[QT] QXcbConnection: Could not connect to display

解決方法：

#helloworld -platform linuxfb

[Python] Eric IDE For Python On Ubuntu16.04

• 安装QT5
  Ref : http://smlie-blog.blogspot.tw/2017/11/qtqt5ubuntu1604.html
• 安装相關套
• pip3
  
  #apt-get install python3-pip
  #pip3 install --upgrade pip
• PyQt5
  
  #pip3 install PyQt5
• Mercurial
  
  #apt-get install mercurial
• QScintilla
  
  #pip3 install QScintilla
• Eric
  
  #hg clone hg clone https://die-offenbachs.homelinux.org/hg/eric
  #cd eric
  #python3 install.py

Reference :
[1] https://eric-ide.python-projects.org/index.html
[2]Install Eric 6 for Python (Windows/Ubuntu)

[QT]安装QT5至Ubuntu16.04

• Download
  #wget http://download.qt.io/official_releases/qt/5.7/5.7.0/qt-opensource-linux-x64-5.7.0.run
• Install
  #apt-get install build-essential
  #apt-get install libfontconfig1
  #apt-get install mesa-common-dev #for opengl
  #apt-get install libglu1-mesa-dev
  #apt-get install fcitx-frontend-qt5
  #cp /usr/lib/x86_64-linux-gnu/qt5/plugins/platforminputcontexts/libfcitxplatforminputcontextplugin.so /tmp
  #chmod 777 qt-opensource-linux-x64-5.7.0.run
  #./qt-opensource-linux-x64-5.7.0.run #照步骤完成
  #mv /tmp/libfcitxplatforminputcontextplugin.so /opt/Qt5.7.1/Tools/QtCreator/lib/Qt/plugins/platforminputcontexts #设定sogou输入

Reference
[1] https://wiki.qt.io/Install_Qt_5_on_Ubuntu

[QT] 使用QT設定 TCP/IP KeepAlive

在TCP/IP的機制下所建立的連線，如果Server/Client雙方都沒有要求執行結束的 Four-way handshake，則在TCP/IP機制下，雙方會認為對方一直連線中，直到Timeout结束後才會斷線。通常Timeout時間依據不同的作業系統而有些差異，但時間單位都是小時等級的，故就會發生如對方機器電源已斷電，但程式還是一直保持在E「STABLISHED」的狀態。

如果要解決這個問題，就可以使用TCP/IP 的KeepAlive機制，顧名思義，就是雙方定時回報還活著。範例如下，

QTcpSocket *_socket = new QTcpSocket(this);
if ( _socket != NULL ) {
    _socket->connectToHost(_address, _port);
    if ( _socket->waitForConnected(1000)) {
        int fd = _socket->socketDescriptor(); //一定要連線成功才可取得 descriptor
        int enableKeepAlive = 1;
        setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &enableKeepAlive, sizeof(enableKeepAlive));
        int maxIdle = 10;/* seconds */
        setsockopt(fd, SOL_TCP, TCP_KEEPIDLE, &maxIdle, sizeof(maxIdle));
        int count = 3;//send up to 3 keepalive packets out, then disconnect if no response
        setsockopt(fd, SOL_TCP, TCP_KEEPCNT, &count, sizeof(count));
        int interval = 2;//send a keepalive packet out every 2 seconds (after the 5 second idle period)
        setsockopt(fd, SOL_TCP, TCP_KEEPINTVL, &interval, sizeof(interval));
    }
}

而Linux的系统預設值如下，（注意：修改該參數則會影響到所以的Socket設定）

#cat /proc/sys/net/ipv4/tcp_keepalive_time 7200
#cat /proc/sys/net/ipv4/tcp_keepalive_intvl 75
#cat /proc/sys/net/ipv4/tcp_keepalive_probes 9

Reference : 
[1] QTcpSocket狀態總是連接，甚至連接到乙太網

[Qt] Qml与Qml間資料互傳

main.qml

Window{
    id : window
    Text {
        id : display

    }
    Component.onCompleted: {
        Qt.display = display; // Qt.[ObjectName] = [ObjectId]
    }
}

page.qml

Checkbox {
    onClicked : {
        Qt.display.text = "Just Do it" ; // Qt.[ObjectName] = [Operate]
    }
}

[QT] qt.qpa.input: X-less xkbcommon not available, not performing key mapping

錯誤：

qt.qpa.input: X-less xkbcommon not available, not performing key mapping

解法：

環境變數加入
export QT_QPA_EGLFS_NO_LIBINPUT=1

Reference :
[1] https://github.com/carlonluca/pot/issues/54

[QT] QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

Error Message

QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

會出現上述問題是因為重複使用了DB Driver,只要在開啟DB時給于名稱


方法一：使用預設名稱

QSqlDatabase datebase;    
if(QSqlDatabase::contains("qt_sql_default_connection"))
    datebase = QSqlDatabase::database("qt_sql_default_connection");
else
    datebase = QSqlDatabase::addDatabase("QSQLITE");

QSqlQuery query; //不指定，則使用defaults

方法二：自訂名稱

QSqlDatabase datebase;    
if(QSqlDatabase::contains("QSQLITE"))
     datebase = QSqlDatabase::database("QSQLITE");
else
     datebase = QSqlDatabase::addDatabase("QSQLITE", "QSQLITE");

QSqlQuery *query = new QSqlQuery(database); //不指定，則使用defaults

[QT] Undefined reference to vtable. Trying to compile a Qt project

When using Qt Creator:

1. Build → Run qmake
2. Build → Rebuild All

[QT] 型態轉換

• const char * to QString
  
  const char *charstr = "charstring";
  QString qstring = QString(QLatin1String(charstr));


• Integer to QString
  
  int num = 10;
  QString qstring = QString::number(num,10);


• QString to const char *
  
  QString message = "test";
  char * c_message = message.toLatin1().data();


• QByteArray to unsigned char *
  
  QByteArray message;
  message.resize(5);
  message[0] = 0xFF;
  message[1] = 0xFE;
  unsigned char * c_message = (unsigned char *)message.data();