Save previous floating layout when deleting FloatingWindow

FloatingWindow's are deleted all the time but their layouts
are interesting to restore dockwidgets to floating position

For issue #513

src/core/DockWidget.cpp

@@ -24,7 +24,6 @@
 #include "core/TabBar.h"
 #include "core/TabBar_p.h"
 #include "core/MainWindow.h"
-#include "core/Utils_p.h"
 #include "core/WindowBeingDragged_p.h"
 #include "core/Position_p.h"
 #include "core/Platform_p.h"

src/core/DockWidget_p.h

@@ -19,10 +19,13 @@
 #include "core/View_p.h"
 #include "QtCompat_p.h"
 
+#include <memory>
+
 namespace KDDockWidgets {
 
 namespace Core {
 class SideBar;
+class PreviousFloatingLayout;
 
 class DOCKS_EXPORT_FOR_UNIT_TESTS DockWidget::Private
 {
@@ -296,6 +299,7 @@ class DOCKS_EXPORT_FOR_UNIT_TESTS DockWidget::Private
     KDBindings::ScopedConnection m_toggleActionConnection;
     KDBindings::ScopedConnection m_floatActionConnection;
     CloseReason m_lastCloseReason = CloseReason::Unspecified;
+    std::shared_ptr<Core::PreviousFloatingLayout> m_previousFloatingLayout;
 };
 
 }

src/core/DropArea.cpp

@@ -726,3 +726,13 @@ Core::Group *DropArea::centralGroup() const
 {
     return d->m_centralGroup;
 }
+
+PreviousFloatingLayout::PreviousFloatingLayout(Core::DropArea *l)
+    : layout(l)
+{
+}
+
+PreviousFloatingLayout::~PreviousFloatingLayout()
+{
+    delete layout;
+}

src/core/DropArea.h

@@ -24,6 +24,8 @@
 #include "kddockwidgets/KDDockWidgets.h"
 #include "kddockwidgets/core/Layout.h"
 
+#include <memory>
+
 class TestQtWidgets;
 class TestDocks;
 
@@ -184,7 +186,27 @@ class DOCKS_EXPORT DropArea : public Layout
     class Private;
     Private *const d;
 };
+
+/// Class to keep a Core::Layout * alive, so restore as floating window works
+///
+/// DockWidget::show() will try to put the dock widget in its previously known position. That
+/// works well for MainWindow, as it keeps its layout Item alive as a placeholder. However,
+/// FloatingWindow's are deleted all the time, so we need to store it's item Layout somewhere.
+/// We store it inside PreviousFloatingLayout, which takes ownership and is refcounted so we don't leak
+/// unneeded layouts.
+class PreviousFloatingLayout
+{
+public:
+    typedef std::shared_ptr<PreviousFloatingLayout> Ptr;
+    explicit PreviousFloatingLayout(Core::DropArea *);
+    ~PreviousFloatingLayout();
+
+private:
+    Core::DropArea *const layout;
+};
+
 }
+
 }
 
 #endif

src/core/FloatingWindow.cpp

@@ -267,6 +267,11 @@ FloatingWindow::FloatingWindow(Core::Group *group, Rect suggestedGeometry,
 FloatingWindow::~FloatingWindow()
 {
     m_inDtor = true;
+
+    /// Keep the Core::Layout around, so DockWidget::show() can still restore to floating
+    /// and go to the correct layout position
+    saveLastFloatingLayout();
+
     view()->d->setAboutToBeDestroyed();
 
     if (auto da = dropArea()) {
@@ -851,3 +856,31 @@ FloatingWindow::Private::Private(FloatingWindowFlags requestedFlags, FloatingWin
     , m_dropArea(new DropArea(q->view(), MainWindowOption_None))
 {
 }
+
+void FloatingWindow::saveLastFloatingLayout()
+{
+    // ~FloatingWindow() is running, but let's keep the Core::Layout alive
+    // so any dock widget that later is promoted to a FloatingWindow can have the same layout.
+    if (!d->m_dropArea)
+        return;
+
+    if (!m_deleteScheduled) {
+        // This is a direct delete of FloatingWindow, not the regular case we're interested in
+        // where the FloatingWindow is deleted later because it's empty
+        return;
+    }
+
+    // relinquish ownership:
+    d->m_dropArea->setParentView(nullptr);
+
+    // Give it to all dock widgets so they use it the next time they become floating
+    auto layout = std::make_shared<PreviousFloatingLayout>(d->m_dropArea);
+
+    const auto docks = dockWidgets();
+    for (auto dock : docks) {
+        if (dock->inDtor())
+            continue;
+
+        dock->d->m_previousFloatingLayout = layout;
+    }
+}

src/core/FloatingWindow.h

@@ -219,6 +219,7 @@ class DOCKS_EXPORT FloatingWindow : public Controller, public Draggable
     void onVisibleFrameCountChanged(int count);
     void onCloseEvent(CloseEvent *);
     void updateSizeConstraints();
+    void saveLastFloatingLayout();
 
     bool m_disableSetVisible = false;
     bool m_deleteScheduled = false;