Add UR decoding capability to scanners on desktop & Android

[hectorchu]

As title. Only decoding - no encoding was added.

[SomberNight]

(ref #7905)

[SomberNight]

Note: the qtmultimedia toolchain is only used on Windows and macOS. Desktop Linux uses zbar.
But perhaps this is something we should reconsider.
See comment here:

electrum/electrum/gui/qt/qrreader/__init__.py

Lines 5 to 20 in 169227e

	# We have two toolchains to scan qr codes:
	# 1. access camera via QtMultimedia, take picture, feed picture to zbar
	# 2. let zbar handle whole flow (including accessing the camera)
	#
	# notes:
	# - zbar needs to be compiled with platform-dependent extra config options to be able
	# to access the camera
	# - zbar fails to access the camera on macOS
	# - qtmultimedia seems to support more cameras on Windows than zbar
	# - qtmultimedia is often not packaged with PyQt
	# in particular, on debian, you need both "python3-pyqt6" and "python3-pyqt6.qtmultimedia"
	# - older versions of qtmultimedia don't seem to work reliably
	#
	# Considering the above, we use QtMultimedia for Windows and macOS, as there
	# most users run our binaries where we can make sure the packaged versions work well.
	# On Linux where many people run from source, we use zbar.

[SomberNight]

Thanks a lot for this. It looks like a solid start.
Would you be interested in adding also the encode/display qr code parts to the GUIs?

[hectorchu]

I have the encoding change for QRCodeWidget, but it currently generates the parts upfront, and then these are passed to the QRCodeWidget which then cycles them. But this was only for compatibility with Cake Wallet, which makes the assumption there are n-fixed parts. UR codes are actually intended to be used as a fountain code. See the description I wrote up here.

So I would take this change, but instead the QRCodeWidget should accept the data string and an argument encode_ur, if the latter is True then use a UREncoder to generate a new part on every refresh. These changes aren't difficult and I doubt they are difficult for the Android one also, but I haven't looked at the Android code. Could you do it?

[hectorchu]

Actually, the QRCodeWidget should just accept a UR|str, because a UR also includes a type, and it would be better for all that to be created by the caller.
The widget creates encoder = UREncoder(ur), then the widget would just call encoder.next_part() per refresh.

[pony-montana]

HI! Here just to signal two python projects that implements these feature, hoping it could be useful :)

SeedQReader
bitcoin-qr-tools