LibWeb: Implement the DataTransferItemList length attribute

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

@@ -1,4 +1,7 @@
 dropEffect: none
 effectAllowed: none
-stringItem: [object DataTransferItem], types=custom-type
-fileItem: [object DataTransferItem], types=custom-type,Files
+length=0, types=
+stringItem: [object DataTransferItem]
+length=1, types=custom-type
+fileItem: [object DataTransferItem]
+length=2, types=custom-type,Files

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

@@ -6,9 +6,11 @@
         println(`effectAllowed: ${dataTransfer.effectAllowed}`);
 
         let dataTransferItemList = dataTransfer.items;
+        println(`length=${dataTransferItemList.length}, types=${dataTransfer.types}`);
 
         let stringItem = dataTransferItemList.add("well hello friends", "custom-type");
-        println(`stringItem: ${stringItem}, types=${dataTransfer.types}`);
+        println(`stringItem: ${stringItem}`);
+        println(`length=${dataTransferItemList.length}, types=${dataTransfer.types}`);
 
         try {
             dataTransferItemList.add("well hello friends", "custom-type");
@@ -20,6 +22,7 @@
         });
 
         let fileItem = dataTransferItemList.add(file);
-        println(`fileItem: ${fileItem}, types=${dataTransfer.types}`);
+        println(`fileItem: ${fileItem}`);
+        println(`length=${dataTransferItemList.length}, types=${dataTransfer.types}`);
     });
 </script>

Userland/Libraries/LibWeb/HTML/DataTransfer.cpp

@@ -260,6 +260,13 @@ bool DataTransfer::contains_item_with_type(DragDataStoreItem::Kind kind, String
     return false;
 }
 
+size_t DataTransfer::length() const
+{
+    if (m_associated_drag_data_store)
+        return m_associated_drag_data_store->size();
+    return 0;
+}
+
 // https://html.spec.whatwg.org/multipage/dnd.html#concept-datatransfer-types
 void DataTransfer::update_data_transfer_types_list()
 {

Userland/Libraries/LibWeb/HTML/DataTransfer.h

@@ -61,6 +61,7 @@ public:
 
     JS::NonnullGCPtr<DataTransferItem> add_item(DragDataStoreItem item);
     bool contains_item_with_type(DragDataStoreItem::Kind, String const& type) const;
+    size_t length() const;
 
 private:
     DataTransfer(JS::Realm&, NonnullRefPtr<DragDataStore>);

Userland/Libraries/LibWeb/HTML/DataTransferItemList.cpp

@@ -41,6 +41,14 @@ void DataTransferItemList::visit_edges(JS::Cell::Visitor& visitor)
     visitor.visit(m_data_transfer);
 }
 
+// https://html.spec.whatwg.org/multipage/dnd.html#dom-datatransferitemlist-length
+WebIDL::UnsignedLong DataTransferItemList::length() const
+{
+    // The length attribute must return zero if the object is in the disabled mode; otherwise it must return the number
+    // of items in the drag data store item list.
+    return m_data_transfer->length();
+}
+
 // https://html.spec.whatwg.org/multipage/dnd.html#dom-datatransferitemlist-add
 WebIDL::ExceptionOr<JS::GCPtr<DataTransferItem>> DataTransferItemList::add(String const& data, String const& type)
 {

Userland/Libraries/LibWeb/HTML/DataTransferItemList.h

@@ -9,6 +9,7 @@
 #include <LibJS/Forward.h>
 #include <LibWeb/Bindings/PlatformObject.h>
 #include <LibWeb/WebIDL/ExceptionOr.h>
+#include <LibWeb/WebIDL/Types.h>
 
 namespace Web::HTML {
 
@@ -21,6 +22,8 @@ public:
     static JS::NonnullGCPtr<DataTransferItemList> create(JS::Realm&, JS::NonnullGCPtr<DataTransfer>);
     virtual ~DataTransferItemList() override;
 
+    WebIDL::UnsignedLong length() const;
+
     WebIDL::ExceptionOr<JS::GCPtr<DataTransferItem>> add(String const& data, String const& type);
     JS::GCPtr<DataTransferItem> add(JS::NonnullGCPtr<FileAPI::File>);
 

Userland/Libraries/LibWeb/HTML/DataTransferItemList.idl

@@ -4,7 +4,7 @@
 // https://html.spec.whatwg.org/multipage/dnd.html#datatransferitemlist
 [Exposed=Window]
 interface DataTransferItemList {
-    [FIXME] readonly attribute unsigned long length;
+    readonly attribute unsigned long length;
     [FIXME] getter DataTransferItem (unsigned long index);
     DataTransferItem? add(DOMString data, DOMString type);
     DataTransferItem? add(File data);

Userland/Libraries/LibWeb/HTML/DragDataStore.h

@@ -48,6 +48,7 @@ public:
 
     void add_item(DragDataStoreItem item) { m_item_list.append(move(item)); }
     ReadonlySpan<DragDataStoreItem> item_list() const { return m_item_list; }
+    size_t size() const { return m_item_list.size(); }
     bool has_text_item() const;
 
     Mode mode() const { return m_mode; }