From a47db0d60916fda04f7c2912a47722f046c239f3 Mon Sep 17 00:00:00 2001
From: Damien Elmes <gpg@ankiweb.net>
Date: Sun, 19 Jan 2020 10:05:37 +1000
Subject: [PATCH] add helper for background execution

---
 qt/aqt/main.py    |  2 ++
 qt/aqt/taskman.py | 77 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+)
 create mode 100644 qt/aqt/taskman.py

diff --git a/qt/aqt/main.py b/qt/aqt/main.py
index fdfa21672..69bb47da0 100644
--- a/qt/aqt/main.py
+++ b/qt/aqt/main.py
@@ -34,6 +34,7 @@ from aqt import gui_hooks
 from aqt.profiles import ProfileManager as ProfileManagerType
 from aqt.qt import *
 from aqt.qt import sip
+from aqt.taskman import TaskManager
 from aqt.utils import (
     askUser,
     checkInvalidFilename,
@@ -67,6 +68,7 @@ class AnkiQt(QMainWindow):
         self.state = "startup"
         self.opts = opts
         self.col: Optional[_Collection] = None
+        self.taskman = TaskManager()
         aqt.mw = self
         self.app = app
         self.pm = profileManager
diff --git a/qt/aqt/taskman.py b/qt/aqt/taskman.py
new file mode 100644
index 000000000..34325de51
--- /dev/null
+++ b/qt/aqt/taskman.py
@@ -0,0 +1,77 @@
+# Copyright: Ankitects Pty Ltd and contributors
+# License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
+
+"""
+Helper for running tasks on background threads.
+"""
+
+from concurrent.futures import Future
+from concurrent.futures.thread import ThreadPoolExecutor
+from dataclasses import dataclass
+from threading import Lock
+from typing import Any, Callable, Dict, List, Optional
+
+from PyQt5.QtCore import QObject, pyqtSignal
+
+
+@dataclass
+class PendingDoneCallback:
+    callback: Callable[[Any], None]
+    future: Future
+
+
+class TaskManager(QObject):
+    _results_available = pyqtSignal()
+
+    def __init__(self):
+        QObject.__init__(self)
+
+        self._executor = ThreadPoolExecutor()
+
+        self._pending_callbacks: List[PendingDoneCallback] = []
+
+        self._results_lock = Lock()
+        self._results_available.connect(self._drain_results)  # type: ignore
+
+    def run(
+        self,
+        task: Callable,
+        on_done: Optional[Callable],
+        args: Optional[Dict[str, Any]] = None,
+    ) -> Future:
+        """Run task on a background thread, calling on_done on the main thread if provided.
+
+        Args if provided will be passed on as keyword arguments to the task callable."""
+        if args is None:
+            args = {}
+
+        fut = self._executor.submit(task, **args)
+
+        if on_done is not None:
+
+            def done_closure(completed_future: Future) -> None:
+                self._handle_done_callback(completed_future, on_done)
+
+            fut.add_done_callback(done_closure)
+
+        return fut
+
+    def _handle_done_callback(self, future: Future, callback: Callable) -> None:
+        """When future completes, schedule its callback to run on the main thread."""
+        # add result to the queue
+        with self._results_lock:
+            self._pending_callbacks.append(
+                PendingDoneCallback(callback=callback, future=future)
+            )
+
+        # and tell the main thread to flush the queue
+        self._results_available.emit()  # type: ignore
+
+    def _drain_results(self):
+        """Fires pending callbacks in the main thread."""
+        with self._results_lock:
+            results = self._pending_callbacks
+            self._pending_callbacks = []
+
+        for result in results:
+            result.callback(result.future)