How to get if an application has focus on macOS? - c++

Question

I have the need to gather which application has focus. For this, my approach is to: list windows, get the one with focus, and finally, check which process and application shows it. If there were some: getWindowWithFocus(), it would be fantastic.
Requirements:
The program is implemented in C++, but could interface with objective-C if needed.
The program will run with root privileges.
The list of windows listed must include all users applications.
The returned window allows to get properties, such as it process and if it has UI-focus.
Ideally, no 3rd party tool is used, only standard libraries (STL, Unix APIs and macOS APIs, eventually Qt/Boost).
Must support HSierra to Big-Sur.
I managed to list all windows, but now I am struggling in detecting if a window has or not the focus.
The question:
Which API function can be used to check if a window has focus or not? Any sample?
Any better approach to this problem?
Previous research:
I created a POC/sample which list all windows, including some of it properties.
CGWindowListCopyWindowInfo
https://developer.apple.com/documentation/coregraphics/1455137-cgwindowlistcopywindowinfo?language=objc
DISCLAIM: this is a POC, just for demonstration, and miss required code quality for proper projects. For example, CFObjects are not released with the consequent memory leak.
#include <CoreFoundation/CoreFoundation.h>
#include <CoreGraphics/CGWindow.h> // CoreGraphics
#include <iostream>
int main()
{
CFArrayRef ref = CGWindowListCopyWindowInfo(kCGNullWindowID, 0);
CFIndex nameCount = CFArrayGetCount( ref );
std::cout << "NumCounts: " << nameCount << " windows" << std::endl;
for( int i = 0; i < nameCount ; ++i )
{
std::cerr << " -------- " << std::endl;
CFDictionaryRef dict = (CFDictionaryRef)CFArrayGetValueAtIndex( ref, i );
auto printKeys = [](const void* key, const void* value, void* context)
{
CFShow(key);
std::cerr << " ";
CFShow(value);
};
CFDictionaryApplyFunction(dict, printKeys, nullptr);
// Process PID can be extracted with key:kCGWindowOwnerPID
// DOES THIS WINDOW HAS FOCUS?
}
}

Answer 1

Here is an example, based on this solution, wrapped in C++ (well, actually mostly C).
The only found problem with it is, it must run in main thread, which is not convenient, but this is another topic.
main.cpp:
#include "focus_oc_wrapper.hpp"
#include <thread>
int main(int argc, const char * argv[])
{
FocusDetector::AppFocus focus;
focus.run();
//std::thread threadListener(&FocusDetector::AppFocus::run, &focus); //Does not works
//if (threadListener.joinable())
//{
// threadListener.join();
//}
}
focus_oc_wrapper.hpp
namespace FocusDetector
{
struct AppFocusImpl;
struct AppFocus
{
AppFocusImpl* impl=nullptr;
AppFocus() noexcept;
~AppFocus();
void run();
};
}
focus_oc_wrapper.mm
#include "focus_oc_wrapper.hpp"
#import <Foundation/Foundation.h>
#import <AppKit/AppKit.h>
#import "focus_oc.h"
namespace FocusDetector
{
struct AppFocusImpl
{
OCAppFocus* wrapped=nullptr;
};
AppFocus::AppFocus() noexcept: impl(new AppFocusImpl)
{
impl->wrapped = [[OCAppFocus alloc] init];
}
AppFocus::~AppFocus()
{
if (impl)
{
[impl->wrapped release];
}
delete impl;
}
void AppFocus::run()
{
[NSApplication sharedApplication];
[NSApp setDelegate:impl->wrapped];
[NSApp run];
}
}
focus_oc.h
#import <Foundation/Foundation.h>
#interface OCAppFocus : NSObject <NSApplicationDelegate>
{
NSRunningApplication *currentApp;
}
#property (retain) NSRunningApplication *currentApp;
#end
#implementation OCAppFocus
#synthesize currentApp;
- (id)init
{
if ((self = [super init]))
{
[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver:self
selector:#selector(activeAppDidChange:)
name:NSWorkspaceDidActivateApplicationNotification object:nil];
}
return self;
}
- (void)dealloc
{
[[[NSWorkspace sharedWorkspace] notificationCenter] removeObserver:self];
[super dealloc];
}
- (void)activeAppDidChange:(NSNotification *)notification
{
self.currentApp = [[notification userInfo] objectForKey:NSWorkspaceApplicationKey];
NSLog(#"App: %#", [currentApp localizedName]);
NSLog(#"Bundle: %#", [currentApp bundleIdentifier]);
NSLog(#"Exec Url: %#", [currentApp executableURL]);
NSLog(#"PID: %d", [currentApp processIdentifier]);
}
#end
CMakeLists.txt
cmake_minimum_required(VERSION 3.0)
set(CMAKE_OSX_DEPLOYMENT_TARGET "10.13" CACHE STRING "Minimum OS X deployment version")
project("focus_detection")
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -framework CoreFoundation -framework AppKit")
set ( TESTCPP main.cpp focus_oc_wrapper.mm )
add_executable( ${PROJECT_NAME} ${TESTCPP} )