從C ++對象動態創建qml對象（通過使用setContextProperty）

Question

我正在嘗試使用c ++類的對象在c ++中動態創建一個qml對象。 下面是我的方法的最小代碼。 執行此代碼后，單擊后，應用程序崩潰了（請參閱main.qml中的注釋）。

我已經粘貼了下面的代碼，可以在這里下載。

main.cpp中

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "scene.h"
int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    scene sc(engine);

    QQmlContext* context = engine.rootContext();
    context->setContextProperty("sc", &sc);

    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

scene.h

#ifndef SCENE_H
#define SCENE_H

#include <QObject>
#include <QQmlApplicationEngine>
#include <QQmlComponent>
class scene : public QObject
{
    Q_OBJECT
public:
    explicit scene(QQmlApplicationEngine& engine, QObject *parent = nullptr);
    QQmlApplicationEngine& engine;

public slots:
    void create_rect_object();
};

#endif // SCENE_H

scene.cpp

#include "scene.h"

scene::scene(QQmlApplicationEngine &engine, QObject *parent) : engine(this->engine),QObject(parent)
{

}    
void scene::create_rect_object()
{
    QQmlComponent component(&engine, QUrl::fromLocalFile("myrect.qml"));
    QObject *object = component.create();
    object->setProperty("width", 200);
    object->setProperty("height", 150);
    object->setProperty("color", "blue");
}

main.qml

import QtQuick 2.11
import QtQuick.Window 2.11

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    Rectangle{
        anchors.fill: parent
        color: "red"
        MouseArea{
            anchors.fill: parent
            onClicked: {
                console.log("Before click");
                sc.create_rect_object(); // application is crashing here
                console.log("after click");
            }
        }
    }
}

myrect.qml

import QtQuick 2.0

Rectangle {
    id:id_rec
    width: 100
    height: 100
    color: "green"
    x:0
    y:0
}

---

更新

要創建的對象不是主窗口根目錄的子項，而是主窗口根目錄的子項鏈中的一項的子項。 偽結構如下所示。

main.qml

Window {       

    customitem1{
        id:id_ci1

    }    
    customitem2{
        id:id_ci1        
    }       
}

customitem1.qml

Item {       

    customitem3{
        id:id_ci3

    }    
    customitem3{
        id:id_ci4        
    }       
}

Answer 1

[ 更新 ]

您有兩個崩潰的錯誤和一個不顯示矩形的錯誤

1.您的scene的構造函數成員初始化器列表虛假，導致應用崩潰

（ 提示：通過為類成員添加m_前綴為類的成員使用不同的命名，例如： m_engine表示READABILITY而不引起混淆 ）

//Correct WAY
class Something
{
 private:
    int m_value1;
    double m_value2;
    char m_value3;

 public:
    //#################  YOUR CASE  ###############################
    Something(int number) : m_value1(number), m_value2(2.2), m_value3('c') // directly initialize our member variables
    {
    // No need for assignment here
    }
    //#############################################################
    Something() : m_value1(1), m_value2(2.2), m_value3('c') // directly initialize our member variables
    {
    // No need for assignment here
    }
    void print()
    {
         std::cout << "Something(" << m_value1 << ", " << m_value2 << ", " << m_value3 << ")\n";
    }
}

它應該是這樣的：

scene::scene(QQmlApplicationEngine &engine, QObject *parent) : engine(engine),QObject(parent)

代替

scene::scene(QQmlApplicationEngine &engine, QObject *parent) : engine(this->engine),QObject(parent)

2.從運行時 未找到的本地文件中獲取的myrect.qml url導致應用程序崩潰，也補救措施之一是從qrc文件中加載

 QQmlComponent component(&engine, QUrl("qrc:/myrect.qml"));

3，在單擊之后您會發現沒有矩形，這是因為創建的矩形沒有父對象，並且通過更改create_rect_object() （在此示例中，父對象是我們窗口contentItem的不可見根）得到一些矩形:)

//A QQuickWindow always has a single invisible root item containing all of its content.
//To add items to this window, reparent the items to the contentItem or to an existing item in the scene.
//http://doc.qt.io/qt-5/qquickwindow.html#contentItem-prop



void scene::create_rect_object()
{
    QQmlComponent component(&engine, QUrl("qrc:/myrect.qml"));

    QObject *object = component.create();
    QQuickItem *item = qobject_cast<QQuickItem*>(object);
    // Set the parent of our created qml rect
    item->setParentItem((QQuickItem*)((QQuickWindow *) engine.rootObjects()[0])->contentItem());
    //Set some random position and color
    item->setProperty("color", QColor::fromRgb(QRandomGenerator::global()->generate()));
    item->setX(20+qFloor(QRandomGenerator::global()->generateDouble()*20));
    item->setY(20+qFloor(QRandomGenerator::global()->generateDouble()*20));

}

從C ++查找QML對象

為了找到對象，並使用它們作為parentItem ，你必須設置objectName的QML對象

Rectangle {
             ...
             objectName : "rect_1"
             ...
}

和在C ++中

QObject* obj = dynamic_cast<QObject*>(engine.rootObjects()[0]).findChild("rect_1");