LibCore+LibGUI: Allow inserting a CObject/GWidget before another

When adding a widget to a parent, you don't always want to append it to
the set of existing children, but instead insert it before one of them.

This patch makes that possible by adding CObject::insert_child_before()
which also produces a ChildAdded event with an additional before_child
pointer. This pointer is then used by GWidget to make sure that any
layout present maintains the correct order. (Without doing that, newly
added children would still be appended into the layout order, despite
having a different widget sibling order.)

1 files changed, 9 insertions, 0 deletions

diff --git a/Libraries/LibCore/CObject.cpp b/Libraries/LibCore/CObject.cpp
index 0058de5b39..7de5a5a044 100644
--- a/Libraries/LibCore/CObject.cpp
+++ b/Libraries/LibCore/CObject.cpp
@@ -64,6 +64,15 @@ void CObject::add_child(CObject& object)
     event(*make<CChildEvent>(CEvent::ChildAdded, object));
 }
 
+void CObject::insert_child_before(CObject& new_child, CObject& before_child)
+{
+    // FIXME: Should we support reparenting objects?
+    ASSERT(!new_child.parent() || new_child.parent() == this);
+    new_child.m_parent = this;
+    m_children.insert_before_matching(new_child, [&](auto& existing_child) { return existing_child.ptr() == &before_child; });
+    event(*make<CChildEvent>(CEvent::ChildAdded, new_child, &before_child));
+}
+
 void CObject::remove_child(CObject& object)
 {
     for (int i = 0; i < m_children.size(); ++i) {