Qt Tutorial #1 Add-on

[ShaChris23]

Hi guys,

For this tutorial here,

http://doc.trolltech.com/4.2/tutorial-t11.html

Does someone know how to do this?

"Modify the program such that the X Y location of each shot bullet is displayed on the GUI on the top right. X Y should become 0, 0 once the shot is completely gone. The definition of gone is not just the bullet being off screen since it's possible for a bullet to go off the screen and fall down."

And, no, this is not a homework! I'm just curious how such feature would be implemented!

[marcel]

Sure...
First you have to create two labels in the GUI ( or just one ) - one for X and one for Y.
The moving of the bullet is made in CannonField::moveShot(). You should emit a signal from here with the x and y position. The signal should be connected to a slot in the dialog that just updates the labels with the X and Y values.

void CannonField::moveShot()
 {
     QRegion region = shotRect();
     ++timerCount;
 
     QRect shotR = shotRect();
 
     if (shotR.x() > width() || shotR.y() > height()) {
         autoShootTimer->stop();
     } else {
         region = region.unite(shotR);
     }
     update(region);
     emit updateBulletPosition(shotRect().topLeft().x(), shotRect().topLeft().y() );
 }

void CannonField::moveShot()
 {
     QRegion region = shotRect();
     ++timerCount;

     QRect shotR = shotRect();

     if (shotR.x() > width() || shotR.y() > height()) {
         autoShootTimer->stop();
     } else {
         region = region.unite(shotR);
     }
     update(region);
     emit updateBulletPosition(shotRect().topLeft().x(), shotRect().topLeft().y() );
 }

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Regards

[marcel]

You have to create the labels in main.cpp:

QHBoxLayout *topLayout = new QHBoxLayout;
     topLayout->addWidget(shoot);
     topLayout->addStretch(1);
     QLabel* mxLabel = new QLabel( this );
     QLabel* myLabel = new QLabel( this );
     topLayout->addWidget( xLabel );
     topLayout->addWidget( yLabel );
     connect( cannonField, SIGNAL( updateBulletPosition(int,int) ), this, SLOT( setBulletPosition(int, int ) ) );
 
     QVBoxLayout *leftLayout = new QVBoxLayout;
     leftLayout->addWidget(angle);
     leftLayout->addWidget(force);
 
     QGridLayout *gridLayout = new QGridLayout;
     gridLayout->addWidget(quit, 0, 0);
     gridLayout->addLayout(topLayout, 0, 1);
     gridLayout->addLayout(leftLayout, 1, 0);
     gridLayout->addWidget(cannonField, 1, 1, 2, 1);
     gridLayout->setColumnStretch(1, 10);
     setLayout(gridLayout);
 
     angle->setValue(60);
     force->setValue(25);
     angle->setFocus();

QHBoxLayout *topLayout = new QHBoxLayout;
     topLayout->addWidget(shoot);
     topLayout->addStretch(1);
     QLabel* mxLabel = new QLabel( this );
     QLabel* myLabel = new QLabel( this );
     topLayout->addWidget( xLabel );
     topLayout->addWidget( yLabel );
     connect( cannonField, SIGNAL( updateBulletPosition(int,int) ), this, SLOT( setBulletPosition(int, int ) ) );

     QVBoxLayout *leftLayout = new QVBoxLayout;
     leftLayout->addWidget(angle);
     leftLayout->addWidget(force);

     QGridLayout *gridLayout = new QGridLayout;
     gridLayout->addWidget(quit, 0, 0);
     gridLayout->addLayout(topLayout, 0, 1);
     gridLayout->addLayout(leftLayout, 1, 0);
     gridLayout->addWidget(cannonField, 1, 1, 2, 1);
     gridLayout->setColumnStretch(1, 10);
     setLayout(gridLayout);

     angle->setValue(60);
     force->setValue(25);
     angle->setFocus();

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In the MyWidget class declaration:

class MyWidget : public QWidget
 {
 public:
     MyWidget(QWidget *parent = 0);
 
 public slots:
     void setBulletPosition( int x, int y );
 
 private:
     QLabel* mxLabel;
     QLabel* myLabel;
 };

class MyWidget : public QWidget
 {
 public:
     MyWidget(QWidget *parent = 0);

 public slots:
     void setBulletPosition( int x, int y );
 
 private:
     QLabel* mxLabel;
     QLabel* myLabel;
 };

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the slot:

void MyWidget::setBulletPostion( int x, int y )
{
     mxLabel->setText( QVariant(x).toString() ); 
     myLabel->setText( QVariant(y).toString() ); 
}

void MyWidget::setBulletPostion( int x, int y )
{
     mxLabel->setText( QVariant(x).toString() ); 
     myLabel->setText( QVariant(y).toString() ); 
}

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This is it, pretty much. I'm not sure if the labels will appear correctly ( int the top-right corner ), but that can be fixed easily.

