c++ - Qt QML 数据模型似乎不适用于 C++

Question

我一直在使用http://doc.qt.digia.com/4.7/qdeclarativemodels.html中的示例，这是 QML 声明性数据模型的 Qt 页面。特别是，我正在使用objectlistmodelQt SDK 附带的示例（在示例/声明/模型视图/对象列表模型中）。在我尝试将它与http://www.developer.nokia.com/Community/Wiki/How_to_create_a_Page_Control_component_in_QML上的 QMLPageControl 示例结合起来之前，这一切似乎都运行得很好。

当我尝试使用这样的 QML ListView 显示基于 QML 的 ListModel（填充有 QML ListElements）时：

import QtQuick 1.0

Rectangle {
   width: 200; height: 200

   ListModel {
       id: qmlModel
       ListElement { name: "qml entry1 (red)"; colour: "red" }
       ListElement { name: "qml entry2 (orange)"; colour: "orange" }
       ListElement { name: "qml entry3 (yellow)"; colour: "yellow" }
       ListElement { name: "qml entry4 (green)"; colour: "green" }
       ListElement { name: "qml entry5 (blue)"; colour: "blue" }
       ListElement { name: "qml entry6 (purple)"; colour: "purple" }
   }


   ListView {

       id: list_view

       anchors.fill: parent
       model: qmlModel
       delegate: Rectangle {
           height: 20
           width: 200
           color: colour
           Text { text: name }

       }
    }
}

...一切都很好。这完全按预期工作 - 弹出一个窗口，其中包含带状彩色背景的一些文本。

然后，我可以做一些更复杂的事情，比如使用 PathView：

import QtQuick 1.0

Rectangle {
    width: 200; height: 200

    ListModel {
        id: qmlModel
        ListElement { name: "qml entry1 (red)"; colour: "red" }
        ListElement { name: "qml entry2 (orange)"; colour: "orange" }
        ListElement { name: "qml entry3 (yellow)"; colour: "yellow" }
        ListElement { name: "qml entry4 (green)"; colour: "green" }
        ListElement { name: "qml entry5 (blue)"; colour: "blue" }
        ListElement { name: "qml entry6 (purple)"; colour: "purple" }
    }


    //       ListView {
    //           id: list_view
    //           anchors.fill: parent
    //           model: qmlModel
    //           delegate: Rectangle {
    //               height: 20
    //               width: 200
    //               color: colour
    //               Text { text: name }
    //           }
    //       }

    PathView {
        id: my_path_view

        anchors.fill: parent

        Keys.onRightPressed: if (!moving && interactive) incrementCurrentIndex()
        Keys.onLeftPressed: if (!moving && interactive) decrementCurrentIndex()

        flickDeceleration: 500

        preferredHighlightBegin: 0.5
        preferredHighlightEnd: 0.5
        focus: true
        interactive: true
        model: qmlModel

        delegate: Rectangle {
            width: 100
            height: 100
            color: colour
            Text {
                anchors.centerIn: parent
                text: name
            }
        }


        path: Path {
            startX: - my_path_view.width * my_path_view.model.count / 2 + my_path_view.width / 2
            startY: my_path_view.height / 2
            PathLine {
                x: my_path_view.width * my_path_view.model.count / 2 + my_path_view.width / 2
                y: my_path_view.height / 2
            }
        }
    }
}

同样，这一切都按预期工作 - 弹出一个窗口，其中包含可轻弹、可拖动的彩色框列表。

备份，然后我可以像这样在 C++ 中定义一个数据对象：

数据对象.h

#ifndef DATAOBJECT_H
#define DATAOBJECT_H

#include <QObject>

class DataObject : public QObject
{
    Q_OBJECT

    Q_PROPERTY( QString name READ name WRITE setName NOTIFY nameChanged )
    Q_PROPERTY( QString colour READ colour WRITE setColour NOTIFY colourChanged )


public:
    DataObject( QObject * parent = 0 );
    DataObject( const QString &_name, const QString &_color, QObject * parent=0 );

    QString name() const;
    void setName(const QString &);

    QString colour() const;
    void setColour(const QString &);

signals:
    void nameChanged();
    void colourChanged();


private:
    QString m_name;
    QString m_colour;
};


