LibGUI: Add a simple GSplitter container widget.

This allows you to put multiple widgets in a container and makes the space
in between them draggable to resize the two adjacent widgets.

Applications/FileManager/main.cpp

@@ -13,6 +13,7 @@
 #include <LibGUI/GProgressBar.h>
 #include <LibGUI/GTreeView.h>
 #include <LibGUI/GFileSystemModel.h>
+#include <LibGUI/GSplitter.h>
 #include <unistd.h>
 #include <signal.h>
 #include <stdio.h>
@@ -50,9 +51,7 @@ int main(int argc, char** argv)
 
     auto* location_textbox = new GTextEditor(GTextEditor::SingleLine, location_toolbar);
 
-    // FIXME: Implement an actual GSplitter widget.
+    auto* splitter = new GSplitter(Orientation::Horizontal, widget);
-    auto* splitter = new GWidget(widget);
-    splitter->set_layout(make<GBoxLayout>(Orientation::Horizontal));
     auto* tree_view = new GTreeView(splitter);
     auto file_system_model = GFileSystemModel::create("/", GFileSystemModel::Mode::DirectoriesOnly);
     tree_view->set_model(file_system_model.copy_ref());

LibGUI/GSplitter.cpp

@@ -0,0 +1,99 @@
+#include <LibGUI/GSplitter.h>
+#include <LibGUI/GBoxLayout.h>
+
+GSplitter::GSplitter(Orientation orientation, GWidget* parent)
+    : GFrame(parent)
+    , m_orientation(orientation)
+{
+    set_layout(make<GBoxLayout>(orientation));
+    set_fill_with_background_color(true);
+    set_background_color(Color::LightGray);
+    layout()->set_spacing(4);
+}
+
+GSplitter::~GSplitter()
+{
+}
+
+void GSplitter::mousedown_event(GMouseEvent& event)
+{
+    if (event.button() != GMouseButton::Left)
+        return;
+    m_resizing = true;
+    int x_or_y = m_orientation == Orientation::Horizontal ? event.x() : event.y();
+    GWidget* first_resizee { nullptr };
+    GWidget* second_resizee { nullptr };
+    int fudge = layout()->spacing();
+    for (auto* child : children()) {
+        if (!child->is_widget())
+            continue;
+        auto& child_widget = *static_cast<GWidget*>(child);
+        int child_start = m_orientation == Orientation::Horizontal ? child_widget.relative_rect().left() : child_widget.relative_rect().top();
+        int child_end = m_orientation == Orientation::Horizontal ? child_widget.relative_rect().right() : child_widget.relative_rect().bottom();
+        if (x_or_y > child_end && (x_or_y - fudge) <= child_end)
+            first_resizee = &child_widget;
+        if (x_or_y < child_start && (x_or_y + fudge) >= child_start)
+            second_resizee = &child_widget;
+    }
+    ASSERT(first_resizee && second_resizee);
+    m_first_resizee = first_resizee->make_weak_ptr();
+    m_second_resizee = second_resizee->make_weak_ptr();
+    m_first_resizee_start_size = first_resizee->size();
+    m_second_resizee_start_size = second_resizee->size();
+    m_resize_origin = event.position();
+}
+
+void GSplitter::mousemove_event(GMouseEvent& event)
+{
+    if (!m_resizing)
+        return;
+    auto delta = event.position() - m_resize_origin;
+    if (!m_first_resizee || !m_second_resizee) {
+        // One or both of the resizees were deleted during an ongoing resize, screw this.
+        m_resizing = false;
+        return;;
+    }
+    int minimum_size = 0;
+    auto new_first_resizee_size = m_first_resizee_start_size;
+    auto new_second_resizee_size = m_second_resizee_start_size;
+    if (m_orientation == Orientation::Horizontal) {
+        new_first_resizee_size.set_width(new_first_resizee_size.width() + delta.x());
+        new_second_resizee_size.set_width(new_second_resizee_size.width() - delta.x());
+
+        if (new_first_resizee_size.width() < minimum_size) {
+            int correction = minimum_size - new_first_resizee_size.width();
+            new_first_resizee_size.set_width(new_first_resizee_size.width() + correction);
+            new_second_resizee_size.set_width(new_second_resizee_size.width() - correction);
+        }
+        if (new_second_resizee_size.width() < minimum_size) {
+            int correction = minimum_size - new_second_resizee_size.width();
+            new_second_resizee_size.set_width(new_second_resizee_size.width() + correction);
+            new_first_resizee_size.set_width(new_first_resizee_size.width() - correction);
+        }
+    } else {
+        new_first_resizee_size.set_height(new_first_resizee_size.height() + delta.y());
+        new_second_resizee_size.set_height(new_second_resizee_size.height() - delta.y());
+
+        if (new_first_resizee_size.height() < minimum_size) {
+            int correction = minimum_size - new_first_resizee_size.height();
+            new_first_resizee_size.set_height(new_first_resizee_size.height() + correction);
+            new_second_resizee_size.set_height(new_second_resizee_size.height() - correction);
+        }
+        if (new_second_resizee_size.height() < minimum_size) {
+            int correction = minimum_size - new_second_resizee_size.height();
+            new_second_resizee_size.set_height(new_second_resizee_size.height() + correction);
+            new_first_resizee_size.set_height(new_first_resizee_size.height() - correction);
+        }
+    }
+    m_first_resizee->set_preferred_size(new_first_resizee_size);
+    m_second_resizee->set_preferred_size(new_second_resizee_size);
+
+    invalidate_layout();
+}
+
+void GSplitter::mouseup_event(GMouseEvent& event)
+{
+    if (event.button() != GMouseButton::Left)
+        return;
+    m_resizing = false;
+}

