Question

概述

我的问题涉及QQmlComponent::create()创建的QObject的有效期。 create()返回的对象是QQmlComponent的实例化，我将其添加到QML StackView。我用C ++创建对象并将其传递给QML以显示在StackView中。问题是当我从堆栈中弹出一个项目时出现错误。我写了一个演示应用来说明发生了什么。

免责声明：是的，我知道从C ++进入QML不是“最佳实践”。是的，我知道你应该在QML中做UI的东西。但是，在生产环境中，需要与UI共享大量的C ++代码，因此需要在C ++和CML之间进行一些互操作。我正在使用的主要机制是Q_PROPERTY绑定，方法是在C ++端设置上下文。

此屏幕是演示开始时的样子：

StackView位于中心，背景为灰色，其中有一个项目（文本“默认视图”）;此项目由QML实例化和管理。现在，如果按推送按钮，则C ++后端会从ViewA.qml创建一个对象并将其放在堆栈中...这是一个显示以下内容的屏幕截图：

此时，我按弹出从StackView删除“查看A”（上图中的红色）。 C ++调用QML从堆栈中弹出项目，然后删除它创建的对象。问题是QML需要这个对象用于过渡动画（我正在使用StackView的默认动画），当我从C ++中删除它时它会抱怨。所以我想我明白为什么会这样，但我不知道如何找出QML完成对象的时间，以便我可以删除它。 如何确保使用我在C ++中创建的对象完成QML，以便我可以安全地删除它？

总结一下，以下是重现我所描述的问题的步骤：

启动程序
点击推送
点击弹出

以下输出显示了在上面的步骤3中弹出项目时发生的TypeError：

输出

在下面的输出中，我按“推”一次，然后按“弹出”。注意调用TypeError时的两个~ViewA()。

root object name =  "appWindow"
[c++] pushView() called
qml: [qml] pushView called with QQuickRectangle(0xdf4c00, "my view")
[c++] popView() called
qml: [qml] popView called
[c++] deleting view
~ViewA() called
file:///opt/Qt5.8.0/5.8/gcc_64/qml/QtQuick/Controls/Private/StackViewSlideDelegate.qml:97: TypeError: Cannot read property 'width' of null
file:///opt/Qt5.8.0/5.8/gcc_64/qml/QtQuick/Controls/StackView.qml:899: TypeError: Type error

必须从C ++

显然，正在发生的事情是StackView正在使用的对象（项）正被C ++删除，但QML仍需要此项用于过渡动画。我想我可以在QML中创建对象并让QML引擎管理生命周期，但我需要设置对象的QQmlContext以将QML视图绑定到C ++端的Q_PROPERTY。 / p>

在Who owns object returned by QQmlIncubator上查看我的相关问题。

代码示例

我已经生成了一个最简单的完整示例来说明问题。所有文件都列在下面。请特别注意~ViewA()中的代码注释。

// main.qml
import QtQuick 2.3
import QtQuick.Controls 1.4

Item {
    id: myItem
    objectName: "appWindow"

    signal signalPushView;
    signal signalPopView;

    visible: true
    width: 400
    height: 400

    Button {
        id: buttonPushView
        text: "Push"
        anchors.left: parent.left
        anchors.top: parent.top
        onClicked: signalPushView()
    }

    Button {
        id: buttonPopView
        text: "Pop"
        anchors.left: buttonPushView.left
        anchors.top: buttonPushView.bottom
        onClicked: signalPopView()
    }

    Rectangle {
        x: 100
        y: 50
        width: 250
        height: width
        border.width: 1

        StackView {
            id: stackView
            initialItem: view
            anchors.fill: parent

            Component {
                id: view

                Rectangle {
                    color: "#DDDDDD"

                    Text {
                        anchors.centerIn: parent
                        text: "Default View"
                    }
                }
            }
        }
    }

    function pushView(item) {
        console.log("[qml] pushView called with " + item)
        stackView.push(item)
    }

    function popView() {
        console.log("[qml] popView called")
        stackView.pop()
    }
}

// ViewA.qml
import QtQuick 2.0

Rectangle {
    id: myView
    objectName: "my view"

    color: "#FF4a4a"

    Text {
        text: "View A"
        anchors.centerIn: parent
    }
}

// viewa.h
    #include <QObject>

class QQmlContext;
class QQmlEngine;
class QObject;

class ViewA : public QObject
{
    Q_OBJECT
public:
    explicit ViewA(QQmlEngine* engine, QQmlContext* context, QObject *parent = 0);
    virtual ~ViewA();

    // imagine that this view has property bindings used by 'context'
    // Q_PROPERTY(type name READ name WRITE setName NOTIFY nameChanged)

    QQmlContext* context = nullptr;
    QObject* object = nullptr;
};

// viewa.cpp
#include "viewa.h"
#include <QQmlEngine>
#include <QQmlContext>
#include <QQmlComponent>
#include <QDebug>

ViewA::ViewA(QQmlEngine* engine, QQmlContext *context, QObject *parent) :
    QObject(parent),
    context(context)
{
    // make property bindings visible to created component
    this->context->setContextProperty("ViewAContext", this);

    QQmlComponent component(engine, QUrl(QLatin1String("qrc:/ViewA.qml")));
    object = component.create(context);
}

ViewA::~ViewA()
{
    qDebug() << "~ViewA() called";
    // Deleting 'object' in this destructor causes errors
    // because it is an instance of a QML component that is
    // being used in a transition. Deleting it here causes a
    // TypeError in both StackViewSlideDelegate.qml and
    // StackView.qml. If 'object' is not deleted here, then
    // no TypeError happens, but then 'object' is leaked.
    // How should 'object' be safely deleted?

