c++ - Qt QItemSelection::indexes() 返回错误

Question

我从 QAbstractItemModel 实现了一个派生类，它似乎工作正常。然后我创建了一个 QItemSelectionModel 并将其分配给提到的 QAbstractItemModel。

我的 QAbstractItemModel 不是小部件，也不会显示，只管理一个层次结构。When the selection is changed, and the QItemSelection model emits the selection changed signal the QItemSelections selected and deselected seem to contain the right data.

当我调用他们的::indexes() 函数来获取所选项目的索引时，问题就出现了，它没有返回任何项目，即使我知道项目被选中并且::width() 和::height()函数返回正确的值。

基本示例代码:(以下是演示问题的工作示例和文件)

class DerivedModel : public QAbstractItemModel {
    DerivedModel(QObject* parent) : QAbstractItemModel(parent)
                                   ,m_selectionModel(nullptr)
    {
        //create the selection model and assign this model to it
        m_selectionModel = new QItemSelectionModel(this, this);
    }
...
//all needed overload functions
//the DerivedModel works great
...
private:
    QItemSelectionModel* m_selectionModel;
}

//in a different object called SceneModel (a QGraphicsScene which shows graphical items based on the DerivedModel) which is connected to the selection models selectionChanged() signal I query the new selection

SceneModel::setSelectedItems(const QItemSelection& selected, const QItemSelection& deselected){

int selectionSize_A = selected.size(); //this returns correct number of selected items
int selectionSize_B = selected.indexes().size(); //this returns 0 -> WRONG
int selectionSize_C = selected.value(0).indexes().size(); //this returns 0 -> WRONG
int selectionSize_CA = selected.value(0).width(); //this returns correct
int selectionSize_CB = selected.value(0).height(); //this returns correct

//if I purposefully try to access the 1st selected index via QItemSelectionRange::topLeft() all is good and I get the index:
QItemSelectionRange range = selected.value(0);
QModelIndex topLeft = range.topLeft(); //cool, i get the 1st selected index

//it seems there is a problem with the ::indexes function, so dived into the Qt5 source and basically implemented again whats done there and it works.
}

包含 cmake 构建文件的链接: https://drive.google.com/file/d/0Bz03DnXr46WXYXRCeExtaHZadUU/view?usp=sharing

那里发生了什么:创建一个 DerivedModel 并在根项 (ROOT) 下保存 2 个项(A 和 B)。按下按钮会向 QItemSelectionModel 发出选择/取消选择 A 或 B 的信号。如果在模型中找到该项目，则打印“Found Item :)”，表明该项目存在并且可用于模型。QGraphicsView 包含一个场景(派生自 QGraphicsScene)。该场景是空的，仅代表一个从选择模型接收 selectionChange 信号的对象。当它收到该信号时，它会打印“Scene received item selection change”，这样我们就可以看到信号已经通过。然后是真正的东西:

1. 我们计算出传递的“selected”变量中有多少 QItemRanges，这是正确的。
2. 我们计算了传递的“selected”变量 (selected.indexes()) 中所有范围内有多少索引，该变量返回 0，我们很快就会看到这是错误的
3. 我们手动访问“selected”变量 (selected.value(0).topLeft()) 中第一个范围内的第一个索引，发现它确实包含指向正确项目的索引，显示了问题。

如果有人知道某事，或者发现我的方法有错误，请告诉我。谢谢!

Linux Manjaro Gcc 4.9.1 Qt5.3

派生模型.h:

#ifndef DERIVEDMODEL_H
#define DERIVEDMODEL_H 

#include <QAbstractItemModel>

//fwd declaration
QT_FORWARD_DECLARE_CLASS(QItemSelectionModel)

class Item;

class DerivedModel : public QAbstractItemModel{
    Q_OBJECT
public:
    //model is a singleton, function to get instance
    static DerivedModel& instance();

    explicit DerivedModel(QObject* parent);
    virtual ~DerivedModel();

    /////////////////model overloads//////////////////////////////
    QVariant data(const QModelIndex& index, int role) const;
    Qt::ItemFlags flags(const QModelIndex& index) const;
    QVariant headerData(int section, Qt::Orientation orientation, int role = Qt::DisplayRole) const;
    QModelIndex index(int row, int column, const QModelIndex& parent = QModelIndex()) const;
    QModelIndex parent(const QModelIndex& index) const;
    int rowCount(const QModelIndex& parent = QModelIndex()) const;
    int columnCount(const QModelIndex& parent = QModelIndex()) const {Q_UNUSED(parent); return 1;}
    //////////////////////////////////////////////////////////////

