aidysun / qt

QT

• qtchooser
• Qt Modules
• .pro
• Concrete QWidget
• UIC
• MOC (meta-object compiler)
  • Signal
  • Slots
  • slot/signal in Qt 4
  • QObject
• Q_OBJECT v.s. Q_GADGET
• UI
  • Stylesheet
  • Layout
    • 1. Anchor
    • 2. Layout
• Event
  • event filter
• Paint
  • Device
• File
• Logging
  • Logging Rules

qtchooser

vi /usr/lib/x86_64-linux-gun/qt-chooser/default.conf
  <qt>/bin
  <qt>/lib

Qt Modules

• Basic
  • Qt Core
  • Qt GUI
  • Qt Quick
  • Qt QML
  • Qt Test
  • Qt Network
  • Qt Multimedia
  • Qt SQL
  • Qt Webkit
  • Qt WebKit Widgets
  • Qt Widgets
• Extended
  • Qt 3D / Qt Bluetooth / Qt Contact
  • Qt Location / Qt Organizer / Qt Sensors / Qt Publis and Subscribe
  • Qt Service Framework / System Info / JSON DB / Versit / Wayland / Feekback

.pro

CONFIG += C++11

Concrete QWidget

class MyWidget: public QWidget {
public:
	MyWidget() : QWidget() {}
	~MyWidget() {}

public slots:	// optional parameters, dependents on signal functions, // void return, need implementation
	void button_click();

signals: 	// no return value, optional parameters // only declare, no implementation
	void show_hide_signal(int a);
};

UIC

• MOC is not the only code generator. Another code generator that Qt uses is uic (User Interface Compiler).
• UIC reads user interface described in XML and creates C++ code that sets up the form.

MOC (meta-object compiler)

• The program that handles Qt's C++ extensions.
  • moc reads C++ source, if it contains Q_OBJECT macor, it produces another C++ file which contain the meta object code for those classes.

Signal

• one signal func can connect to multiple slots function, invoke order is undefined
• multiple signal can connect to one slot
• one signal can connect to another signal
  • the slot func of the signal can emit another signal

Slots

• member func, static func, global func, lambda func(since QT5)
  • Lambda

    int a = 20;
    [=]() mutable -> int {
    	a = 100;
    	return a;
    }
    

• slots function parameter could be less than the parameter in signal func
  • if slots func has parameters, it should be the same order / type with signal func
• when one object is deleted, QT disconnect all slots connected to the object automatically

slot/signal in Qt 4

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

• Qt5

  • QObject::connect(&button, &QPushButton::clicked, &app, &QApplication::quit);

• Lambda

  • QObject::connect(button, &QPushButton::clicked, secondWindow, [secondWindow]() { secondWindow->setText("clicked"); });
  • the 3rd parameter secondWindow is optional, but to specific it would prevent crash issue once secondWindow is deleted earlier than current connect.

QObject

• adds features to C++ (MOC)
  • signals and slots
  • Properties
  • Event handlong
  • Memory Management
    • destruct all children first in its destructor

Q_OBJECT v.s. Q_GADGET

Q_GADGET is lighter version of Q_OBJECT, it doesn't have signal and slot.

The Q_GADGET macro is a lighter version of the Q_OBJECT macro for classes that do not inherit from QObject but still want to use some of the reflection capabilities offered by QMetaObject. Just like the Q_OBJECT macro, it must appear in the private section of a class definition.
Q_GADGETs can have Q_ENUM, Q_PROPERTY and Q_INVOKABLE, but they cannot have signals or slots.
Q_GADGET makes a class member, staticMetaObject, available. staticMetaObject is of type QMetaObject and provides access to the enums declared with Q_ENUMS.

UI

• relative path used in UI is related to Makefile
  • not UI file, not source file, not exe
• when adding resource/image to resouce file, it must be inside the folder of project

Stylesheet

Layout

1. Anchor

• x, y, width, height could be affected by anchor

Rectangle {
	id: bg

	Rectangle {
		width: 40; height: 40
		y: 20
		anchor.right: bg.right
		anchor.top: bg.top 		// override y:20
	}
}

2. Layout

Event

• e.ignore() generally is not used in override event functions, it may used in dialog close event.

class MyButton: public QPushButton {
protected:
	void MyButton::mousePressedEvent(MouseEvent *e) {
		if (e->button() == Qt::LeftButton) {
			qDebug() << "left button pressed";
			// e.ignore(); // this would pass the event to parent widget, not super class (inhereted class)
		}
		QPushButton::mousePressedEvent(e);
	}

/// event filter

private: 
	MyLabel *label;
	/*
	label->installEventFilter(this);

	bool eventFilter(QObject *obj, QEvent *e) {
		if (obj == lable) {
			if (e->type() == QEvent::MouseMove) {
				// ...
			}
		}
		return QWidget::eventFilter(obj, e);
	}
	*/
};

event filter

• In QObject
• return true : it won't forward anymore; return false : it will continue forward
• event filter and the object/widget it filters MUST be in same thread

Paint

Device

1. QPixmap: optimized for scree, platform-related, cannot modify image
2. QImage : platfor-unrelated, can modify pixes of image, can be drawn in thread
3. QPicture : save paiting status in binary

File

QFileInfo q(path);
qDebug << q.fileName().utf8().data(); // multil-bytes

Logging

• Def Logging category in code

bool Highscore::load(const QString &filepath)
{
    QLoggingCategory fcIo("fc.io");
    qCDebug(fcIo) << "Loading highscore ...";

• Def Logging category in one place

// fclogging.h
#include <QLoggingCategory>
Q_DECLARE_LOGGING_CATEGORY(fcIo)

// fclogging.cpp
#include "fclogging.h"
Q_LOGGING_CATEGORY(fcIo, "fc.io")

Logging Rules

• Rules can be defined in config file or in code

int main(int argc, char **argv)
{
    QGuiApplication app(argc, argv);
    QLoggingCategory::setFilterRules("*.debug=false\n"
                                     "fc.io.debug=true");
    // ...
}

// ~/.config/QtProject/qtlogging.ini:

[rules]
*.debug=false
fc.io.debug=true