LibWeb: Implement separate DataTransfer factories for IDL / internal use

The IDL constructor has to take separate steps than a DataTransfer that
is internally constructed. Notably, an IDL-created object has its own
drag data store, and that store is placed in a read-write mode.

Tests/LibWeb/Text/expected/HTML/data-transfer.txt

@@ -0,0 +1,2 @@
+dropEffect: none
+effectAllowed: none

Tests/LibWeb/Text/input/HTML/data-transfer.html

@@ -0,0 +1,8 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let dataTransfer = new DataTransfer();
+        println(`dropEffect: ${dataTransfer.dropEffect}`);
+        println(`effectAllowed: ${dataTransfer.effectAllowed}`);
+    });
+</script>

Userland/Libraries/LibWeb/HTML/DataTransfer.cpp

@@ -27,14 +27,31 @@ ENUMERATE_DATA_TRANSFER_EFFECTS
 
 }
 
-JS::NonnullGCPtr<DataTransfer> DataTransfer::construct_impl(JS::Realm& realm)
+JS::NonnullGCPtr<DataTransfer> DataTransfer::create(JS::Realm& realm, NonnullRefPtr<DragDataStore> drag_data_store)
 {
-    return realm.heap().allocate<DataTransfer>(realm, realm);
+    return realm.heap().allocate<DataTransfer>(realm, realm, move(drag_data_store));
 }
 
-DataTransfer::DataTransfer(JS::Realm& realm)
+// https://html.spec.whatwg.org/multipage/dnd.html#dom-datatransfer
-    : PlatformObject(realm)
+JS::NonnullGCPtr<DataTransfer> DataTransfer::construct_impl(JS::Realm& realm)
 {
+    // 1. Set the drag data store's item list to be an empty list.
+    auto drag_data_store = DragDataStore::create();
+
+    // 2. Set the drag data store's mode to read/write mode.
+    drag_data_store->set_mode(DragDataStore::Mode::ReadWrite);
+
+    // 3. Set the dropEffect and effectAllowed to "none".
+    // NOTE: This is done by the default-initializers.
+
+    return realm.heap().allocate<DataTransfer>(realm, realm, move(drag_data_store));
+}
+
+DataTransfer::DataTransfer(JS::Realm& realm, NonnullRefPtr<DragDataStore> drag_data_store)
+    : PlatformObject(realm)
+    , m_associated_drag_data_store(move(drag_data_store))
+{
+    update_data_transfer_types_list();
 }
 
 DataTransfer::~DataTransfer() = default;
@@ -195,12 +212,6 @@ JS::NonnullGCPtr<FileAPI::FileList> DataTransfer::files() const
     return files;
 }
 
-void DataTransfer::associate_with_drag_data_store(NonnullRefPtr<DragDataStore> drag_data_store)
-{
-    m_associated_drag_data_store = move(drag_data_store);
-    update_data_transfer_types_list();
-}
-
 void DataTransfer::disassociate_with_drag_data_store()
 {
     m_associated_drag_data_store.clear();

Userland/Libraries/LibWeb/HTML/DataTransfer.h

@@ -37,6 +37,7 @@ class DataTransfer : public Bindings::PlatformObject {
     JS_DECLARE_ALLOCATOR(DataTransfer);
 
 public:
+    static JS::NonnullGCPtr<DataTransfer> create(JS::Realm&, NonnullRefPtr<DragDataStore>);
     static JS::NonnullGCPtr<DataTransfer> construct_impl(JS::Realm&);
     virtual ~DataTransfer() override;
 
@@ -55,11 +56,10 @@ public:
     String get_data(String const& format) const;
     JS::NonnullGCPtr<FileAPI::FileList> files() const;
 
-    void associate_with_drag_data_store(NonnullRefPtr<DragDataStore> drag_data_store);
     void disassociate_with_drag_data_store();
 
 private:
-    DataTransfer(JS::Realm&);
+    DataTransfer(JS::Realm&, NonnullRefPtr<DragDataStore>);
 
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(JS::Cell::Visitor&) override;

Userland/Libraries/LibWeb/Page/DragAndDropEventHandler.cpp

@@ -506,8 +506,7 @@ JS::NonnullGCPtr<HTML::DragEvent> DragAndDropEventHandler::fire_a_drag_and_drop_
     }
 
     // 6. Let dataTransfer be a newly created DataTransfer object associated with the given drag data store.
-    auto data_transfer = HTML::DataTransfer::construct_impl(realm);
+    auto data_transfer = HTML::DataTransfer::create(realm, *m_drag_data_store);
-    data_transfer->associate_with_drag_data_store(*m_drag_data_store);
 
     // 7. Set the effectAllowed attribute to the drag data store's drag data store allowed effects state.
     data_transfer->set_effect_allowed_internal(m_drag_data_store->allowed_effects_state());