Userland: Rename LibThread => LibThreading

Also rename the "LibThread" namespace to "Threading"

Userland/Libraries/LibThreading/BackgroundAction.h

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2019-2020, Sergey Bugaev <bugaevc@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/Optional.h>
+#include <AK/Queue.h>
+#include <LibCore/Event.h>
+#include <LibCore/EventLoop.h>
+#include <LibCore/Object.h>
+#include <LibThreading/Lock.h>
+#include <LibThreading/Thread.h>
+
+namespace Threading {
+
+template<typename Result>
+class BackgroundAction;
+
+class BackgroundActionBase {
+    template<typename Result>
+    friend class BackgroundAction;
+
+private:
+    BackgroundActionBase() { }
+
+    static Lockable<Queue<Function<void()>>>& all_actions();
+    static Thread& background_thread();
+};
+
+template<typename Result>
+class BackgroundAction final : public Core::Object
+    , private BackgroundActionBase {
+    C_OBJECT(BackgroundAction);
+
+public:
+    static NonnullRefPtr<BackgroundAction<Result>> create(
+        Function<Result()> action,
+        Function<void(Result)> on_complete = nullptr)
+    {
+        return adopt_ref(*new BackgroundAction(move(action), move(on_complete)));
+    }
+
+    virtual ~BackgroundAction() { }
+
+private:
+    BackgroundAction(Function<Result()> action, Function<void(Result)> on_complete)
+        : Core::Object(&background_thread())
+        , m_action(move(action))
+        , m_on_complete(move(on_complete))
+    {
+        Locker locker(all_actions().lock());
+
+        all_actions().resource().enqueue([this] {
+            m_result = m_action();
+            if (m_on_complete) {
+                Core::EventLoop::current().post_event(*this, make<Core::DeferredInvocationEvent>([this](auto&) {
+                    m_on_complete(m_result.release_value());
+                    this->remove_from_parent();
+                }));
+                Core::EventLoop::wake();
+            } else {
+                this->remove_from_parent();
+            }
+        });
+    }
+
+    Function<Result()> m_action;
+    Function<void(Result)> m_on_complete;
+    Optional<Result> m_result;
+};
+
+}