QML: Using cpp signal in QML always results in “Cannot assign to non-existent property”

问题:

I just want to connect a cpp signal to a qml slot and tried different ways, but it always results in the same QML-Error at runtime: Cannot assign to non-existent property "onProcessed"! Why?

This is my Cpp Object:

#include <QObject>  class ImageProcessor : public QObject {     Q_OBJECT public:     explicit ImageProcessor(QObject *parent = 0);  signals:     void Processed(const QString str); public slots:     void processImage(const QString& image); };  ImageProcessor::ImageProcessor(QObject *parent) :     QObject(parent) { }  void ImageProcessor::processImage(const QString &path) {     Processed("test"); } 

This is my main.cpp

#include <QGuiApplication> #include <QQmlApplicationEngine> #include <QtQml>  #include "imageprocessor.h"  int main(int argc, char *argv[]) {     qmlRegisterType<ImageProcessor>("ImageProcessor", 1, 0, "ImageProcessor");      QGuiApplication app(argc, argv);     QQmlApplicationEngine engine;     engine.load(QUrl(QStringLiteral("qrc:///main.qml")));      return app.exec(); } 

And this is my QML file

import QtQuick 2.2 import QtQuick.Window 2.1 import QtMultimedia 5.0  import ImageProcessor 1.0  Window {     visible: true     width: maximumWidth     height: maximumHeight      Text {         id: output         text: qsTr("Hello World")         anchors.centerIn: parent     }      VideoOutput {         anchors.fill: parent         source: camera     }      Camera {         id: camera         // You can adjust various settings in here          imageCapture {             onImageCaptured: {                 imageProcessor.processImage(preview);             }         }     }      MouseArea {         anchors.fill: parent         onClicked: {             camera.imageCapture.capture();         }     }      ImageProcessor{         id: imageProcessor         onProcessed: {             output.text = str;         }     } } 

I am using QT 5.3.0 with Qt Creator 3.1.1, which is even suggesting me onProcessed and highlights it correctly.

回答1:

For exposing signals from C++ Object you must follow some naming conventions:

• Signal must begin by a lowercase letter in your C++ code, i.e void yourLongSignal()
• Signal handler in QML will be named on<YourLongSignal>

So, the only thing you have to edit in your code is to change

signals:     void processed(const QString& str); 

文章来源: QML: Using cpp signal in QML always results in “Cannot assign to non-existent property”