Qt - use QListWidgetItem user data

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''This page describes how the user data of a QListWidgetItem can be used to store a custom class.''
 
''This page describes how the user data of a QListWidgetItem can be used to store a custom class.''
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== Introduction ==
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The <tt>QListWidgetItem</tt><ref>[http://developer.qt.nokia.com/doc/qt-4.8/qlistwidgetitem.html QListWidget Class Reference - Qt 4.8]</ref> has an attribute:
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<syntaxhighlight lang="cpp-qt">
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QVariant QListWidgetItem data(int role)
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</syntaxhighlight>
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This can be used to store a custom class. How this is done is described on this page.
  
 
== Custom class definition ==
 
== Custom class definition ==
  
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A custom class is defined as follows:
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Suppose a custom class is defined as follows:
  
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<syntaxhighlight lang="cpp">
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<syntaxhighlight lang="cpp-qt">
 
class MyCustomClass
 
class MyCustomClass
 
{
 
{
 
public:
 
public:
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     MyCustomClass() { value_a=0; }
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     MyCustomClass();
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     ~MyCustomClass() {}
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     ~MyCustomClass();
 
     int a;
 
     int a;
 
};
 
};
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</syntaxhighlight>
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To ensure that the class is compatible with QVariant, the following is added<ref>[http://developer.qt.nokia.com/doc/qt-4.8//qmetatype.html QMetaType Class Reference - Qt 4.8 reference]</ref>:
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<syntaxhighlight lang="cpp-qt">
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#include <QMetaType>
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Q_DECLARE_METATYPE(MyCustomClass)
 
</syntaxhighlight>
 
</syntaxhighlight>
  
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== Adding the QListWidget ==
 
== Adding the QListWidget ==
  
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Suppose two edit fields hold the values for <tt>value_a</tt>. If a button is clicked, a new <tt>QListWidgetItem</tt> must be added with the text from one of the line edits and an instance of the <tt>MyCustomClass</tt> must be attached with the values set by the other two line edit fields.
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Suppose one edit fields hold the value for <tt>a</tt> and the other the text associated with the list widget item. If a button is clicked, a new <tt>QListWidgetItem</tt> must be added with the text retrieve from one of the line edits and an instance of the <tt>MyCustomClass</tt> must be attached with the <tt>a</tt> attribute set by the integer conversion of the other line edit field.
  
 
When clicked on an item in the list, the date of the item clicked must be received and put in the corresponding line edit fields.
 
When clicked on an item in the list, the date of the item clicked must be received and put in the corresponding line edit fields.
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== Implementation ==
 
== Implementation ==
  
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<syntaxhighlight lang="cpp">
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TODO: create actual small application.
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void MainWindow::on_pushButton_clicked()
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=== Add item ===
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<syntaxhighlight lang="cpp-qt">
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void MainWindow::addItem()
 
{
 
{
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    // Local temporary variables.
 
     MyCustomClass myClass;
 
     MyCustomClass myClass;
 
     QVariant      qv;
 
     QVariant      qv;
  
 
     myClass.a = ui->valueLineEdit->text().toInt();
 
     myClass.a = ui->valueLineEdit->text().toInt();
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    // Use content of local variable and copy.
 
     qv.setValue(myClass);
 
     qv.setValue(myClass);
  
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     qlwi->setText(ui->textLineEdit->text());
 
     qlwi->setText(ui->textLineEdit->text());
  
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    // Copy the local qvariant and store with list item.
 
     qlwi->setData(Qt::UserRole,qv);
 
     qlwi->setData(Qt::UserRole,qv);
 
     ui->listWidget->addItem(qlwi);
 
     ui->listWidget->addItem(qlwi);
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</syntaxhighlight>
 
</syntaxhighlight>
  
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=== Display data ===
  
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<syntaxhighlight lang="cpp-qt">
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<syntaxhighlight lang="cpp">
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void MainWindow::displayData(QListWidgetItem *item)
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void MainWindow::on_listWidget_currentItemChanged(QListWidgetItem *current, QListWidgetItem *previous)
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{
 
{
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     textLineEdit->setText(current->text());
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     textLineEdit->setText(item->text());
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     MyCustomClass myClass = current->data(Qt::UserRole).value<MyCustomClass>();
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     MyCustomClass myClass = item->data(Qt::UserRole).value<MyCustomClass>();
 
     ui->lineEdit->setText(QString("%1").arg(myClass.a));
 
     ui->lineEdit->setText(QString("%1").arg(myClass.a));
 
}
 
}
 
</syntaxhighlight>
 
</syntaxhighlight>
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=== Change data ===
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<syntaxhighlight lang="cpp-qt">
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void MainWindow::changeData(QListWidgetItem *item)
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{
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    QVariant *qvPtr = item->data(Qt::UserRole);
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    MyCustomClass myClass = qvPtr->value<MyCustomClass>();
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    myClass.a += 1;
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    qvPtr->setValue(myClass);
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}
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</syntaxhighlight>
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== References ==
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<references/>

Latest revision as of 09:30, 24 December 2011

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