- Getting Qt
- Writing Dialogs in Code
- Visual Dialog Design with Qt Designer
- Conclusion
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Writing Dialogs in Code
Writing a dialog entirely in code using Qt is much easier than in most other toolkits. Qt provides three layout manager classes that take care of positioning widgets (called controls on some platforms) on screen, as shown in Figure 1:
QHBoxLayout (left)
QVBoxLayout (center)
QGridLayout (right)
The layout managers can be nested to create complex dialogs.
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Figure
1 QHBoxLayout, QVBoxLayout, and QGridLayout
classes.
Dialogs that use the layout managers automatically enjoy the following benefits (see Figure 2):
The dialogs are user-resizable and have a sensible minimum size.
They look good no matter what font, platform, theme, or direction of writing (left-to-right or right-to-left) is applied.
They adapt smoothly when the text changes—for example, if the application is translated to another language.
Layout managers relieve the programmer of the tedious task of specifying hard-coded coordinates for the dialog's widgets. Although Qt also offers absolute positioning as an alternative, very few developers use that approach.
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Figure 2 A layout-managed Find dialog with different platforms, languages, and directions of writing.
In addition to layout managers, one reason why it's easy to develop Qt dialogs is Qt's "signals and slots" mechanism. Widgets on the screen emit signals in response to user actions. For example, QPushButton, the Qt class for button widgets, emits a clicked() signal whenever the user clicks the button. Similarly, QSpinBox has a valueChanged(int) signal that is emitted when the user changes the value held by the spin box, either by typing a new value or by clicking the tiny arrows on the widget. Signals can be connected to member functions of other objects. Member functions that signals can connect to are called slots.
Here's a simple example of a signal-to-slot connection:
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QPushButton *quitButton = new QPushButton("Quit", 0);
connect(quitButton, SIGNAL(clicked()),
&app, SLOT(quit()));
...
}
In this example, we connect the Quit button's clicked() signal to the QApplication object's quit() slot.
NOTE
QApplication is a class that provides global-like functionality. Every Qt application must create one instance of that class, typically in main().
To see how this all works in practice, let's implement the Find dialog depicted earlier in Figure 2. This involves subclassing QDialog and writing the code for a few functions. We'll start by reviewing the header file:
#ifndef FINDDIALOG_H
#define FINDDIALOG_H
#include <qbuttongroup.h>
#include <qcheckbox.h>
#include <qdialog.h>
#include <qlabel.h>
#include <qlineedit.h>
#include <qpushbutton.h>
#include <qradiobutton.h>
class FindDialog : public QDialog
{
Q_OBJECT
public:
FindDialog(QWidget *parent = 0);
signals:
void findNext(const QString &text, bool caseSensitive,
bool forward);
private slots:
void findNextButtonClicked();
private:
QLabel *label;
QLineEdit *lineEdit;
QCheckBox *caseCheckBox;
QButtonGroup *buttonGroup;
QRadioButton *upRadioButton;
QRadioButton *downRadioButton;
QPushButton *findNextButton;
QPushButton *closeButton;
};
#endif
This header file looks like a standard C++ header file, with a few exceptions:
The class definition contains the Q_OBJECT macro at the very top. This macro is necessary for all classes that provide their own signals and slots.
The signals keyword is used to specify that findNext() is a signal.
The slots keyword is used to specify that findNextButtonClicked() is a slot.
The signals and slots keywords are converted into standard C++ by the C++ preprocessor. They serve as markers for Qt's meta-object compiler (moc) tool, which gathers meta-information about Qt classes and generates glue code necessary for the "signals and slots" mechanism.
Let's review the implementation file, starting with the constructor:
#include <qlayout.h>
#include "finddialog.h"
FindDialog::FindDialog(QWidget *parent)
: QDialog(parent)
{
setCaption(tr("Find"));
label = new QLabel(tr("Find &what:"), this);
lineEdit = new QLineEdit(this);
label->setBuddy(lineEdit);
buttonGroup = new QButtonGroup(tr("Direction"), this);
buttonGroup->setColumns(2);
upRadioButton = new QRadioButton(tr("&Up"), buttonGroup);
downRadioButton = new QRadioButton(tr("&Down"), buttonGroup);
downRadioButton->setChecked(true);
caseCheckBox = new QCheckBox(tr("&Case sensitive"), this);
findNextButton = new QPushButton(tr("&Find Next"), this);
closeButton = new QPushButton(tr("Close"), this);
// more follows
The FindDialog constructor takes a QWidget * parameter that specifies the dialog's parent window. If a parent is specified (that is, parent isn't a null pointer), the dialog automatically centers itself on top of that window and shares that window's taskbar entry; otherwise, the dialog acts as an independent window inside the application.