[ShaChris23]

Hi Marcel,

Thanks for a very helpful post. For some reason though, when I run the program, Qt reports the following:

Object::connect: No such slot QWidget::setBulletPosition(int,int)

I also dont see the x y label on the top right. Here's my current code:


From MyWidget constructor

QHBoxLayout* topLayout = new QHBoxLayout;
   topLayout->addWidget(shoot);
   topLayout->addStretch(1);
 
   QLabel* mxLabel = new QLabel( this );
   QLabel* myLabel = new QLabel( this );
 
   topLayout->addWidget( mxLabel );
   topLayout->addWidget( myLabel );
   connect( cannonField, SIGNAL( updateBulletPosition( int, int ) ), this, SLOT( setBulletPosition( int, int ) ) );

QHBoxLayout* topLayout = new QHBoxLayout;
   topLayout->addWidget(shoot);
   topLayout->addStretch(1);

   QLabel* mxLabel = new QLabel( this );
   QLabel* myLabel = new QLabel( this );

   topLayout->addWidget( mxLabel );
   topLayout->addWidget( myLabel );
   connect( cannonField, SIGNAL( updateBulletPosition( int, int ) ), this, SLOT( setBulletPosition( int, int ) ) );

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From MyWidget class declaration

class MyWidget : public QWidget
{
public:
   MyWidget( QWidget* parent = 0 );
 
public slots:
   void setBulletPosition( int x, int y );
 
private:
   QLabel* mxLabel;
   QLabel* myLabel;
};

class MyWidget : public QWidget
{
public:
   MyWidget( QWidget* parent = 0 );

public slots:
   void setBulletPosition( int x, int y );

private:
   QLabel* mxLabel;
   QLabel* myLabel;
};

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From MyWidget::setBulletPosition( int x, int y )

void MyWidget::setBulletPosition( int x, int y )
{
   mxLabel->setText( QVariant(x).toString() );
   myLabel->setText( QVariant(y).toString() );
}

void MyWidget::setBulletPosition( int x, int y )
{
   mxLabel->setText( QVariant(x).toString() );
   myLabel->setText( QVariant(y).toString() );
}

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From CannonField::moveShot()

void CannonField::moveShot()
{
   // 
   QRegion region = shotRect();
   ++timerCount;
 
   // Get the new shot
   QRect   shotR  = shotRect();
 
   if( shotR.x() > width() || shotR.y() > height() )
   {
      autoShootTimer->stop();
   }
   else
   {
      region = region.unite(shotR);
   }
 
   // Repaint the QRegion
   update(region);
   emit updateBulletPosition( shotRect().topLeft().x(), shotRect().topLeft().y() );
}

void CannonField::moveShot()
{
   // 
   QRegion region = shotRect();
   ++timerCount;

   // Get the new shot
   QRect   shotR  = shotRect();

   if( shotR.x() > width() || shotR.y() > height() )
   {
      autoShootTimer->stop();
   }
   else
   {
      region = region.unite(shotR);
   }

   // Repaint the QRegion
   update(region);
   emit updateBulletPosition( shotRect().topLeft().x(), shotRect().topLeft().y() );
}

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Please let me know if I missed anything...I cant think of a reason why this doesnt work.

[jacek]

Add the Q_OBJECT macro:

class MyWidget : public QWidget
{
   Q_OBJECT
   public:
...

class MyWidget : public QWidget
{
   Q_OBJECT
   public:
...

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and re-run qmake and make.

