feat: add QQmlImageProviderBase

[dphaldes]

supercedes #1142

[codecov]

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ee1849d) to head (e66959c).
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[dphaldes]

I have "working" implementation here https://github.com/dphaldes/compose/blob/master/src/bridge.rs.

Not particularly fond of the solution since it uses an unchecked cast since I wasn't able to figure out how to cast Pin<&mut QQmlImageProviderBase> into *mut QQmlImageProviderBase. The application also segfaults when closing probably because it is trying to release the ImageProvider.

Any ideas appreciated!

[ahayzen-kdab]

How are you making your QQmlImageProviderBase implementation? As looks like you have a UnqiuePtr<PreviewProvider> which you can do as_mut_ptr() or into_inner() to get a *mut PreviewProvider which then could be cast to a *mut QQmlImageProviderBase ? Or do you need to do it in a place where you only have a Pin ?

And generally things here are looking like they are heading in the right direction :-)

https://docs.rs/cxx/latest/cxx/struct.UniquePtr.html#method.as_mut_ptr

[dphaldes]

I didn't see that method. I will look into using that. It would work but it still wouldnt work as a safe cast will it? as in using the Upcast trait instead of as keyword

[ahayzen-kdab]

Hmm right we don't have any safe API for casting pointers, there is upcast_ptr (basically static_cast) and from_base_ptr (basically dynamic_cast) https://github.com/KDAB/cxx-qt/blob/main/crates/cxx-qt/src/casting.rs#L23 ... but both of these are *const T not *mut T.

I think for now just use an as or the cast_mut() from std https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.cast_mut Raw pointers are always going to be tricky with Rust, so we should consider whether we can improve any help there or if writing .cast_mut() is fine

[dphaldes]

That works pretty well on the implementation side. https://github.com/dphaldes/compose/blob/38019867036cce5d156e786884567c565fdd7903/src/main.rs#L46-L51

Had to use into_raw() so that we release the UniquePtr early and avoid the segfault when QmlEngine takes ownership of the Provider.
Didnt require anything changed from cxx-qt side. So this PR should be ready for review and cleanup

[ahayzen-kdab]

these should be on the QQmlEngine rather than the QQmlApplicationEngine (which inherits from the QQmlEngine)

[ahayzen-kdab]

In Qt 5 these enums are slightly different either have a cfg like #[cfg(cxxqt_qt_version_major = "6")] and #[cfg(cxxqt_qt_version_major = "5")] to have different enums or disable all of this for Qt 5.

In Qt 5

• Image = 0
• Pixmap = 1
• Texture = 2
• ImageResponse = 4
  https://doc.qt.io/archives/qt-5.15/qqmlimageproviderbase.html#ImageType-enum

In Qt 6

• Image = 1
• Pixmap = 2
• Texture = 3
• ImageResponse = 4
  https://doc.qt.io/qt-6/qqmlimageproviderbase.html#ImageType-enum

Note in Qt 5 there is a gap of no 3 :-) And do we need the Invalid = 0 ? As that is not defined on the Qt side ?

[ahayzen-kdab]

In Qt 5, this is just a normal class and does no inherit from QObject, so again might need a cfg block to have two different types if we want to make it work with Qt 5

[ahayzen-kdab]

remove empty line :-)

[ahayzen-kdab]

Looking overall good, just need to either decide to fix the Qt 5 support or opt-out for these methods/types