Fading away widget in QT C++ - c++

Question

I want to make a simple QWidget which is a simple rectangle fade away. The main problem is that the paint event paint at the same place every time in fact making the effect opposite, it make the colour stronger. Is there any way of achieving this functionality? Could you maybe provide some simple example?
My code:
`
#ifndef WIDGET_H
#define WIDGET_H
#include <QWidget>
#include <QTimer>
namespace Ui {
class Widget;
}
class Widget : public QWidget
{
Q_OBJECT
public:
explicit Widget(QWidget *parent = 0);
void paintEvent(QPaintEvent*);
~Widget();
private:
Ui::Widget *ui;
int alfa;
int i;
QTimer time;
};
#endif // WIDGET_H
`
and the cpp:
#include "widget.h"
#include "ui_widget.h"
#include <QColor>
#include <QPainter>
#include <QBrush>
#include <QTimer>
#include <QDebug>
Widget::Widget(QWidget *parent) :
QWidget(parent,Qt::Popup | Qt::FramelessWindowHint),
ui(new Ui::Widget),
time()
{
ui->setupUi(this);
setAttribute(Qt::WA_NoSystemBackground, true);
setAttribute(Qt::WA_TranslucentBackground, true);
setAttribute(Qt::WA_PaintOnScreen);
time.setInterval(500);
time.start();
connect(&time,SIGNAL(timeout()),this,SLOT(update()));
alfa=100;
i=0;
}
void Widget::paintEvent(QPaintEvent *)
{
QPainter painter(this);
int rectBase = height();
QColor c(255,0,255);
alfa=alfa-(i*10);
c.setAlpha(alfa);
qDebug()<<c.alpha();
i++;
painter.setBrush(QBrush(c));
painter.drawRect(0, 0, width(),height());
}
Widget::~Widget()
{
delete ui;
}

Answer 1

You shouldn't rely on QWidget::paintEvent() to change your alpha level, since it can be called less or more than you want (multiple update() calls may result in only one paintEvent() and paintEvent() may be called when you don't expect it).
So a more reliable way to get to the result, is have a separate slot where you decrease the alpha level and then call update(). Your class definition might look like this:
class Widget : public QWidget
{
Q_OBJECT
public:
Widget( QWidget * inParent );
private:
void paintEvent(QPaintEvent *);
private slots:
void animate();
private:
QTimer * mTimer;
int mAlpha;
};
And the declaration:
Widget::Widget( QWidget * inParent )
:
QWidget( inParent ),
mTimer( new QTimer( this ) ),
mAlpha( 255 )
{
setAttribute(Qt::WA_NoSystemBackground, true);
setAttribute(Qt::WA_TranslucentBackground, true);
setAttribute(Qt::WA_PaintOnScreen);
mTimer->setInterval( 40 );
mTimer->setSingleShot( false );
connect( mTimer, SIGNAL(timeout()), this, SLOT(animate()) );
mTimer->start();
}
void
Widget::paintEvent(QPaintEvent *)
{
QPainter painter(this);
painter.setBrush( QColor(255,0,255,mAlpha ) );
painter.drawRect(rect());
}
void
Widget::animate()
{
if ( mAlpha > 0 )
{
mAlpha -= 3;
}
else
{
mTimer->stop();
}
update();
}
Notice that I did decrease the interval of the timer. You only called an update() every half a second. That typically does not result in a smooth animation.

Answer 2

I get a warning with this code on Kubuntu Linux, under Qt5:
QWidget::paintEngine: should no longer be called
The originating line is in qWarning("QWidget::paintEngine: Should no longer be called");, in src/widgets/kernel/qwidget_qpa.cpp and discussed a bit in this ticket:
https://qt.gitorious.org/qt/qtbase-harmattan/commit/3037525
You can get the warning to stop by removing the setAttribute(Qt::WA_PaintOnScreen);, so I did that. After taking that line out, it works for me--although your subtraction model is strange. You are modifying the alpha as well as changing the subtraction value on each iteration; you probably didn't intend both. So either change it to:
QColor c (255, 0, 255);
alfa = alfa - 10;
if (alfa >= 0) {
c.setAlpha(alfa);
} else {
time.stop();
}
...or:
QColor c(255,0,255);
if (alfa - i * 10 >= 0) {
c.setAlpha(alfa - i * 10);
i++;
} else {
time.stop();
}
Etc. (See also #PrisonMonkeys note on your timer not necessarily being the only source of update() calls.) Regarding getting these warnings to be more vocal so you don't miss them, you might look at The Essential Noisy Debug Hook For Qt, which I should update.
If with the change, an alpha blended window doesn't work on your platform at all, you should mention explicitly what your circumstance is...as it is working for me.