[marcel]

Did you declare the signal updateBulletPosition, in the CannonField class declaration?
Should be something like:

class CanonField : ....
{
...
signals:
     void updateBulletPosition( int, int );
...
}

class CanonField : ....
{
...
signals:
     void updateBulletPosition( int, int );
...
}

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Regards

[ShaChris23]

Hi Marcel,

Yes, here's my CannonField class declaration:

class CannonField : public QWidget
{
   Q_OBJECT
 
public:
   CannonField( QWidget* parent = 0 );
 
   int angle() const { return currentAngle; }
   int force() const { return currentForce; }
 
public slots:
   void setAngle( int angle );
   void setForce( int force );
   void shoot();
 
private slots:
   void moveShot();
 
signals:
   void angleChanged( int newAngle );
   void forceChanged( int newForce );
   void updateBulletPosition( int x, int y );
 
protected:
   void paintEvent( QPaintEvent* event );
 
private:
   void  paintShot( QPainter& painter );
   void  paintCannon( QPainter& painter );
 
   QRect cannonRect() const;
   QRect shotRect() const;
 
   int currentAngle;
   int currentForce;
 
   int      timerCount;
   QTimer*  autoShootTimer;
   float    shootAngle;
   float    shootForce;
};

class CannonField : public QWidget
{
   Q_OBJECT

public:
   CannonField( QWidget* parent = 0 );

   int angle() const { return currentAngle; }
   int force() const { return currentForce; }

public slots:
   void setAngle( int angle );
   void setForce( int force );
   void shoot();

private slots:
   void moveShot();

signals:
   void angleChanged( int newAngle );
   void forceChanged( int newForce );
   void updateBulletPosition( int x, int y );

protected:
   void paintEvent( QPaintEvent* event );

private:
   void  paintShot( QPainter& painter );
   void  paintCannon( QPainter& painter );

   QRect cannonRect() const;
   QRect shotRect() const;

   int currentAngle;
   int currentForce;

   int      timerCount;
   QTimer*  autoShootTimer;
   float    shootAngle;
   float    shootForce;
};

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[ShaChris23]

Hmm..this is strange. I put in Q_OBJECT into MyWidget class declaration, and it doesnt complain anymore. But when I press shoot, the program crashes at

qstring.h

here -->

inline int size() const { return d->size; }

inline int size() const { return d->size; }

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under class

class Q_CORE_EXPORT QString

class Q_CORE_EXPORT QString

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But when I take out Q_OBJECT, the program doesnt crash. Of course, the x y coordinate still doesnt show up when I press the shoot button.

I then looked around a little bit, turns out these 2 lines caused the crash:

void MyWidget::setBulletPosition( int x, int y )
{
   mxLabel->setText( QVariant(x).toString() );
   myLabel->setText( QVariant(y).toString() );
}

void MyWidget::setBulletPosition( int x, int y )
{
   mxLabel->setText( QVariant(x).toString() );
   myLabel->setText( QVariant(y).toString() );
}

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Once I comment out those two lines, it doesnt crash anymore. Do you happen to know why?

I also tried putting a cout debugging like this:

void MyWidget::setBulletPosition( int x, int y )
{
   cout << x << " " << y << "\n";
}

void MyWidget::setBulletPosition( int x, int y )
{
   cout << x << " " << y << "\n";
}

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and I saw that the x y are "real" numbers, not some garbage values.

I'm using Qt 4.2.3 with VS STUDIO 05.

[marcel]

Are mxLabel and myLabel valid pointers when you try to set their value?
Do you delete them somehow after you create them and add them to the layout?

Because I can't really see how it could crash in those lines...

Try not setting the actual coordinate. Call the setText methods for a constant string, "1" for example. See what it does then. If it still crashes, then your two pointers gost somehow corrupted.

Regards

[ShaChris23]

Hi Marcel,

I'm sorry to waste your time...the bug was in my MyWidget's constructor

where

QLabel* mxLabel = new QLabel( this );
QLabel* myLabel = new QLabel( this );

QLabel* mxLabel = new QLabel( this );
QLabel* myLabel = new QLabel( this );

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should have been

mxLabel = new QLabel( this );
myLabel = new QLabel( this );

mxLabel = new QLabel( this );
myLabel = new QLabel( this );

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Since I made mxLabel and myLabel part of the class private data members. +_=. So you were right. Sorry about that.

Anyway, how can I make the labels reset to 0 0 when the shot is completely off the screen?

[ShaChris23]

Also, does anyone know how to do the following?

"Re-paint the cannon to red when the shot is in the air. Then paint the cannon blue when the shot is gone."

Thanks!