At the top of the constructor, we call setCaption() to set the window title to Find. The tr() function that surrounds the "Find" string marks the string as translatable.
Then we create a label widget showing the text Find what:, an empty line editor widget, a button group with the heading text Direction, two radio buttons (Up and Down), a check box with the text Case sensitive, and two push buttons (Find Next and Close).
Most of the widgets are created with this as the last argument, to specify that they're children of the FindDialog object. Child widgets are shown onscreen inside their parent. The two radio buttons are created with buttonGroup, not this, as the parent, because they're displayed inside the button group's area. Qt allows widgets to be nested to any depth.
Let's continue reviewing the constructor code. Now that we've created the child widgets, we need to create layout managers to manage them:
QHBoxLayout *topLeftLayout = new QHBoxLayout; topLeftLayout->addWidget(label); topLeftLayout->addWidget(lineEdit); QVBoxLayout *leftLayout = new QVBoxLayout; leftLayout->addLayout(topLeftLayout); leftLayout->addWidget(buttonGroup); leftLayout->addWidget(caseCheckBox); QVBoxLayout *rightLayout = new QVBoxLayout; rightLayout->addWidget(findNextButton); rightLayout->addWidget(closeButton); rightLayout->addStretch(1); QHBoxLayout *mainLayout = new QHBoxLayout(this); mainLayout->setMargin(11); mainLayout->setSpacing(6); mainLayout->addLayout(leftLayout); mainLayout->addLayout(rightLayout); // more follows
We create four layout manager objects, as shown in Figure 3:
The first one, topLeftLayout, is a horizontal layout that contains the Find what label and the line edit side-by-side.
leftLayout is a vertical layout that contains topLeftLayout, buttonGroup, and caseCheckBox.
Similarly, rightLayout is a vertical layout that contains the two push buttons and a stretch (represented by a blue "spring" in Figure 3). The stretch fills empty space in a layout, and ensures that the Find Next and Close buttons stay aligned at the top. Finally, mainLayout is a horizontal layout that contains leftLayout and rightLayout side-by-side.
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Figure
3 The Find dialog's layout.
Notice that we need no layout for the two radio buttons inside the QButtonGroup. Thanks to the setColumns(2) call on the QButtonGroup earlier in the constructor, QButtonGroup will automatically arrange its child widgets as a grid.
Once we're done with the layouts, the only thing that's left of the constructor code is the following signal/slot connections:
connect(findNextButton, SIGNAL(clicked()),
this, SLOT(findNextButtonClicked()));
connect(closeButton, SIGNAL(clicked()),
this, SLOT(close()));
}
We connect the Find Next button's clicked() signal to the Find dialog's findNextButtonClicked() slot. This ensures that findNextButtonClicked() will be called whenever the user clicks the button, or presses the Spacebar when the button has the focus.
Then we connect the Close button to the Find dialog's close() slot. This slot is provided by QDialog, FindDialog's base class.
Apart from the constructor, the only function we need to implement is the findNextButtonClicked() slot:
void FindDialog::findNextButtonClicked()
{
emit findNext(lineEdit->text(), caseCheckBox->isOn(),
upRadioButton->isOn());
}
When the user clicks Find Next, we emit the dialog's findNext() signal with three arguments:
Text typed by the user
Boolean value indicating whether the search is case sensitive
Boolean value indicating whether the search should go up or down
The emit keyword is Qt-specific; like the signals and slots keywords, it's converted to standard C++ by the C++ preprocessor.
This completes the FindDialog class. The result is a fully functional Find dialog that can be used from a Qt application. The application simply needs to connect the findNext() signal to a slot that takes three parameters of the same types as the signal, and implement the actual search functionality in that slot.