How to display QML view( with scroll) to load and display multiline Text around 10 million lines from file [duplicate] - qt

Question

I have a Qt application and I'd like to show some log. I use a TextArea. However, if the log is large or the events come too fast the GUI can't draw Textarea fast enough.
I have analyzed this problem with Qt Creator (QML Profiler) and if the log is large it takes 300 ms to draw the GUI. I use this software on a Raspberry Pi2.
Any ideas how to solve this? Should I use other QML controls? Thanks.
QML code:
TextArea {
text: appHandler.rawCommunication
readOnly: true
}
C++ code:
Q_PROPERTY(QString rawCommunication READ rawCommunication WRITE setrawCommunication NOTIFY rawCommunicationChanged)
void setrawCommunication(QString val)
{
val.append("\n");
val.append(m_rawCommunication);
m_rawCommunication = val;
emit rawCommunicationChanged(m_rawCommunication);
}

Answer 1

Use a view, like ListView. They instantiate their delegates as needed, based on which data the view says it needs to show depending on the position the user is at in the list. This means that they perform much better for visualising large amounts of data than items like TextArea, which in your case is going to keep a massive, ever-growing string in memory.
Your delegate could then be a TextArea, so you'd have one editable block of text per log line. However, if you don't need styling, I'd recommend going with something a bit lighter, like TextEdit. Taking it one step further: if you don't need editable text, use plain old Text. Switching to these might not make much of a difference, but if you're still seeing slowness (and have lots of delegates visible at a time), it's worth a try.

Answer 2

I tried the ListView suggestion but it has several drawbacks:
No easy way to keep the view positioned at the bottom when new output is added
No selection across lines/delegates
So I ended up using a cached TextArea, updating once every second:
TextArea {
id: outputArea_text
wrapMode: TextArea.Wrap
readOnly: true
font.family: "Ubuntu Mono, times"
function appendText(text){
logCache += text + "\n";
update_timer.start();
}
property string logCache: ""
Timer {
id: update_timer
// Update every second
interval: 1000
running: false
repeat: false
onTriggered: {
outputArea_text.append(outputArea_text.logCache);
outputArea_text.logCache = "";
}
}
Component.onCompleted: {
my_signal.connect(outputArea_text.appendText)
}
}

Answer 3

try this approach:
create a c++ logger class
append all logs to this class
and print them using some action, example a button click
this will solve your performance issue
Example of code:
Logger.h
#ifndef LOGGER_H
#define LOGGER_H
#include <QQmlContext>
#include <QObject>
#include <QStringList>
#include <QQmlEngine>
#include <QString>
#include <QtCore>
#include <QDebug>
class Logger : public QObject
{
Q_OBJECT
public:
explicit Logger(QObject *parent = 0);
~Logger();
Q_INVOKABLE QStringList *getLogStream();
Q_INVOKABLE void printLogStream();
Q_INVOKABLE void appendLog(QString log);
Q_INVOKABLE void log(QString log="");
Q_INVOKABLE void log(QString fileName, QString log);
signals:
public slots:
private:
QStringList* stringStream_;
};
#endif // LOGGER_H
Logger.cpp
#include "logger.h"
Logger::Logger(QObject *parent) :
QObject(parent),
stringStream_(new QStringList)
{
}
~Logger(){
if(stringStream_ != NULL)
{
delete stringStream_;
stringStream_ = NULL;
}
}
QStringList* Logger::getLogStream(){
return stringStream_;
}
void Logger::printLogStream()
{
QStringListIterator itr(*stringStream_);
while (itr.hasNext())
qDebug()<< itr.next()<<"\n";
}
void Logger::appendLog(QString log){
stringStream_->push_back(log) ;
}
void Logger::log(QString fileName,QString log)
{
#ifdef ENABLElogs
fileName.push_front(" [");
if(!fileName.contains(".qml"))
{
fileName.append(".qml]:");
}
qDebug()<<fileName<<log;
#else
Q_UNUSED(log);
Q_UNUSED(fileName);
#endif
}
void Logger::log(QString log)
{
#ifdef ENABLElogs
qDebug()<<log;
#else
Q_UNUSED(log);
#endif
}
main.cpp
#include <QtGui/QGuiApplication>
#include "qtquick2applicationviewer.h"
#include "logger.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QtQuick2ApplicationViewer *viewer = new QtQuick2ApplicationViewer;
Logger* stream = new Logger;
viewer->rootContext()->setContextProperty("Stream",stream);
viewer->setMainQmlFile(QStringLiteral("qml/project/main.qml"));
viewer->showExpanded();
return app.exec();
}
main.qml
import QtQuick 2.0
import QtQuick.Controls 1.1
Rectangle {
width: 800
height: 480
Text {
text: qsTr("Hello World")
anchors.centerIn: parent
Component.onCompleted: Stream.appendLog("Text object is completed")
}
Column{
x:300
Button{
text:"append"
onClicked: {
Stream.appendLog("MouseArea object clicked")
}
Component.onCompleted: Stream.appendLog("Button object is completed")
}
Button{
text:"logger"
onClicked: {
Stream.printLogStream()
}
Component.onCompleted: Stream.appendLog("Button logger object is completed")
}
}
TextArea{
text:"blablabla"
Component.onCompleted: Stream.appendLog("TextArea object is completed")
}
Component.onCompleted: Stream.appendLog("the main object is completed")
}
project.pro
#add this line
# comment it, run qmake and recompile to disable logs
DEFINES += ENABLElogs
using this approch you can stop all logs with a single line change when you want to release your soft

Answer 4

However, I have included complete code, using "QAbstractListModel" for a Logging heavy data to QML
listmodel.h
#ifndef LISTMODEL_H
#define LISTMODEL_H
#include <QAbstractListModel>
class ListModel: public QAbstractListModel
{
Q_OBJECT
public:
ListModel();
// Q_PROPERTY(QStringList logs READ name WRITE nameChanged)
int rowCount(const QModelIndex & parent = QModelIndex()) const;
QVariant data(const QModelIndex & index, int role = Qt::DisplayRole) const;
Q_INVOKABLE QVariant activate(int i);
private:
QStringList m_list;
};
#endif // LISTMODEL_H
listmodel.cpp
#include "listmodel.h"
#include <QFile>
#include <QHash>
ListModel::ListModel()
{
QFile file("/home/ashif/LogFile");
if(!file.open(QIODevice::ReadOnly))
{
qDebug( "Log file open failed" );
}
bool isContinue = true;
do
{
if(file.atEnd())
{
isContinue = false;
}
m_list.append(file.readLine());
}
while( isContinue);
}
int ListModel::rowCount(const QModelIndex & parent ) const
{
return m_list.count();
}
QVariant ListModel::data(const QModelIndex & index, int role ) const
{
if(!index.isValid()) {
return QVariant("temp");
}
return m_list.value(index.row());
}
QVariant ListModel::activate(int i)
{
return m_list[i];
}
main.qml
import QtQuick 2.3
import QtQuick.Window 2.2
import QtQuick.Controls 1.4
Window {
visible: true
ListView
{
width: 200; height: 250
anchors.centerIn: parent
model:mylistModel
delegate: Text
{
text:mylistModel.activate(index)
}
}
}
main.cpp
#include <QGuiApplication>
#include <QQmlContext>
#include <QQmlApplicationEngine>
#include "logger.h"
#include "listmodel.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
Logger myLogger;
ListModel listModel;
engine.rootContext()->setContextProperty("mylistModel", &listModel);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}