WinUI3：如何知道在 UIElement.drop() 事件中删除了哪个 uielement

Question

I'm working on a WinUI3 desktop application with c++. I was trying to perform a drag-and-drop operation. When I drop a textbox on a canvas I get a drop event from the canvas. In that drop event, as a sender, we can get a reference to the canvas instance[ the UIElement on which the drop happened ], but how to know which element was dropped on the canvas?

I came across this documentation on how to process drop events, but it's unclear how to pass data between dragstaring and drop events.

It would be of great help if you could help me understand which UIElement was dropped when we receive UIElement.drop() event

Thank you

Answer 1

Although the Drag and drop documentation (which is a UWP doc, not a WinUI3 doc which in many cases cannot be found...) says

In most cases, the system will construct a data package for you. The system automatically handles: Images, Text

This doesn't work for WinUI3 for some reason (maybe it depends on app identity, packaging, security, etc.), there's nothing constructed automatically in the target data object.

So you must hook the DragStarting event and put what you want in it so you will get it back in the Drop event.

You can't put/get the source element itself as an object from a drag & drop operation, you must put yourself some information you find suitable about it in the Data object. D&D as a general OS feature only supports serializable or well-known data (text, image bits, a stream, etc.).

To do that you can use one of the SetXXX methods of the DataPackage class , like SetText for example.