[Development] Proposing CMake as build tool for Qt 6

[Alexandru Croitor]

It was partially supported for iOS actually. I was working on that front to make it work properly when building qtbase, and there are still WIP patches for that, but it wasn't completely finished.

But yes, in principle qbs is the only other modern build tool that i know that could do that. Excluding things like writing custom makefiles.

> On 6. Jun 2019, at 15:44, Bogdan Vatra <bogdan.vatra at kdab.com> wrote:
> 
> Hi,
> 
>  Multi-arch builds was the biggest win of QBS which supported them not only 
> for iOS but for Android and I think for any platform.
>  Again multi-arch builds are out for years but no cmake support :).
> 
> Cheers,
> BogDan.
> 
> În ziua de joi, 6 iunie 2019, la 16:28:20 EEST, Alexandru Croitor a scris:
>> Hi,
>> 
>> There are 3rd party solutions for PCH in CMake, but we have not tested those
>> yet.
> 
>> 
>> Regarding iOS, I did some investigation work 2-ish months ago and here's
>> what I can say:
> 
>> - CMake still has bugs here and there regarding iOS, but the patches I
>> submitted so far to upstream CMake were merged. So I hope along the way we
>> can iron out the issues before Qt 6 is released.
> 
>> - CMake has no proper support for doing multi-arch builds (simulator x64 +
>> arm64 at the same time). We would either have to find a way to fix it
>> upstream, possibly find some hack, or resort to building multiple types for
>> different architectures, and then lipo-ing things together. 
> 
>> Note that the current support for multi-arch builds in qmake I would call
>> hack-ish, given that no other build tool that I know of, does the same
>> things that qmake does.
> 
>> - We will have to figure out which 3rd party package provide we want to use
>> for CI, for packages like PCRE2, Freetype, etc, because for all the ones i
>> tried (Conan, vcpkg, hunter), all of them need custom patches to support
>> iOS builds.
> 
>> Other than that, with some WIP non-merged patches in qtbase, I was able to
>> build parts of qtbase with CMake, targeting armv8 iOS.
> 
>> So there's still some work to be done, but in principle it should be
>> doable.
> 
>> 
>>> On 6. Jun 2019, at 15:13, Bogdan Vatra via Development
>>> <development at qt-project.org> wrote:
> 
>>> În ziua de joi, 6 iunie 2019, la 14:45:14 EEST, Simon Hausmann a scris:
>>> 
>>>> Hi,
>>>> 
>>>> In the past months we, some developers from the Qt Company and KDAB,
>>>> have made good progress on the port of Qt to use CMake as build tool.
>>>> Since the initial prototype, the port has advanced very well and its
>>>> current state can be summarized roughly like this:
>>>> 
>>>> 
>>>>   * Builds on
>>>> 
>>>>     * Windows (desktop)
>>>>     * macOS
>>>>     * Linux (desktop and embedded)
>>>>     * Android (running not tested yet)
>>>> 
>>>> 
>>> 
>>> 
>>> Any news about iOS/tvOS/watchOS and PCH?
>>> 
>>> Cheers,
>>> BogDan.
>>> 
>>> 
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>> 
>> 
> 
> 
>