Developer Douglas Rudd has a drag and drop tutorial post up for Firemonkey Delphi 10 Seattle. The blog post contains sample source code, an explanation of how it is implemented, and a example project download. At it’s core it creates a list of TRectangles that are stored in an array. The TRectangles are parented to a TLayout and have a couple components inside them as well (like a TLabel). In the OnMouseDown event of each TRectangle it uses a trick via the TLayout.Root.Captured property to send the mouse movements to the TLayout. TLayout handles the mouse movements to move the TRectangle around the form. The example code works on all four supported platforms which are Android, IOS, OSX, and Windows. The code should also work with Appmethod and can probably be converted to C++Builder as well. We have another older post about drag and drop as well.
Head over and check out the full blog post about dragging and dropping controls in Firemonkey with Delphi 10 Seattle before downloading the example.