Qml/QtQuick2中的QSignalBlocker？

Question

我想在不触发附加信号的情况下动态设置ComboBox的currentIndex。

我目前正在使用bool属性来设置一个标志，当我的信号被调用时，该标志将被清除。

Item {
    property bool ignoreNextChanged: false

    CustomCppItem  {
        id: customItem

        onSomethingHappened: {
            ignoreNextChanged = true
            comboBox.currentIndex = 42
        }
    }

    ComboBox {
        id: comboBox

        currentIndex: -1
        onCurrentIndexChanged: {
            if(ignoreNextChanged) {
                ignoreNextChanged = false
                return
            }

            // Removed code here
        }
    }
}

来自Qt/C++，我想在这里使用某种QSignalBlocker，而不是我自己的属性。

Qml/QtQuick2中有没有等价物？

Answer 1

为此，我将在c++中实现一个attached type：

signalblockerattachedtype.h：

#ifndef SIGNALBLOCKERATTACHEDTYPE_H
#define SIGNALBLOCKERATTACHEDTYPE_H

#include <QObject>
#include <qqml.h>

class SignalBlockerAttachedType : public QObject
{
    Q_OBJECT
    Q_PROPERTY(bool enabled READ enabled WRITE setEnabled NOTIFY enabledChanged)

public:
    explicit SignalBlockerAttachedType(QObject *object = nullptr) : QObject(object), m_object(object)
    {
    }

    ~SignalBlockerAttachedType() {
        if (m_object)
            m_object->blockSignals(false);
    }

    bool enabled() const
    {
        return m_enabled;
    }
    void setEnabled(bool enabled)
    {
        if (m_enabled == enabled)
            return;

        if (m_object)
            m_object->blockSignals(enabled);

        m_enabled = enabled;
        emit enabledChanged();
    }

    static SignalBlockerAttachedType *qmlAttachedProperties(QObject *object) {
        return new SignalBlockerAttachedType(object);
    }

signals:
    void enabledChanged();

private:
    bool m_enabled = false;
    QObject *m_object;
};

QML_DECLARE_TYPEINFO(SignalBlockerAttachedType, QML_HAS_ATTACHED_PROPERTIES)

#endif // SIGNALBLOCKERATTACHEDTYPE_H

main.cpp：

#include "signalblockerattachedtype.h"
// ...
qmlRegisterUncreatableType<SignalBlockerAttachedType>("fr.grecko.SignalBlocker", 1, 0, "SignalBlocker", "SignalBlocker is only available as an attached type.");
// ...
engine.load(QUrl(QLatin1String("qrc:/usage.qml")));

usage.qml：

import fr.grecko.SignalBlocker 1.0
// ...

Item {
    id: rootItem
    property alias currentIndex: comboBox.currentIndex
    onCurrentIndexChanged: {
        // this won't get called from a change in onSomethingHappened
    }
    CustomCppItem  {
        id: customItem

        onSomethingHappened: {
            rootItem.SignalBlocker.enabled = true;
            rootItem.currentIndex = 42
            rootItem.SignalBlocker.enabled = false;
        }
    }

    ComboBox {
        id: comboBox
        currentIndex: -1
    }
}

即使SignalBlocker可以在ComboBox上运行，您也不希望将其放在ComboBox上。这样做将阻止它在索引更改时更新其显示。

在您的情况下，我不认为这种解决方案比使用简单的标志更好。