LibCards: Add support for card dragging

Solitaire and Spider have almost identical code for dragging cards from
one stack to another, so it makes sense to move that here. But using
the term "moving" for them, instead of "focused" which (to me at least)
was not clear what it meant.

Userland/Libraries/LibCards/CardGame.cpp

@@ -1,5 +1,7 @@
 /*
+ * Copyright (c) 2020, Till Mayer <till.mayer@web.de>
  * Copyright (c) 2022, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -32,6 +34,65 @@ void CardGame::mark_intersecting_stacks_dirty(Cards::Card const& intersecting_ca
     update(intersecting_card.rect());
 }
 
+Gfx::IntRect CardGame::moving_cards_bounds() const
+{
+    if (!is_moving_cards())
+        return {};
+
+    // Note: This assumes that the cards are arranged in a line.
+    return m_moving_cards.first().rect().united(m_moving_cards.last().rect());
+}
+
+void CardGame::pick_up_cards_from_stack(Cards::CardStack& stack, Gfx::IntPoint click_location, CardStack::MovementRule movement_rule)
+{
+    if (m_moving_cards_source_stack)
+        m_moving_cards_source_stack->set_focused(false);
+    stack.add_all_grabbed_cards(click_location, m_moving_cards, movement_rule);
+    stack.set_focused(true);
+    m_moving_cards_source_stack = stack;
+}
+
+RefPtr<CardStack> CardGame::find_stack_to_drop_on(CardStack::MovementRule movement_rule) const
+{
+    auto bounds_to_check = moving_cards_bounds();
+
+    // FIXME: This currently returns only the first stack we overlap,
+    //        but what we want is the stack we overlap the most.
+    for (auto const& stack : stacks()) {
+        if (stack.is_focused())
+            continue;
+
+        if (stack.bounding_box().intersects(bounds_to_check)) {
+            if (stack.is_allowed_to_push(moving_cards().at(0), moving_cards().size(), movement_rule)) {
+                return stack;
+            }
+        }
+    }
+
+    return nullptr;
+}
+
+void CardGame::drop_cards_on_stack(Cards::CardStack& stack, CardStack::MovementRule movement_rule)
+{
+    VERIFY(stack.is_allowed_to_push(m_moving_cards.at(0), m_moving_cards.size(), movement_rule));
+    for (auto& to_intersect : moving_cards()) {
+        mark_intersecting_stacks_dirty(to_intersect);
+        stack.push(to_intersect);
+        (void)moving_cards_source_stack()->pop();
+    }
+
+    update(moving_cards_source_stack()->bounding_box());
+    update(stack.bounding_box());
+}
+
+void CardGame::clear_moving_cards()
+{
+    if (!m_moving_cards_source_stack.is_null())
+        m_moving_cards_source_stack->set_focused(false);
+    m_moving_cards_source_stack.clear();
+    m_moving_cards.clear();
+}
+
 void CardGame::dump_layout() const
 {
     dbgln("------------------------------");

Userland/Libraries/LibCards/CardGame.h

@@ -27,6 +27,16 @@ public:
     void add_stack(NonnullRefPtr<CardStack>);
     void mark_intersecting_stacks_dirty(Card const& intersecting_card);
 
+    bool is_moving_cards() const { return !m_moving_cards.is_empty(); }
+    NonnullRefPtrVector<Card>& moving_cards() { return m_moving_cards; }
+    NonnullRefPtrVector<Card> const& moving_cards() const { return m_moving_cards; }
+    Gfx::IntRect moving_cards_bounds() const;
+    RefPtr<CardStack> moving_cards_source_stack() const { return m_moving_cards_source_stack; }
+    void pick_up_cards_from_stack(CardStack&, Gfx::IntPoint click_location, CardStack::MovementRule);
+    RefPtr<CardStack> find_stack_to_drop_on(CardStack::MovementRule) const;
+    void drop_cards_on_stack(CardStack&, CardStack::MovementRule);
+    void clear_moving_cards();
+
     void dump_layout() const;
 
 protected:
@@ -36,6 +46,9 @@ private:
     virtual void config_string_did_change(String const& domain, String const& group, String const& key, String const& value) override;
 
     NonnullRefPtrVector<CardStack> m_stacks;
+
+    NonnullRefPtrVector<Card> m_moving_cards;
+    RefPtr<CardStack> m_moving_cards_source_stack;
 };
 
 }