[Development] Proposing CMake as build tool for Qt 6

[Alexandru Croitor]

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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