QT – Dialogs

Blanchette and Summerfield, Ch. 2

Overview

• About Qt – widgets, signals and slots, synchronization, layout

• Windows vs. Qt dialog handling

• Example – findDialog

• Steps in defining and implementing subclass findDialog (objects and c++ file structure)– Define subclass: finddialog.h– Implement subclass: finddialog.cpp– Execute: main.cpp

• More signals and slots

• Qt Designer– Learn by doing

Hello, Qt!

• Quick review of last time

• Qt-ese

Hello, Qt!

#include <QApplication>

#include <QLabel>

int main(int argc, char *argv[])

{

QApplication app(argc, argv);

QLabel *label = new QLabel("Hello Qt!");

label->show();

return app.exec();

}

• A bit for dramatic effect, but there will be advantages• Just a label in a field with no real “main window”

• Will look at some basics tonight• Slots and signals (vs. event queues), and layouts (this is a GUI builder),

Hello, Qt!

#include <QApplication> // Definitions of QApplication and

#include <QLabel> // QLabel classes

// Qt is truly object-oriented

int main(int argc, char *argv[])

{

QApplication app(argc, argv); // Creates a QApplication object to

// manage app-wide resources

QLabel *label = new QLabel("Hello Qt!"); // Creates a QApplication widget (used as

// a window) that displays string

label->show(); // Make label (window) visible

return app.exec(); // Passes control to Qt

} // (would also delete QLabel)

Handling Events: Signals and Slots

• Qt not handle events (or event queue) directly

• Qt widgets emit signals when user action or change of state occurs

• Signal can be connected to a function, or slot– When signal emitted the slot, or function, is executed

• In example, will connect button’s clicked signal to slot - function quit()– quit() is “built-in” QApplication function (object)

A Signal and A Slot

#include <QApplication> // Include class definitions

#include <QPushButton>

int main(int argc, char *argv[])

{

QApplication app(argc, argv); // Create QApplication object

QPushButton *button = new QPushButton("Quit"); // Create labeled button

// Associate (connect) a particular signal of this application with a slot

QObject::connect(button, SIGNAL(clicked()), &app, SLOT(quit()));

button->show();

return app.exec(); // Pass control to QT

}

Synchronization & Layout of 2 Widgets

• Creates three widgets:– QSpinbox, QSlider, QWidget (application’s main window, will be parent of others)

• Using signals and slots of the QSpinbox and QSlider, set value of one depending on value set in the other

– E.g., change spinbox value to 70, slider will move to appropriate position– Change slider to some position, spinbox value will be changed based on position

• “Layout manager” object will set size and position of widgets

“Setting up”, 1/3

#include <QApplication> // Include class definitions,

#include <QHBoxLayout> // including for layout, slider

#include <QSlider> // and spinbox

#include <QSpinBox>

int main(int argc, char *argv[])

{

QApplication app(argc, argv);

QWidget *window = new QWidget; // Again, window will be widget

window->setWindowTitle("Enter Your Age"); // Not good interface design

Create & Synchronize Widgets, 2/3

QSpinBox *spinBox = new QSpinBox; // Create spinbox and slider

QSlider *slider = new QSlider(Qt::Horizontal);

spinBox->setRange(0, 130); // Set/define range of each

slider->setRange(0, 130);

// As before, a particular widget signal causes a function (slot) to be called

// here, when the value in the spinbox is changed, the function to set the value in the

// slider is called, and passed the new value of the spinbox to be the new value of the slider

QObject::connect(spinBox, SIGNAL(valueChanged(int)), slider, SLOT(setValue(int)));

// … and vice versa

QObject::connect(slider, SIGNAL(valueChanged(int)), spinBox, SLOT(setValue(int)));

spinBox->setValue(35); // Set initial value for spinbox, slider will then get set

Dialogs, and Qt

• “Dialog” in ui programming refers to exchange of information of user and program

– See Dix et al. and others for thorough treatment

• Dialog boxes are ui elements that can do this, as are other elements – For Qt-ese, just say “dialogs”

• “GUI applications” consist of main window, menu bar, tool bar, … and many dialog boxes (dialogs)

– From a programming perspective, could create an application that was just a “dialog”, e.g., calculator

Windows vs. Qt Dialog Handling

• Recall, windows api requires:– Creation of form and elements of dialog, or widgets, (resource script or visually –

tool - created)– Display of dialog (wndproc – event handling procedure)– Separate dialog function for dialog event handling– Post dialog function handling (in wndproc)

• Similarities and differences with Qt– Dialog elements (widgets) created (program or visually - tool)– Display of dialog (by program and widget)– Not separate dialog function – slots and signals for control flow– Different post dialog handling (essentially none, dialog widget)

