+                self.setProgress(status)
+        except QgsTaskException as e:
+            self.task_error = e.msg
+            self.stopped()
+        except QgsTaskResult as r:
+            self.task_result = r.result()
+            self.completed()
+        else:
+            self.completed()
+
+    def result(self):
+        return self.task_result
+
+    def error(self):
+        return self.task_error
+
+
+def fromFunction(cls, description, function, extraArgs):
+    return QgsTaskWrapper(description, function, extraArgs)
+
+QgsTask.fromFunction = classmethod(fromFunction)