Qt - use QListWidgetItem user data

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(Created page with "''This page describes how the user data of a QListWidgetItem can be used to store a custom class.'' == Custom class definition == Suppose a custome class is defined as follows:...")
 
(→Implementation)
 
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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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Suppose a custome class is defined as follows:
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Suppose a custom class is defined as follows:
  
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<syntaxhightlight lang="cpp">
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<syntaxhighlight lang="cpp-qt">
 
class MyCustomClass
 
class MyCustomClass
 
{
 
{
 
public:
 
public:
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     MyCustomClass() { a=0; b=0; }
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     MyCustomClass();
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     ~MyCustomClass() {}
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     ~MyCustomClass();
 
     int a;
 
     int a;
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    int b;
 
 
};
 
};
 
</syntaxhighlight>
 
</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)
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</syntaxhighlight>
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== GUI ==
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A main window with a list and two line edit fields and a push-button.
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== Adding the QListWidget ==
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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.
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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 ==
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TODO: create actual small application.
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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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{
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    // Local temporary variables.
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    MyCustomClass myClass;
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    QVariant      qv;
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    myClass.a = ui->valueLineEdit->text().toInt();
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    // Use content of local variable and copy.
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    qv.setValue(myClass);
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    QListWidgetItem *qlwi = new QListWidgetItem();
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    qlwi->setText(ui->textLineEdit->text());
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    // Copy the local qvariant and store with list item.
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    qlwi->setData(Qt::UserRole,qv);
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    ui->listWidget->addItem(qlwi);
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}
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</syntaxhighlight>
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=== Display data ===
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<syntaxhighlight lang="cpp-qt">
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void MainWindow::displayData(QListWidgetItem *item)
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{
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    textLineEdit->setText(item->text());
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    MyCustomClass myClass = item->data(Qt::UserRole).value<MyCustomClass>();
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    ui->lineEdit->setText(QString("%1").arg(myClass.a));
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}
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</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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