如果QML组件是以编程方式从C++创建的，则不显示

Question

我的目标是创建一个自定义组件(让我们称之为ComponentLoader)，它可以通过委托实例化另一个组件(让我们称之为DelegateComponent)。

问题是DelegateComponent实例在创建后不会显示(白色屏幕)。注意:存在tst_component is loaded消息，这意味着实际创建了DelegateComponent实例。

下面是一个很小的例子：

main.qml

ApplicationWindow {
    id: mainWindow
    width: 640
    height: 480
    visible: true

    Component {
        id: tst_component

        Rectangle {
            anchors.fill: parent

            Component.onCompleted: {
                console.debug("tst_component is loaded");
            }

            Label {
                text: "hello world"
                anchors.centerIn: parent
            }
        }
    }

// doesn't work
    ComponentLoader {
        anchors.fill: parent
        delegate: tst_component
    }

// works
//    Loader {
//        anchors.fill: parent
//        sourceComponent: tst_component
//    }
}

组件加载器.h+ componentloader.cpp

#pragma once
#include <QQuickItem>

class ComponentLoader : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY(QQmlComponent* delegate READ delegate WRITE setDelegate NOTIFY delegateChanged)

public:
    QQmlComponent* delegate() const;
    void setDelegate(QQmlComponent*);

signals:
    void delegateChanged();

private:
    void generate();

protected:
    void componentComplete() override;

private:
    QQmlComponent* mDelegate = nullptr;
};

// componentloader.cpp
#include "componentloader.h"
#include <QtWidgets/QtWidgets>
#include <QtQmlModels/QtQmlModels>
#include <QQuickWindow>

QQmlComponent* ComponentLoader::delegate() const
{
    return mDelegate;
}

void ComponentLoader::setDelegate(QQmlComponent* delegate)
{
    if (delegate != mDelegate)
    {
        mDelegate = delegate;
        emit delegateChanged();
    }
}

void ComponentLoader::componentComplete()
{
    QQuickItem::componentComplete();
    generate();
}

void ComponentLoader::generate()
{
    QQmlEngine* engine = qmlEngine(this);
    QQmlContext* root_ctx = engine->rootContext();
    QQmlContext* ctx = new QQmlContext(root_ctx);

    QObject* item = mDelegate->create(ctx);
    QQuickItem* quickItem = qobject_cast<QQuickItem*>(item);

    QQuickItem* quickParent = qobject_cast<QQuickItem*>(parent());
    quickItem->setParent(quickParent);
    quickItem->setParentItem(quickParent);
}

Answer 1

quickParent指针采用的是QObject父()值，而不是QQuickItem parentItem()值。

当我尝试使用您的代码时，简单地将它更改为这样对我是有效的：

QQuickItem* quickParent = qobject_cast<QQuickItem*>(parentItem());

我也不认为您需要调用setParent()。只有setParentItem()。