Drag-and-Drop (Files)

Relevant official examples: drag_and_drop.

Bevy supports the Drag-and-Drop gesture common on most desktop operating systems, but only for files, not arbitrary data / objects.

If you drag a file (say, from the file manager app) into a Bevy app, Bevy will produce a FileDragAndDrop event, containing the path of the file that was dropped in.

fn file_drop(
    mut evr_dnd: EventReader<FileDragAndDrop>,
) {
    for ev in evr_dnd.read() {
        if let FileDragAndDrop::DroppedFile { window, path_buf } = ev {
            println!("Dropped file with path: {:?}, in window id: {:?}", path_buf, window);
        }
    }
}

Detecting the Position of the Drop

You may want to do different things depending on where the cursor was when the drop gesture ended. For example, add the file to some collection, if it was dropped over a specific UI element/panel.

Unfortunately, this is currently somewhat tricky to implement, due to winit bug #1550. Bevy does not get CursorMoved events while the drag gesture is ongoing, and therefore does not respond to the mouse cursor. Bevy completely loses track of the cursor position.

Checking the cursor position from the Window will also not work.

Systems that use cursor events to respond to cursor movements will not work during a drag gesture. This includes Bevy UI's Interaction detection, which is the usual way of detecting when a UI element is hovered over.

Workaround

The only way to workaround this issue is to store the file path somewhere temporarily after receiving the drop event. Then, wait until the next CursorMoved event, and then process the file.

Note that this might not even be on the next frame update. The next cursor update will happen whenever the user moves the cursor. If the user does not immediately move the mouse after dropping the file and leaves the cursor in the same place for a while, there will be no events and your app will have no way of knowing the cursor position.