Dialogs –1st Example

• Implement dialog as (sub)class– Independent, self-contained component– Signals and slots all in one– No “window”– QWidget > QDialog > FindDialog

• At right, from last time – QDialog not shown

• Dialog consists of 5 widgets– Edit box, 2 check boxes, 2 buttons

• Program will do class setups, definition and placement of widgets

• Three files: – finddialog.h, finddialog.cpp, main.cpp

Steps in Defining and Implementing Subclass findDialog

• Will look at the code in detail – “survival kit” for creating dialogs

1. finddialog.h– Set up: include, forward declarations– FindDialog subclass definition

2. finddialog.cpp– Will implement findDialog class:– Create dialog elements (widgets)– Connect signals and slots– Lay out widgets using layout managers– Define slots, or functions

3. main.cpp– As before, create new object, show it, and execute

1. finddialog.hSet up: include, forward

declarations

#ifndef FINDDIALOG_H // For c++ preprocessor – only include once

#define FINDDIALOG_H

#include <QDialog> // QDialog is Qt base class (it inherits Qwidget)

// Again, below from last time

class QCheckBox; // forward declarations of the Qt classes used

class QLabel; // tells C++ compiler exists, later defined

class QLineEdit;

class QPushButton;

finddialog.h FindDialog - Subclass

Definition

class FindDialog : public QDialog // FindDialog is a subclass of QDialog

{

Q_OBJECT // Required macro for classes defining sgnls or slots

public:

FindDialog (QWidget *parent = 0); // Qt widget class constructor, here, no parent

// (“signals” and “slots” are macros)

signals: // Define signal emitted when “Find” button clicked

void findNext (const QString &str, Qt::CaseSensitivity cs); // depends on match case

void findPrevious (const QString &str, Qt::CaseSensitivity cs); // Qt::Ca… is an enum type

private slots: // Define slots

void findClicked ();

void enableFindButton (const QString &text);

private: // Forward declarations of classes

QLabel *label; QLineEdit *lineEdit;

QCheckBox *caseCheckBox; QCheckBox *backwardCheckBox;

QPushButton *findButton; QPushButton *closeButton;

}

2. finddialog.cppImplement finddialog class:Create dialog elts (widgets)

#include <QtGui> // Definitions of Qt’s GUI classes - hundreds

#include "finddialog.h“

FindDialog::FindDialog (QWidget *parent) : Qdialog (parent) // pass parent to base class constructor

{

label = new Qlabel (tr ("Find &what")); // ----- Create dialog element, Qlabel, etc. --------

lineEdit = new QLineEdit; // tr func for translation, & indicates shortcut key

label->setBuddy (lineEdit); // “buddy” is widget that accepts focus w/shortcut

caseCheckBox = new QCheckBox (tr ("Match &case")); // ------ Qcheckbox ------

backwardCheckBox = new QCheckBox (tr ("Search &backward")); // ------ Qcheckbox ------

findButton = new QPushButton (tr ("&Find")); // ------ QPushbutton ------

findButton->setDefault (true); // “default” indicates executed on key Enter

findButton->setEnabled (false); // not enabled, grayed out

closeButton = new QPushButton (tr ("Close"));

finddialog.cppConnect signals and slots

// Signal “textchanged” from lineEdit widget enables button labeled “find” by private slot enableFindButton

connect ( lineEdit, SIGNAL (textChanged (const QString &)),

this, SLOT (enableFindButton (const QString &)));

// Again, private slot, findClicked, called when signal “clicked” emitted by widget findbutton

connect ( findButton, SIGNAL (clicked ()),

this, SLOT (findClicked ()));

// As before, Qt function close called when widget closeButton emits clicked signal

connect ( closeButton, SIGNAL (clicked ()),

this, SLOT (close ()));

finddialog.cpp Lay out widgets using

layout managers

• Layout managers are Qt’s “system” for positioning display elements, e.g., widgets, spacers

• MUCH more flexible than windows

• Easily, and naturally, supports incremental development

• … and have their own terminology and rules, e.g., nested, boxes

• Will work through example

finddialog.cpp Lay out widgets using

layout managers

// layouts are nested – constrain widgets and other layouts

QHBoxLayout *topLeftLayout = new QHBoxLayout;

topLeftLayout->addWidget(label);

topLeftLayout->addWidget(lineEdit);

QVBoxLayout *leftLayout = new QVBoxLayout;

leftLayout->addLayout(topLeftLayout);

leftLayout->addWidget(caseCheckBox);

leftLayout->addWidget(backwardCheckBox);

finddialog.cpp Lay out widgets using

layout managers

// layouts are nested – constrain widgets and other layouts

QHBoxLayout *topLeftLayout = new QHBoxLayout;

topLeftLayout->addWidget(label);

topLeftLayout->addWidget(lineEdit);

