Allow Windows builds with system-installed Qt

We can't provide binary packages of Qt for the public and compiling Qt from source takes a long time and is cumbersome. To solve that problem this introduces the possibility to use a system-installed Qt as we support it on Linux for a long time. This new support is optional and we will continue building our own Qt for the official Stadia version of Orbit. But for external contributors that should simplify getting started on Windows.
google · Dec 1, 2020 · 44d193a · 44d193a
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diff --git a/DEVELOPMENT.md b/DEVELOPMENT.md
@@ -25,31 +25,16 @@ release from GitHub's [releases page](https://github.com/google/orbit/releases).
 
 To build Orbit you need a compiler capable of C++17. The following ones should be fine.
 
-* GCC 8 and above on Linux
-* Clang 7 and above on Linux
-* MSVC 2017, 2019 and above on Windows
-
-## Building Orbit
-
-Orbit relies on `conan` as its package manager.  Conan is written in Python3,
-so make sure you have either Conan installed or at least have Python3 installed.
-
-The `bootstrap-orbit.{sh,ps1}` will try to install `conan` via `pip3` if not
-installed and reachable via `PATH`. Afterwards it calls `build.{sh,ps1}` which
-will compile Orbit for you.
-
-On Linux, `python3` should be preinstalled anyway, but you might need to install
-pip (package name: `python3-pip`).
-
-On Windows, one option to install Python is via the Visual Studio Installer.
-Alternatively you can download prebuilts from [python.org](https://www.python.org/)
-(In both cases verify that `pip3.exe` is in the path, otherwise the bootstrap
-script will not be able to install conan for you.)
+- GCC 8 and above on Linux
+- Clang 7 and above on Linux
+- MSVC 2017, 2019 and above on Windows
 
 ## Dependencies
 
 All our third-party libraries and dependencies are managed by conan.
 
+### Qt on Linux
+
 There are some exceptions. On Linux, we rely by default on the distribution's Qt5
 and Mesa installation. This can be changed by modifying the conan package options
 `system_qt` and `system_mesa`, but we recommend to go with the distribution provided
@@ -61,6 +46,96 @@ to change the default values of these two options.
 Check out `conanfile.py`. There is a python dictionary called `default_options`
 defined in the python class `OrbitConan`.
 
+### Qt on Windows
+
+On Windows you have the choice to either let Conan compile Qt from source or use
+one of the prebuilt distributions from [The Qt Company](https://qt.io/). (Note that
+as of writing this, you need to register to download the distribution packages.)
+
+We recommend to use a prebuilt distribution since compiling from source can take
+several hours.
+
+If you decide to compile from source, you don't have to prepare anything.
+You can skip over the next paragraph and go to "Building Orbit".
+
+If you decide to use a prebuilt Qt distribution, please download and install it
+yourself. Keep in mind the prebuilt has to match your Visual Studio version and
+architecture. You also have to install the QtWebEngine component which is usually
+not selected by default in the installer.
+
+As of writing this the minimum supported Qt version is 5.12.4 but this might change.
+We recommend the version which we also compile from source. It can be found by checking
+`conanfile.py`. Search for `self.requires("qt/`. The version can be found after that string.
+
+As a next step you have to tell the Orbit build system where Qt is installed by
+setting an environment variable `Qt5_DIR`. It has to point to the directory
+containing the CMake config file `Qt5Config.cmake`. For Qt 5.15.0 installed to
+the default location the path is `C:\Qt\5.15.0\msvc2019_64\lib\cmake\Qt5`;
+
+You don't have to set that variable globally. It's fine to set it in a local
+PowerShell when starting the bootstrapping script:
+
+```powershell
+$Env:Qt5_DIR="C:\Qt\5.15.0\msvc2019_64\lib\cmake\Qt5"
+.\bootstrap-orbit.ps1
+```
+
+The value of `Qt5_DIR` is persisted in the default conan profiles. You can
+call `conan profile show default_relwithdebinfo` (after running bootstrap)
+to see the value of `Qt5_DIR`.
+
+If you have pre-existing `default_*`-profiles and want to switch to prebuilt
+Qt distributions you have to either delete these profiles - the build script
+will regenerate them - or you can manually edit them.
+
+A default profile with Qt compiled from source is pretty much empty:
+
+```
+# default_relwithdebinfo
+
+include(msvc2019_relwithdebinfo)
+
+[settings]
+[options]
+[build_requires]
+[env]
+
+```
+
+A default profile prepared for a prebuilt Qt package has two extra lines:
+
+```
+# default_relwithdebinfo
+
+include(msvc2019_relwithdebinfo)
+
+[settings]
+[options]
 OrbitProfiler:system_qt=True
+[build_requires]
+[env]
+OrbitProfiler:Qt5_DIR="C:\Qt\5.15.0\msvc2019_64\lib\cmake\Qt5"
+
+```
+
+You can find all the conan profiles in `%USERPROFILE%\.conan\profiles`.
+
+## Building Orbit
+
+Orbit relies on `conan` as its package manager. Conan is written in Python3,
+so make sure you have either Conan installed or at least have Python3 installed.
+
+The `bootstrap-orbit.{sh,ps1}` will try to install `conan` via `pip3` if not
+installed and reachable via `PATH`. Afterwards it calls `build.{sh,ps1}` which
+will compile Orbit for you.
+
+On Linux, `python3` should be preinstalled anyway, but you might need to install
+pip (package name: `python3-pip`).
+
+On Windows, one option to install Python is via the Visual Studio Installer.
+Alternatively you can download prebuilts from [python.org](https://www.python.org/)
+(In both cases verify that `pip3.exe` is in the path, otherwise the bootstrap
+script will not be able to install conan for you.)
 
 ## Running Orbit
 

diff --git a/build.ps1 b/build.ps1
@@ -31,9 +31,28 @@ function conan_create_profile($profile) {
   $build_type = $profile -replace "default_"
   $profile_path = "$conan_dir/profiles/$profile"
 
+  if (!$Env:Qt5_DIR) {
+    Write-Host ""
+    Write-Host "Compile Qt from source or use official distribution?"
+    Write-Host "====================================================`r`n"
+    Write-Host "Orbit depends on the Qt framework which can be either automatically compiled from source (can take several hours!)"
+    Write-Host "or can be provided by an official Qt distribution. We recommend the latter which is less cumbersome."
+    Write-Host "Check out DEVELOPMENT.md for more details on how to use an official Qt distribution.`r`n"
+    Write-Host "Press Ctrl+C to stop here and install Qt first. Press Enter to continue with compiling Qt from source."
+    Write-Host "Google-internal devs: Please press Enter!`r`n"
+    Read-Host  "Answer"
+  }
+
   "include(${compiler_version}_${build_type})`r`n" | Set-Content -Path $profile_path
   "[settings]`r`n[options]" | Add-Content -Path $profile_path
-  "[build_requires]`r`n[env]" | Add-Content -Path $profile_path
+
+  if ($Env:Qt5_DIR) {
+    Write-Host "Found Qt5_DIR environment variable. Using system provided Qt distribution."
+    "OrbitProfiler:system_qt=True`r`n[build_requires]`r`n[env]" | Add-Content -Path $profile_path
+    "OrbitProfiler:Qt5_DIR=`"$Env:Qt5_DIR`"" | Add-Content -Path $profile_path
+  } else {
+    "[build_requires]`r`n[env]" | Add-Content -Path $profile_path
+  }
 }
 
 $profiles = if ($args.Count) { $args } else { @("default_relwithdebinfo") }