#endif // DATAOBJECT_H

数据对象.cpp

#include "dataobject.h"
#include <QDebug>

DataObject::DataObject( QObject * parent )
    : QObject( parent )
{
    qDebug() << "DataObject::DataObject() has been called.\n";

}

DataObject::DataObject( const QString &_name, const QString &_colour, QObject * parent )
    : QObject( parent )
    , m_name( _name )
    , m_colour( _colour )
{
    qDebug() << "DataObject::DataObject(name, color) has been called.\n";

}


QString DataObject::name() const {
    qDebug() << "name() has been called.\n";
    return m_name;
}

void DataObject::setName(const QString &name) {
    qDebug() << "setName has been called.\n";
    if ( name != m_name ) {
        m_name = name;
        emit nameChanged();
    }
}

QString DataObject::colour() const {
    qDebug() << "colour() has been called.\n";
    return m_colour;
}

void DataObject::setColour(const QString &colour) {
    qDebug() << "setColour has been called.\n";
    if ( colour != m_colour ) {
        m_colour = colour;
        emit colourChanged();
    }
}

然后我将它添加到 QML 上下文中：

#include <QApplication>
#include <QDialog>
#include <QDeclarativeView>
#include <QDeclarativeContext>
#include <QLayout>
#include <QDir>
#include "qmlapplicationviewer.h"
#include "dataobject.h"

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

    QList<QObject*> dataList;
    dataList.append( new DataObject( "c++ entry1 (red)", "red" ) );
    dataList.append( new DataObject( "c++ entry2 (orange)", "orange" ) );
    dataList.append( new DataObject( "c++ entry3 (yellow)", "yellow" ) );
    dataList.append( new DataObject( "c++ entry4 (green)", "green" ) );
    dataList.append( new DataObject( "c++ entry5 (blue)", "blue" ) );
    dataList.append( new DataObject( "c++ entry6 (purple)", "purple" ) );

    QmlApplicationViewer viewer;
    viewer.rootContext()->setContextProperty( "cppModel", QVariant::fromValue(dataList) );
    viewer.setOrientation(QmlApplicationViewer::ScreenOrientationAuto);
#if defined( Q_OS_MAC )
    viewer.setMainQmlFile("../Resources/qml/main.qml");
#elif defined( Q_OS_WIN32 )
    viewer.setMainQmlFile("qml/main.qml");
#else
#error - unknown platform
#endif
    viewer.showExpanded();

    return app.exec();
}

最后，在 QML 中，我将这个 C++ 模型添加到 ListView：

import QtQuick 1.0

Rectangle {
    width: 200; height: 200

    ListModel {
        id: qmlModel
        ListElement { name: "qml entry1 (red)"; colour: "red" }
        ListElement { name: "qml entry2 (orange)"; colour: "orange" }
        ListElement { name: "qml entry3 (yellow)"; colour: "yellow" }
        ListElement { name: "qml entry4 (green)"; colour: "green" }
        ListElement { name: "qml entry5 (blue)"; colour: "blue" }
        ListElement { name: "qml entry6 (purple)"; colour: "purple" }
    }


           ListView {

               id: list_view

               anchors.fill: parent
               //model: qmlModel
               model: cppModel
               delegate: Rectangle {
                   height: 20
                   width: 200
                   color: colour
                   Text { text: name }

               }
           }

}

再一次，这工作得很好 - 出现一个对话框，其中包含以带状排列的彩色背景的文本。显示由 C++ 模型支持的 ListView 似乎每一点都有效，以及显示由 QML ListModel 支持的 ListView。

我想要工作的是一个支持 PathView 的 C++ 模型，如下所示：

import QtQuick 1.0

Rectangle {
    width: 200; height: 200

    ListModel {
        id: qmlModel
        ListElement { name: "qml entry1 (red)"; colour: "red" }
        ListElement { name: "qml entry2 (orange)"; colour: "orange" }
        ListElement { name: "qml entry3 (yellow)"; colour: "yellow" }
        ListElement { name: "qml entry4 (green)"; colour: "green" }
        ListElement { name: "qml entry5 (blue)"; colour: "blue" }
        ListElement { name: "qml entry6 (purple)"; colour: "purple" }
    }