QVBoxLayout *leftLayout = new QVBoxLayout;

leftLayout->addLayout(topLeftLayout);

leftLayout->addWidget(caseCheckBox);

leftLayout->addWidget(backwardCheckBox);

QVBoxLayout *rightLayout = new QVBoxLayout;

rightLayout->addWidget(findButton);

rightLayout->addWidget(closeButton);

rightLayout->addStretch(); // fill – here, below

finddialog.cpp Lay out widgets using

layout managers

// layouts are nested – constrain widgets and other layouts

QHBoxLayout *topLeftLayout = new QHBoxLayout;

topLeftLayout->addWidget(label);

topLeftLayout->addWidget(lineEdit);

QVBoxLayout *leftLayout = new QVBoxLayout;

leftLayout->addLayout(topLeftLayout);

leftLayout->addWidget(caseCheckBox);

leftLayout->addWidget(backwardCheckBox);

QVBoxLayout *rightLayout = new QVBoxLayout;

rightLayout->addWidget(findButton);

rightLayout->addWidget(closeButton);

rightLayout->addStretch(); // fill – here, below

QHBoxLayout *mainLayout = new QHBoxLayout;

mainLayout->addLayout(leftLayout);

mainLayout->addLayout(rightLayout);

setLayout(mainLayout);

setWindowTitle (tr ("Find")); setFixedHeight (sizeHint().height()); }

FindDialog Constructors -Parent-child Relationships

text

Define finddialog slots, or functions

// Called when user clicks Find button, emits signal findPrevious() or findNext

void FindDialog :: findClicked()

{

QString text = lineEdit->text();

Qt::CaseSensitivity cs =

caseCheckBox->isChecked() ? Qt::CaseSensitive

: Qt::CaseInsensitive;

if (backwardCheckBox->isChecked()) {

emit findPrevious(text, cs);

} else {

emit findNext(text, cs); // emit keyword (macro) specific to Qt

}

}

// Called whenever user changes text in line editor, enables button if there is text in line editor

void FindDialog :: enableFindButton(const QString &text)

{

findButton->setEnabled (!text.isEmpty ());

}

3. main.cpp

// As before, simply create a new object, here FindDialog, show it, and execuute

#include <QApplication>

#include "finddialog.h“

int main (int argc, char *argv[])

{

QApplication app (argc, argv);

FindDialog *dialog = new FindDialog;

dialog->show();

return app.exec();

}

More about Signals and Slots

• “Signals and slots enables application programmer to bind objects together without objects knowing anything about each other”

– Essentially c++ member functions– Can be virtual, overloaded, public, protected, private, invoked indirectly– Connected to a signal and automatically called each time the signal is emitted

• connect (sender, SIGNAL (signal), receiver, SLOT (slot) )

• So far, only different signals to different slots

• Other ways, as well

Signals and Slots

• Again, form is:

connect (sender, SIGNAL (signal), receiver, SLOT (slot) )

1. One signal can be connected to many slots:– connect(slider, SIGNAL(valueChanged(int)),

spinBox, SLOT(setValue(int)));– connect(slider, SIGNAL(valueChanged(int)),

this, SLOT(updateStatusBarIndicator(int)));– When signal emitted, slots called one after the other, in arbitrary order

2. Many signals can be connected to the same slot:– connect(lcd, SIGNAL(overflow()),

this, SLOT(handleMathError()));– connect(calculator, SIGNAL(divisionByZero()),

this, SLOT(handleMathError()));– When either signal is emitted, the slot is called.

Signals and Slots

• connect (sender, SIGNAL (signal), receiver, SLOT (slot) )

3. A signal can be connected to another signal:– connect(lineEdit, SIGNAL(textChanged(const QString &)),

this, SIGNAL(updateRecord(const QString &)));– When the first signal is emitted, the second signal is emitted as well.

4. Connections can be removed:– disconnect(lcd, SIGNAL(overflow()),

this, SLOT(handleMathError()));

Can Use Signals not only with Widgets, but as More General Mechanism

// Define class

class Employee : public QObject

{

Q_OBJECT

public:

Employee() { mySalary = 0; }

int salary() const { return mySalary; }

public slots:

void setSalary (int newSalary);

signals:

void salaryChanged (int newSalary);

private:

int mySalary;

};

// Use signal to communicate with any/all slots

// (functions) connected to salaryChanged

// signal!

void Employee::setSalary (int newSalary)

{

if (newSalary != mySalary) {

mySalary = newSalary;

emit salaryChanged (mySalary);

}

}

Qt Designer – Qt’s Visual Tool: Layout, widgets, …

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Qt Designer – Qt’s Visual Tool: Layout, widgets, …

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Qt Designer

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End

• … and you’re off and running …