C++ – Windows API – Windows Focus (with Qt)

A simple class to know when a window gets or losses focus on windows with Qt.

Edit : I finally use another system (the isActive function) but this post can be useful anyway to work with windows API.

Header (with many in lining functions)

#ifndef LOSTFOCUSHANDLER_H
#define LOSTFOCUSHANDLER_H

#include <QObject>
#include <QTimer>
#include <windows.h>

class LostFocusHandler : public QObject
{
	Q_OBJECT

public:
    LostFocusHandler(QObject *parent = 0);
    ~LostFocusHandler();

	void setTarget(HWND itarget){
		target = itarget;
	}

	bool hasCurrentFocus(){
		return GetActiveWindow() == target;
	}

	void start(){
		hasFocus = (GetActiveWindow() == target);
		timer.start();
	}

	void stop(){
		timer.stop();
	}

	static bool hasCurrentFocus(HWND itarget){
		return GetActiveWindow() == itarget;
	}

signals:
	void LostFocus();
	void GetFocus();

protected slots:
	void refresh();

private:
    QTimer timer;
	bool hasFocus;
	HWND target;
};

#endif // LOSTFOCUSHANDLER_H

Implementation

#include "LostFocusHandler.h"

LostFocusHandler::LostFocusHandler(QObject *parent)
	: QObject(parent)
{
	target = 0;

	timer.setInterval( 250 );
	connect(&timer,SIGNAL(timeout()),this,SLOT(refresh()));
}

LostFocusHandler::~LostFocusHandler()
{
	timer.stop();
}

void LostFocusHandler::refresh(){
	if(hasFocus && GetActiveWindow() != target){
		hasFocus = false;
		emit LostFocus();
	}
	else if(!hasFocus && GetActiveWindow() == target){
		hasFocus = true;
		emit GetFocus();
	}
}

Utilization

//in a QWidget
protected slots:
	void run(){
		setWindowTitle("Run");
	}
	void end(){
		setWindowTitle("Stop");
	}

...

void MyProg::create(){
	focusManager.setTarget(winId());

	connect(&focusManager,SIGNAL(LostFocus()),this,SLOT(run()));
	connect(&focusManager,SIGNAL(GetFocus()),this,SLOT(end()));

	focusManager.start();
}