QSyntaxHighlighter rehighlight() ignores changes triggered from another thread - qt

Question

I have a spellchecking thread which fires spellcheck() signals from time to time which are connected to my highlighter's rehighlight() method. The latter sets the whole block to have red foreground.
This used to work in Qt 5.6.2 and ceased to work in newer versions. I hopelessly waited for it to get fixed in Qt 5.9.5, but it still does not work (neither in Windows 10, nor in OS X).
Small example which reproduces the problem could be obtained from here https://bitbucket.org/ribtoks/qt-highlighting-issue (in order to repro, type something in the input. rehighlight() will be triggered each 7 seconds from background thread)
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QQuickTextDocument>
#include <QSyntaxHighlighter>
#include <QThread>
#include <QString>
class SpellCheckWorker : public QObject
{
Q_OBJECT
public:
explicit SpellCheckWorker(QObject *parent = 0) : QObject(parent), m_Counter(0), m_IsOK(false)
{ }
public:
bool isOK() { return m_IsOK; }
signals:
void spellcheck();
public slots:
void process() {
qInfo() << "Worker Thread is" << QThread::currentThreadId();
while (1) {
m_Counter++;
m_IsOK = m_Counter % 7 == 0;
QThread::sleep(1);
emit spellcheck();
}
}
private:
int m_Counter;
volatile bool m_IsOK;
};
class SpellCheckErrorsHighlighter : public QSyntaxHighlighter
{
Q_OBJECT
public:
SpellCheckErrorsHighlighter(SpellCheckWorker *worker, QTextDocument *document):
QSyntaxHighlighter(document),
m_Worker(worker)
{ }
virtual ~SpellCheckErrorsHighlighter() {}
protected:
virtual void highlightBlock(const QString &text) override {
if (!m_Worker->isOK()) {
qDebug() << "Worker is not OK" << text;
return;
}
qInfo() << "Reapplied formatting for" << text;
qInfo() << "Highlight thread is" << QThread::currentThreadId();
this->setFormat(0, text.length(), QColor(0xff, 0, 0));
}
private:
SpellCheckWorker *m_Worker;
};
class MainModel : public QObject
{
Q_OBJECT
Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)
public:
explicit MainModel(QObject *parent = 0) : QObject(parent), m_Worker(nullptr)
{ }
public:
QString name() const { return m_name; }
void startChecking() {
qInfo() << "Main thread is" << QThread::currentThreadId();
m_Worker = new SpellCheckWorker();
QThread *thread = new QThread();
m_Worker->moveToThread(thread);
QObject::connect(thread, &QThread::started, m_Worker, &SpellCheckWorker::process);
thread->start();
}
Q_INVOKABLE void initNameHighlighting(QQuickTextDocument *document) {
SpellCheckErrorsHighlighter *highlighter = new SpellCheckErrorsHighlighter(m_Worker, document->textDocument());
QObject::connect(m_Worker, &SpellCheckWorker::spellcheck,
highlighter, &SpellCheckErrorsHighlighter::rehighlight);
}
signals:
void nameChanged();
public slots:
void setName(QString name)
{
if (m_name == name)
return;
m_name = name;
emit nameChanged();
}
private:
SpellCheckWorker *m_Worker;
QString m_name;
};
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
MainModel mainModel;
mainModel.startChecking();
QQmlApplicationEngine engine;
QQmlContext *rootContext = engine.rootContext();
rootContext->setContextProperty("mainModel", &mainModel);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}
#include "main.moc"
and main.qml
import QtQuick 2.6
import QtQuick.Controls 1.0
import QtQuick.Layouts 1.3
import QtQuick.Window 2.0
ApplicationWindow {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
StackView {
id: mainStackView
anchors.fill: parent
focus: true
initialItem: Rectangle {
anchors.fill: parent
Rectangle {
id: titleRect
height: 30
width: 300
anchors.centerIn: parent
color: "#ffffff"
border.color: "#000000"
border.width: titleTextInput.activeFocus ? 1 : 0
clip: true
focus: false
Flickable {
id: titleFlick
contentWidth: titleTextInput.paintedWidth
contentHeight: titleTextInput.paintedHeight
height: parent.height
anchors.verticalCenter: parent.verticalCenter
anchors.left: parent.left
anchors.right: parent.right
anchors.leftMargin: 5
anchors.rightMargin: 5
clip: true
flickableDirection: Flickable.HorizontalFlick
interactive: false
focus: false
function ensureVisible(r) {
if (contentX >= r.x)
contentX = r.x;
else if (contentX+width <= r.x+r.width)
contentX = r.x+r.width-width;
}
TextEdit {
id: titleTextInput
width: paintedWidth > titleFlick.width ? paintedWidth : titleFlick.width
height: titleFlick.height
text: mainModel.name
focus: true
onTextChanged: mainModel.name = text
Component.onCompleted: mainModel.initNameHighlighting(titleTextInput.textDocument)
onCursorRectangleChanged: titleFlick.ensureVisible(cursorRectangle)
}
}
}
}
}
}
Is there any way to get it working with workarounds? I need to keep spellchecking logic in the background thread so it's not possible to move it to main thread.

Answer 1

You seem to be referring to two different root causes with the following two statements:
There's NO problem in delivering slot call to Highlighter.
and
m_Worker->isOK() check is there for a reason to execute highlighting only once per 7 seconds. The bug in Qt can only be demonstrated with this.
If I just focus on the second statement, it looks like the problem is that you are not able to hit the statement if (!m_Worker->isOK()) right?
That may well be a problem due to your code or a Qt issue on a specific platform.
Can you change the code to avoid the condition i.e. emit the spellcheck signal only when 7 seconds have passed to avoid making this check from another thread?

Answer 2

Ok, so after messing with your code, The issue is that you are not calling this line inside of the hilightBlock directive in main.cpp near line 55 in order to keep the format from before.
Add this in to fix it (I think) The question was still fairly unclear...
if (!m_Worker->isOK()) {
qDebug() << "Worker is not OK" << text;
this->setFormat(0, text.length(), this->format(0));
return;
}
The other part of the issue is that you don't have any rules defined for the syntax highlighter... so it will always be red once it turns red.