Add KWindowStateSaver

This is basically the C++ counter-part to
https://invent.kde.org/frameworks/kconfig/-/merge_requests/94
and allows to easily retrofit window size persistence on existing windows/
dialogs, replacing e.g. code like
https://invent.kde.org/pim/pimcommon/-/blob/master/src/pimcommon/widgets/kpimprintpreviewdialog.cpp.

This is a bit more complicated than one might expect, as KWindowConfig
works with QWindows, but that's something freshly created QWidget windows/
dialogs don't have yet. Additionally, we are in a library here that doesn't
depend on Qt::Widgets. To overcome this we move the widget-dependent code
(basically just a call to QWidget::windowHandle()) to inline template code
(and thus into the consumer), use std::function's type erasure to pass it
into the library code, and an event filter on the widget to wait for the
QWindow to become available.

1 files changed, 136 insertions, 0 deletions

diff --git a/src/gui/kwindowstatesaver.cpp b/src/gui/kwindowstatesaver.cpp
new file mode 100644
index 00000000..d3eaa99e
--- /dev/null
+++ b/src/gui/kwindowstatesaver.cpp
@@ -0,0 +1,136 @@
+/*
+    SPDX-FileCopyrightText: 2022 Volker Krause <vkrause@kde.org>
+    SPDX-License-Identifier: LGPL-2.0-or-later
+*/
+
+#include "kwindowstatesaver.h"
+#include "ksharedconfig.h"
+#include "kwindowconfig.h"
+
+#include <QWindow>
+
+class KWindowStateSaverPrivate
+{
+public:
+    QWindow *window = nullptr;
+    KConfigGroup configGroup;
+    std::function<QWindow *()> windowHandleCallback;
+    int timerId = 0;
+
+    void init(KWindowStateSaver *q);
+    void initWidget(QObject *widget, KWindowStateSaver *q);
+};
+
+void KWindowStateSaverPrivate::init(KWindowStateSaver *q)
+{
+    if (!window) {
+        return;
+    }
+
+    KWindowConfig::restoreWindowSize(window, configGroup);
+    KWindowConfig::restoreWindowPosition(window, configGroup);
+
+    const auto deferredSave = [q, this]() {
+        if (!timerId) {
+            timerId = q->startTimer(250);
+        }
+    };
+    QObject::connect(window, &QWindow::widthChanged, q, deferredSave);
+    QObject::connect(window, &QWindow::heightChanged, q, deferredSave);
+    QObject::connect(window, &QWindow::xChanged, q, deferredSave);
+    QObject::connect(window, &QWindow::yChanged, q, deferredSave);
+}
+
+void KWindowStateSaverPrivate::initWidget(QObject *widget, KWindowStateSaver *q)
+{
+    if (!window && windowHandleCallback) {
+        window = windowHandleCallback();
+    }
+    if (window) {
+        init(q);
+    } else {
+        widget->installEventFilter(q);
+    }
+}
+
+KWindowStateSaver::KWindowStateSaver(QWindow *window, const KConfigGroup &configGroup)
+    : QObject(window)
+    , d(new KWindowStateSaverPrivate)
+{
+    Q_ASSERT(window);
+    d->window = window;
+    d->configGroup = configGroup;
+    d->init(this);
+}
+
+KWindowStateSaver::KWindowStateSaver(QWindow *window, const QString &configGroupName)
+    : QObject(window)
+    , d(new KWindowStateSaverPrivate)
+{
+    Q_ASSERT(window);
+    d->window = window;
+    d->configGroup = KConfigGroup(KSharedConfig::openStateConfig(), configGroupName);
+    d->init(this);
+}
+
+KWindowStateSaver::KWindowStateSaver(QWindow *window, const char *configGroupName)
+    : QObject(window)
+    , d(new KWindowStateSaverPrivate)
+{
+    Q_ASSERT(window);
+    d->window = window;
+    d->configGroup = KConfigGroup(KSharedConfig::openStateConfig(), configGroupName);
+    d->init(this);
+}
+
+KWindowStateSaver::~KWindowStateSaver()
+{
+    delete d;
+}
+
+void KWindowStateSaver::timerEvent(QTimerEvent *event)
+{
+    killTimer(event->timerId());
+    KWindowConfig::saveWindowPosition(d->window, d->configGroup);
+    KWindowConfig::saveWindowSize(d->window, d->configGroup);
+    d->timerId = 0;
+}
+
+bool KWindowStateSaver::eventFilter(QObject *watched, QEvent *event)
+{
+    // QEvent::PlatformSurface would give us a valid window, but if there are
+    // intial resizings (explicitly or via layout constraints) those would then
+    // already overwrite our restored values. So wait until all that is done
+    // and only restore afterwards.
+    if (event->type() == QEvent::ShowToParent && !d->window) {
+        watched->removeEventFilter(this);
+        d->window = d->windowHandleCallback();
+        d->init(this);
+    }
+
+    return QObject::eventFilter(watched, event);
+}
+
+void KWindowStateSaver::initWidget(QObject *widget, const std::function<QWindow *()> &windowHandleCallback, const KConfigGroup &configGroup)
+{
+    d = new KWindowStateSaverPrivate;
+    d->windowHandleCallback = windowHandleCallback;
+    d->configGroup = configGroup;
+    d->initWidget(widget, this);
+}
+
+void KWindowStateSaver::initWidget(QObject *widget, const std::function<QWindow *()> &windowHandleCallback, const QString &configGroupName)
+{
+    d = new KWindowStateSaverPrivate;
+    d->windowHandleCallback = windowHandleCallback;
+    d->configGroup = KConfigGroup(KSharedConfig::openStateConfig(), configGroupName);
+    d->initWidget(widget, this);
+}
+
+void KWindowStateSaver::initWidget(QObject *widget, const std::function<QWindow *()> &windowHandleCallback, const char *configGroupName)
+{
+    d = new KWindowStateSaverPrivate;
+    d->windowHandleCallback = windowHandleCallback;
+    d->configGroup = KConfigGroup(KSharedConfig::openStateConfig(), configGroupName);
+    d->initWidget(widget, this);
+}