Details
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Bug
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Resolution: Unresolved
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P2: Important
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6.5, 6.6, 6.7
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None
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All
Description
When a C++ classed exposed to QML has a property of type QVariantList and this property is serialized in QML via JSON.stringify() the property is read over again for each element in the list. This can be a serious performance issue and worked properly in Qt 6.2 and earlier. The expected behaviour is that the property is only read/evaluated once and the output of the provided example is just 1 instead of numbers 1…1001. As a workaround the type of the property can be changed to QJsonArray.
Minimal working example:
main.cpp
#include <QCoreApplication> #include <QQmlContext> #include <QQmlEngine> #include <QQmlExpression> #include "Test.h" int main(int argc, char *argv[]) { QCoreApplication app(argc, argv); QQmlEngine engine; Test test; engine.rootContext()->setContextProperty(QStringLiteral("test"), &test); QQmlExpression(engine.rootContext(), nullptr, QStringLiteral("JSON.stringify(test.foo)")).evaluate(); return 0; }
Test.h
#pragma once #include <QQmlEngine> class Test : public QObject { Q_OBJECT QML_ELEMENT Q_PROPERTY(QVariantList foo READ foo) public: QVariantList foo() const { static int callCount = 0; qCritical() << ++callCount; QVariantList list; for (int i = 0; i < 1000; i++) { list.append(i); } return list; } };
CMakeLists.txt
cmake_minimum_required(VERSION 3.16) project(test VERSION 0.1 LANGUAGES CXX) find_package(Qt6 REQUIRED COMPONENTS Qml) set(CMAKE_AUTOMOC TRUE) qt_add_executable(test main.cpp Test.h) target_link_libraries(test PRIVATE Qt6::Qml)
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