//    ListView {

//       id: list_view

//       anchors.fill: parent
//       model: qmlModel
//       //model: cppModel
//       delegate: Rectangle {
//           height: 20
//           width: 200
//           color: colour
//           Text { text: name }

//       }
//    }

    PathView {
        id: my_path_view

        anchors.fill: parent

        Keys.onRightPressed: if (!moving && interactive) incrementCurrentIndex()
        Keys.onLeftPressed: if (!moving && interactive) decrementCurrentIndex()

        flickDeceleration: 500

        preferredHighlightBegin: 0.5
        preferredHighlightEnd: 0.5
        focus: true
        interactive: true
        //model: qmlModel
        model: cppModel

        delegate: Rectangle {
            width: 100
            height: 100
            color: colour
            Text {
                anchors.centerIn: parent
                text: name
            }
        }


        path: Path {
            startX: - my_path_view.width * my_path_view.model.count / 2 + my_path_view.width / 2
            startY: my_path_view.height / 2
            PathLine {
                x: my_path_view.width * my_path_view.model.count / 2 + my_path_view.width / 2
                y: my_path_view.height / 2
            }
        }
    }
}

这不起作用。我看到的是彩色矩形，但它们不能与鼠标交互，并且它们不在 qmlviewer 对话框中居中。

在调试控制台上，我看到了这个：

QDeclarativeDebugServer: Waiting for connection on port 3768...
QDeclarativeDebugServer: Connection established
QPainterPath::lineTo: Adding point where x or y is NaN or Inf, ignoring call
QPainterPath::lineTo: Adding point where x or y is NaN or Inf, ignoring call
colour() has been called.

name() has been called.

colour() has been called.

name() has been called.

colour() has been called.

name() has been called.

colour() has been called.

name() has been called.

colour() has been called.

name() has been called.

colour() has been called.

name() has been called.

QPainterPath::lineTo: Adding point where x or y is NaN or Inf, ignoring call
QPainterPath::lineTo: Adding point where x or y is NaN or Inf, ignoring call
QPainterPath::lineTo: Adding point where x or y is NaN or Inf, ignoring call
QPainterPath::lineTo: Adding point where x or y is NaN or Inf, ignoring call
QPainterPath::lineTo: Adding point where x or y is NaN or Inf, ignoring call

似乎 QList 的基本形状足够接近 QML ListModel/ListItem 集合以供 ListView 显示，但不足以接近 PathView 来显示。

有谁知道可能出了什么问题？不幸的是，QML 类文档并没有真正与编写符合 C++ 替代的目标放在一起。例如，http://qt-project.org/doc/qt-4.8/qml-pathview.html 上的 PathView 对象文档没有说明其模型需要支持哪些属性。此外，ListModel 文档不是确定性的——它没有准确说明 ListModel 支持哪些属性，也没有明确的文档说明 QList 如何精确地满足这些要求以及它如何不满足这些要求。

更新：我已经在 Windows 上使用 Qt 5 进行了尝试，但我仍然遇到同样的问题。

Answer 1

事实证明， 的count属性cppModel不可用有一个非常简单的原因——因为既没有QAbstractListModel也QList<>没有count属性！

我假设 aListModel可以用基于 C++ 的对象（如 QList<> ）替换这一事实意味着它们是多态的，并且 ListView 或 PathView 将使用count属性来正确处理它们。

首先， aQAbstractListModel也不QList<>是多态的ListModel。事实证明，它们都只是特例——aListView知道它是否具有 aListModel或 aQList<>或 aQAbstractListModel并且具有用于使用每个的单独代码路径。ListView不需要不存在的属性count来管理 aQList<>或 a QAbstractListModel。事实上，我不清楚，ListView甚至PathView使用ListModel'count属性。该count属性似乎主要是为了 QML 程序员的利益。在我的示例中，我使用该count属性Path在PathView. 如果我使用length属性代替，我的示例将完美运行，因为QList<>确实有length属性。

感谢 #qt-qml 上的 blam 和 torgeirl 帮助我解决这个问题（他们都不想通过发布这个答案来收集 stackoverflow 积分，所以我发布它是为了社区的利益）。