LibGUI/GSplitter.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <LibGUI/GFrame.h>
+
+class GSplitter : public GFrame {
+public:
+    GSplitter(Orientation, GWidget* parent);
+    virtual ~GSplitter() override;
+
+protected:
+    virtual void mousedown_event(GMouseEvent&) override;
+    virtual void mousemove_event(GMouseEvent&) override;
+    virtual void mouseup_event(GMouseEvent&) override;
+
+private:
+    Orientation m_orientation;
+    bool m_resizing { false };
+    Point m_resize_origin;
+    WeakPtr<GWidget> m_first_resizee;
+    WeakPtr<GWidget> m_second_resizee;
+    Size m_first_resizee_start_size;
+    Size m_second_resizee_start_size;
+};

LibGUI/GWidget.h

@@ -135,6 +135,7 @@ public:
     bool global_cursor_tracking() const;
 
     void notify_layout_changed(Badge<GLayout>);
+    void invalidate_layout();
 
     bool is_visible() const { return m_visible; }
     void set_visible(bool);
@@ -148,7 +149,6 @@ private:
     void handle_resize_event(GResizeEvent&);
     void handle_mouseup_event(GMouseEvent&);
     void do_layout();
-    void invalidate_layout();
 
     GWindow* m_window { nullptr };
     OwnPtr<GLayout> m_layout;

LibGUI/Makefile

@@ -56,6 +56,7 @@ LIBGUI_OBJS = \
     GFrame.o \
     GTreeView.o \
     GFileSystemModel.o \
+    GSplitter.o \
     GWindow.o
 
 OBJS = $(SHAREDGRAPHICS_OBJS) $(LIBGUI_OBJS)

SharedGraphics/Point.h

@@ -49,6 +49,7 @@ public:
     }
 
     Point operator-() const { return { -m_x, -m_y }; }
+    Point operator-(const Point& other) const { return { m_x - other.m_x, m_y - other.m_y }; }
 
     operator WSAPI_Point() const;
     String to_string() const { return String::format("[%d,%d]", x(), y()); }