Import Qt function to QML file? - qt

Question

How do you call for example QFile::exists(path) inside a QML file in Qt 5.5?
MyFile.qml
import QtQuick 2.5
// These are some various things I've tried, with the error message they
// give related to the row where I call QFile::exists()
#include <QFile> // Expected token `{'
import QtQml 2.5 // Expected token `;'
import io.qt // Expected token `;'
import io.qt.QFile // Expected token `;'
Item {
id: root
property string imageSource
Image {
id: test
source: fileOrFallback(root.imageSource)
}
function fileOrFallback (source) {
return QFile::exists(source)
? preprocessor.getFilePath(source)
: theme.example + 'placeholder.png'
}
}
I've seen some examples on how to import your custom Qt functions, but how do you call built-in Qt functions in QML?

Answer 1

You cannot directly import a C ++ function, in these cases the approach is to create a QObject that exposes the method through a Q_INVOKABLE:
backend.h
#ifndef BACKEND_H
#define BACKEND_H
#include <QObject>
class Backend : public QObject
{
Q_OBJECT
public:
using QObject::QObject;
Q_INVOKABLE bool exists(const QString &fileName);
};
#endif // BACKEND_H
backend.cpp
#include "backend.h"
#include <QFile>
bool Backend::exists(const QString &fileName){
return QFile::exists(fileName);
}
main.cpp
#include "backend.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
Backend backend;
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("backend", &backend);
const QUrl url(QStringLiteral("qrc:/main.qml"));
QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
&app, [url](QObject *obj, const QUrl &objUrl) {
if (!obj && url == objUrl)
QCoreApplication::exit(-1);
}, Qt::QueuedConnection);
engine.load(url);
return app.exec();
}
*.qml
// ...
function fileOrFallback (source) {
return backend.exists(source)
? preprocessor.getFilePath(source)
: theme.example + 'placeholder.png'
}
// ...