Allow dynamic builds of Qt for iOS - Tech Preview

It is possible to configure Qt with -shared flag for iOS and it does produce .dylib and iOS Frameworks.

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But the Qt iOS platform plugin is still static (libqios.a), which makes the build not purely dynamic libs, but mostly dynamic libs with exception of a static libqios.a.

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Note: linking with libqios.a also requires linking with libQt5FontDatabaseSupport.a, libQt5ClipboardSupport.a, libQt5GraphicsSupport.a, libqtfreetype.a and libqtlibpng.a to get rid of unresolved symbols, which makes the process quite cumbersome.

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For example, it is possible to make a few modifications to Qt source code and change it to purely dynamic this way [tested in Qt 5.15.0]:

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1. Remove the line that forces libqios to be static from qtbase/src/plugins/platforms/ios/kernel.pro:

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qtConfig(shared): CONFIG += static

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1. Extend existing qt_main_wrapper() in qtbase/src/plugins/platforms/ios/qioseventdispatcher.mm to have a third parameter, which is a pointer to the actual main(), that must be called when initialisation is complete:

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@@ -213,8 +213,10 @@ void printUsage()
 
 using namespace QT_PREPEND_NAMESPACE(QtPrivate);
 
--extern "C" int qt_main_wrapper(int argc, char *argv[])-
+*int (*main_ptr)(int, char **) = 0;*
+Q_EXTERN_C Q_DECL_EXPORT int qt_main_wrapper(int argc, char *argv[]*, int (*real_main_ptr)(int, char **)*)
 {
+ *main_ptr = real_main_ptr;*
 @autoreleasepool {
 size_t defaultStackSize = 512 * kBytesPerKiloByte; // Same as secondary threads
 
@@ -251,7 +253,7 @@ void printUsage()
 kJumpedFromUserMainTrampoline,
 };
 
--extern "C" int main(int argc, char *argv[]);-
+
 
 static void __attribute__((noinline, noreturn)) user_main_trampoline()
 {
@@ -270,7 +272,7 @@ static void __attribute__((noinline, noreturn)) user_main_trampoline()
 qFatal("Could not convert argv[%d] to C string", i);
 }
 
-- int exitCode = main(argc, argv);-
+ int exitCode = *main_ptr*(argc, argv);
 delete[] argv;
 
 logActivity.applicationDidFinishLaunching.enter();

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1. Add plugins/platforms/libqios.dylib to linked libraries and modify your project entrypoint to be main_wrapper() instead of main() in build options:

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-Wl,-e,_main_wrapper

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1. In your main.cpp add this simple code, which will only work when building for iOS:

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int main(int argc, char **argv) {
 Application app(argc, argv);
 return app.exec();
}

<p></p>

#if TARGET_OS_IPHONE
*extern "C" int qt_main_wrapper(int argc, char *argv[], int (*f)(int, char **));* //this function is inside libqios.dylib
extern "C" int main_wrapper(int argc, char **argv) { //this is the entry point
*qt_main_wrapper*(argc, argv, *main*);//once finished, it will call into the real "main()" above
}
#endif

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How this works:

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• Your app starts executing in main_wrapper().

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• main_wrapper() calls into libqios.dylib's qt_main_wrapper() and passes function pointer to the real main() function.

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• qt_main_wrapper() executes some iOS custom ObjectiveC glue code and waits until 
  applicationDidFinishLaunching notification is received
• Once applicationDidFinishLaunching is received (your app becomes operational), libqios.dylib executes main() in your app via a function pointer that we passed into qt_main_wrapper().
   

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Please add a similar approach or some other solution to allow a purely dynamic build of Qt libraries for iOS.