problem with replacing a background image using signal / slot in qt

Question

Hi

i want to make a simple chess program. So far i've made the board using QTableWidget and loaded the piece pictures in the cells of table. Now i wnat to use signal and slot so that when user clicks a cell and then click another cell the piece picture from first cell goes to second cell, But I don't know how to do it.

Note that i don't want these "piece moves" obey the real chess rules. I only wnat to do the picture replacement between two cells . Later i will make them obey the rules.

here is the code. in this code only table's item(0,0) has a picture. can anyone say how to write a code so that when i click that item then click to item(1,1) , picture "1.bmp" goes to background of item(1,1)?

#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QHBoxLayout>
#include <QTableWidget>
#include <QHeaderView>

class Table : public QWidget
{
    Q_OBJECT

  public:
    Table(QWidget *parent = 0);
slots:
    //??????

};


Table::Table(QWidget *parent)
    : QWidget(parent)
{
  QHBoxLayout *hbox = new QHBoxLayout(this);

  QTableWidget *table = new QTableWidget(8 , 8 , this);

  table->setFixedSize(900,900);
  table->horizontalHeader()->setDefaultSectionSize(100);
  table->verticalHeader()->setDefaultSectionSize(100);

  table->horizontalHeader()->setResizeMode(QHeaderView::Fixed);
  table->verticalHeader()->setResizeMode(QHeaderView::Fixed);

  QString fileName = "/1.bmp";
  QPixmap pic(fileName);

  QBrush brush(pic);

  QTableWidgetItem* item = new QTableWidgetItem();
  item->setBackground(brush);

  table->setItem(0,0,item);

  hbox->addWidget(table);
  setLayout(hbox);
}

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    Table table;

    table.show();


    return app.exec();
}

Answer 1

There are really two questions here.

The first one regarding signals/slots with QTableWidgetItem, and the second regarding handling mouse clicks on the QTableWidget.

Signals Slots on a QTableWidgetItem

• note: I don't recommend doing it this way, read all the way to the bottom*

Using signals and slots requires that the object that emits the signal, have the signal defined in the class definition. Likewise the object that receives a slot, must have that slot declared in the class definition.

You'll notice (on the Qt docs) that QTableWidgetItem doesn't have a signals or slots to set/remove the background brush you are using to draw your picture. So, you will have to subclass QTableWidgetItem, and provide this signals/slots yourself.

Example:

class ChessItem : public QTableWidgetItem
{
    // constructor / destructor
    // other methods

public slots:
    void slotChangeBackground( const QBrush & brush ) 
    {  
        setBackground( brush );
    }
};

Handling Mouse Clicks on the QTableWidget

edit: I removed the event handling paragraph, because using QTableWidget's builtin signals is easier

QTableWidget offers the cell clicked signal:

void QTableWidget::cellClicked ( int row, int column )  

So in your Table class add a slot, then connect it the cellClicked signal to it:

// in your Table's constructor:
connect( table, SIGNAL( cellClicked(int, int) ), this, SLOT( slotCellClicked(int,int) ) ) );
// elsewhere...
void slotCellClicked(int row, int column) {
    // handle mouse clicking here
}

The problem as I see it is you don't want to just connect any ol signal to slotChangeBackground, because that would change every background. So I suggest not using signals/slots for changing the background, and instead use QTableWidget::itemAt ( int ax, int ay ) in your slotCellClicked(x,y) to retrieve the item at a coordinate, then call setBackground on it.