LibGUI: Add GModelSelection to help implementing multiple-select views

Each GAbstractView now has a GModelSelection backed by a simple
HashTable<GModelIndex>. When the selection changes somehow, the view
gets notified via the notify_selection_changed() callback.

In the future it will probably make sense to move to using some kind of
ranges as the internal representation instead.

Libraries/LibGUI/GAbstractView.cpp

@@ -8,6 +8,7 @@
 
 GAbstractView::GAbstractView(GWidget* parent)
     : GScrollableWidget(parent)
+    , m_selection(*this)
 {
 }
 
@@ -96,3 +97,8 @@ void GAbstractView::activate(const GModelIndex& index)
     if (on_activation)
         on_activation(index);
 }
+
+void GAbstractView::notify_selection_changed(Badge<GModelSelection>)
+{
+    update();
+}

Libraries/LibGUI/GAbstractView.h

@@ -2,6 +2,7 @@
 
 #include <AK/Function.h>
 #include <LibGUI/GModel.h>
+#include <LibGUI/GModelSelection.h>
 #include <LibGUI/GScrollableWidget.h>
 
 class GModelEditingDelegate;
@@ -9,6 +10,7 @@ class GModelEditingDelegate;
 class GAbstractView : public GScrollableWidget {
     C_OBJECT(GAbstractView)
     friend class GModel;
+
 public:
     explicit GAbstractView(GWidget* parent);
     virtual ~GAbstractView() override;
@@ -17,6 +19,9 @@ public:
     GModel* model() { return m_model.ptr(); }
     const GModel* model() const { return m_model.ptr(); }
 
+    GModelSelection& selection() { return m_selection; }
+    const GModelSelection& selection() const { return m_selection; }
+
     bool is_editable() const { return m_editable; }
     void set_editable(bool editable) { m_editable = editable; }
 
@@ -37,6 +42,8 @@ public:
 
     Function<OwnPtr<GModelEditingDelegate>(const GModelIndex&)> aid_create_editing_delegate;
 
+    void notify_selection_changed(Badge<GModelSelection>);
+
 protected:
     virtual void did_scroll() override;
     void activate(const GModelIndex&);
@@ -50,5 +57,6 @@ protected:
 private:
     RefPtr<GModel> m_model;
     OwnPtr<GModelEditingDelegate> m_editing_delegate;
+    GModelSelection m_selection;
     bool m_activates_on_selection { false };
 };

Libraries/LibGUI/GModelSelection.cpp

@@ -0,0 +1,39 @@
+#include <LibGUI/GAbstractView.h>
+#include <LibGUI/GModelSelection.h>
+
+void GModelSelection::set(const GModelIndex& index)
+{
+    ASSERT(index.is_valid());
+    if (m_indexes.size() == 1 && m_indexes.contains(index))
+        return;
+    m_indexes.clear();
+    m_indexes.set(index);
+    m_view.notify_selection_changed({});
+}
+
+void GModelSelection::add(const GModelIndex& index)
+{
+    ASSERT(index.is_valid());
+    if (m_indexes.contains(index))
+        return;
+    m_indexes.set(index);
+    m_view.notify_selection_changed({});
+}
+
+bool GModelSelection::remove(const GModelIndex& index)
+{
+    ASSERT(index.is_valid());
+    if (!m_indexes.contains(index))
+        return false;
+    m_indexes.remove(index);
+    m_view.notify_selection_changed({});
+    return true;
+}
+
+void GModelSelection::clear()
+{
+    if (m_indexes.is_empty())
+        return;
+    m_indexes.clear();
+    m_view.notify_selection_changed({});
+}

Libraries/LibGUI/GModelSelection.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <AK/HashTable.h>
+#include <LibGUI/GModelIndex.h>
+
+class GAbstractView;
+
+class GModelSelection {
+public:
+    GModelSelection(GAbstractView& view)
+        : m_view(view)
+    {
+    }
+
+    bool is_empty() const { return m_indexes.is_empty(); }
+    bool contains(const GModelIndex& index) const { return m_indexes.contains(index); }
+
+    void set(const GModelIndex&);
+    void add(const GModelIndex&);
+    bool remove(const GModelIndex&);
+    void clear();
+
+    template<typename Callback>
+    void for_each_index(Callback callback)
+    {
+        for (auto& index : m_indexes)
+            callback(index);
+    }
+
+    // FIXME: This doesn't guarantee that what you get is the lowest or "first" index selected..
+    GModelIndex first() const
+    {
+        if (m_indexes.is_empty())
+            return {};
+        return *m_indexes.begin();
+    }
+
+private:
+    GAbstractView& m_view;
+    HashTable<GModelIndex> m_indexes;
+};

Libraries/LibGUI/Makefile

@@ -54,6 +54,7 @@ OBJS = \
     GComboBox.o \
     GJsonArrayModel.o \
     GAboutDialog.o \
+    GModelSelection.o \
     GWindow.o
 
 LIBRARY = libgui.a