    delete object;  // <--- this line causes errors

    delete context;
}

// viewmanager.h
#include <QObject>

class ViewA;
class QQuickItem;
class QQmlEngine;

class ViewManager : public QObject
{
    Q_OBJECT
public:
    explicit ViewManager(QQmlEngine* engine, QObject* topLevelView, QObject *parent = 0);

    QList<ViewA*> listOfViews;
    QQmlEngine* engine;
    QObject* topLevelView;

public slots:
    void pushView();
    void popView();
};

// viewmanager.cpp
#include "viewmanager.h"
#include "viewa.h"
#include <QQmlEngine>
#include <QQmlContext>
#include <QDebug>
#include <QMetaMethod>

ViewManager::ViewManager(QQmlEngine* engine, QObject* topLevelView, QObject *parent) :
    QObject(parent),
    engine(engine),
    topLevelView(topLevelView)
{
    QObject::connect(topLevelView, SIGNAL(signalPushView()), this, SLOT(pushView()));
    QObject::connect(topLevelView, SIGNAL(signalPopView()), this, SLOT(popView()));
}

void ViewManager::pushView()
{
    qDebug() << "[c++] pushView() called";

    // create child context
    QQmlContext* context = new QQmlContext(engine->rootContext());

    auto view = new ViewA(engine, context);
    listOfViews.append(view);

    QMetaObject::invokeMethod(topLevelView, "pushView",
        Q_ARG(QVariant, QVariant::fromValue(view->object)));
}

void ViewManager::popView()
{
    qDebug() << "[c++] popView() called";

    if (listOfViews.count() <= 0) {
        qDebug() << "[c++] popView(): no views are on the stack.";
        return;
    }

    QMetaObject::invokeMethod(topLevelView, "popView");

    qDebug() << "[c++] deleting view";
    auto view = listOfViews.takeLast();
    delete view;
}

// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QQuickView>
#include <QQuickItem>
#include "viewmanager.h"
#include <QDebug>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQuickView view;
    view.setSource(QUrl(QLatin1String("qrc:/main.qml")));

    QObject* item = view.rootObject();
    qDebug() << "root object name = " << item->objectName();
    ViewManager viewManager(view.engine(), item);

    view.show();
    return app.exec();
}

Answer 1

我发布了自己问题的答案。如果您发布答案，我会考虑接受您的答案，而不是这个答案。但是，这是一种可能的解决方法。

问题在于，用C ++创建的QML对象需要足够长的时间才能使QML引擎完成所有转换。我使用的技巧是将QML对象实例标记为删除，等待几秒钟让QML完成动画，然后删除该对象。＆＃34; hacky＆＃34;这里的一部分是我必须猜测我应该等待多少秒，直到我认为QML完全与对象完成。

首先，我列出了计划销毁的对象。我还制作了一个插槽，在实际删除对象的延迟后调用：

class ViewManager : public QObject {
public:
    ...
    QList<ViewA*> garbageBin;
public slots:
    void deleteAfterDelay();
}

然后，当弹出堆栈项目时，我将项目添加到garbageBin并在2秒内执行单次拍摄信号：

void ViewManager::popView()
{
    if (listOfViews.count() <= 0) {
        qDebug() << "[c++] popView(): no views are on the stack.";
        return;
    }

    QMetaObject::invokeMethod(topLevelView, "popView");

    // schedule the object for deletion in a few seconds
    garbageBin.append(listOfViews.takeLast());
    QTimer::singleShot(2000, this, SLOT(deleteAfterDelay()));
}

几秒钟后，调用deleteAfterDelay()个插槽，＆＃34;垃圾收集＆＃34;该项目：

void ViewManager::deleteAfterDelay()
{
    if (garbageBin.count() > 0) {
        auto view = garbageBin.takeFirst();
        qDebug() << "[c++] delayed delete activated for " << view->objectName();
        delete view;
    }
}

除了没有100％确信等待2秒总是足够长的时候，它似乎在实践中工作得非常好 - 不再有TypeError s并且C ++创建的所有对象都被正确清理。< / p>

Answer 2

我相信我已经找到了一种摆脱@Matthew Kraus建议的垃圾清单的方法。当弹出StackView时，我让QML处理破坏视图。

警告：代码段不完整，仅用于说明对OP帖子的扩展

function pushView(item, id) {
    // Attach option to automate the destruction on pop (called by C++)
    rootStackView.push(item, {}, {"destroyOnPop": true})
}

function popView(id) {
    // Pop immediately (removes transition effects) and verify that the view
    // was deleted (null). Else, delete immediately.
    var old = rootStackView.pop({"item": null, "immediate": true})
    if (old !== null) {
        old.destroy() // Requires C++ assigns QML ownership
    }

    // Tracking views in m_activeList by id. Notify C++ ViewManager that QML has
    // done his job
    viewManager.onViewClosed(id)
}

如果该对象是C ++创建的，并且仍然归其所有，则您很快会发现解释器在删除时对您大吼大叫。

m_pEngine->setObjectOwnership(view, QQmlEngine::JavaScriptOwnership);
QVariant arg = QVariant::fromValue(view);

bool ret = QMetaObject::invokeMethod(
            m_pRootPageObj,
            "pushView",
            Q_ARG(QVariant, arg),
            Q_ARG(QVariant, m_idCnt));

安全删除StackView转换中使用的QML组件

概述

输出

必须从C ++

代码示例

2 个答案: