Gestures + drag & drop

[mikael]

Thanks to the relentless work of @rozaimech and others in this thread, I was inspired and able to add the drag & drop gestures to the package.

The module now supports dragging and dropping both within a Pythonista app and between Pythonista and another app (latter only possible on iPads). These two cases are handled differently:

• For in-app drops, Apple method of relaying objects is skipped completely, and you can refer to any Python object to be dropped to the target view.
• For cross-app drops, we have to conform to Apple method of managing data. Currently only plain text and image drops are supported, in either direction.
• It is also good to note that ui.TextField and ui.TextView views natively act as receivers for both in-app and cross-app plain text drag and drop.

View is set to be a sender for a drap and drop operation with the drag function. Drag starts with a long press, and can end in any view that has been set as a receiver with the drop function. Views show the readiness to receive data with a green "plus" sign. You can accept only specific types of data; incompatible drop targets show a grey "forbidden" sign.

Following example covers setting up an in-app drag and drop operation between two labels. To repeat, in the in-app case, the simple string could replaced by any Python object of any complexity, passed by reference:

drag(sender_label, "Important data")

drop(receiver_label,
    lambda data, sender, receiver: setattr(receiver, 'text', data),
    accept=str)

See the module documentation for the two functions for details.