    //get the item from an index
    Item* item(const QModelIndex& index) const { return static_cast<Item*>(index.internalPointer());}

    //get the index from an item name
    const QModelIndex indexFromName(const QString& name);

    //add an item
    void addItem(const QString& name, Item* parent=nullptr);

    //get the selection model
    QItemSelectionModel* selectionModel() const {return m_selectionModel;}

private:
    //the instance of the singleton to return
    static DerivedModel* m_instance;

    //the root object for the model
    //never actually used
    Item* m_rootItem;

    //selection model for handeling selection
    QItemSelectionModel* m_selectionModel;
};
#endif

派生模型.cpp

#include "DerivedModel.h"

#include "Item.h"

#include <QItemSelectionModel>
#include <QDebug>

//init static member
DerivedModel* DerivedModel::m_instance = nullptr;

DerivedModel& DerivedModel::instance(){
    //check if set
    if(!m_instance){
        qDebug() << "ERROR model instance not set";
        std::abort();
    }
    return *m_instance;
}

DerivedModel::DerivedModel(QObject* parent):
                            QAbstractItemModel(parent)
                            ,m_rootItem(nullptr)
                            ,m_selectionModel(nullptr)
{
    //set the instance
    m_instance = this;

    //creae root item
    m_rootItem = new Item("ROOT");

    //init selection model
    m_selectionModel = new QItemSelectionModel(this, this);
}

DerivedModel::~DerivedModel(){
    //selection model is child so gets deleted
}

QVariant DerivedModel::data(const QModelIndex& index, int role) const {
    //if the index is valid
    if(!index.isValid()) {
        qDebug() << "Index not valid!";
        return QVariant();
    }

    //switch role
    switch(role){
        case Qt::DisplayRole:{
            QString name = static_cast<Item*>(index.internalPointer())->name();
            return name;
            break;
         }
        default:
            return QVariant();
    }
}

Qt::ItemFlags DerivedModel::flags(const QModelIndex& index) const {
    //check valid
    if(!index.isValid()) return 0;

    return static_cast<Item*>(index.internalPointer())->flags();
}

QVariant DerivedModel::headerData(int section, Qt::Orientation orientation, int role) const {
    //unused for now
    Q_UNUSED(section);
    Q_UNUSED(orientation);
    if(role==Qt::DisplayRole) return QVariant("HeaderData");
    else return QVariant();
}

QModelIndex DerivedModel::index(int row, int column, const QModelIndex& parent) const {

    Item* parentItem(nullptr);

    //is valid?
    if(!parent.isValid()) {
        parentItem = m_rootItem;
    }
    else {
        parentItem = item(parent);
    }

    //child pointer holder
    Item* childItem = parentItem->children().value(row);
    //is null?
    if(childItem){
        return createIndex(row, column, childItem);
    }
    else {
        return QModelIndex();
    }
}

QModelIndex DerivedModel::parent(const QModelIndex& index) const {
    //check valid
    if(!index.isValid()) return QModelIndex();

    //get child
    Item* childItem = static_cast<Item*>(index.internalPointer());

    //find parent
    Item* parentItem = childItem->parent();

    //is null?
    if(parentItem == m_rootItem) return QModelIndex();

    return createIndex(parentItem->parent()->children().indexOf(parentItem), 0, parentItem);
}

int DerivedModel::rowCount(const QModelIndex& parent) const {
    //parent holder
    Item* parentItem;

    //check 0 column (not sure why, but is in example, maybe the model iterates also through different columns)
    if(parent.column()>0) return 0;

    //check valid
    if(!parent.isValid()) parentItem = m_rootItem;
    else parentItem = static_cast<Item*>(parent.internalPointer());

    return parentItem->children().length();
}

const QModelIndex DerivedModel::indexFromName(const QString& name){
    //make a match based on the name
    //and return 1st match
    QModelIndex index = match(DerivedModel::index(0,0,QModelIndex()),
            Qt::DisplayRole, name, 1, 
            Qt::MatchFlags(Qt::MatchExactly|Qt::MatchRecursive))
            .value(0);

    return index;
}

void DerivedModel::addItem(const QString& name, Item* parent){
    //check parent
    if(!parent) parent = m_rootItem;

    //create the item
    //will be deleted once parent is deleted
    new Item(name, parent); 
}

项目.h:

#ifndef ITEM_H
#define ITEM_H

#include <QString>
#include <QList>
#include <QDebug>

class Item {

public:
    Item(const QString& name, Item* parent=nullptr) :
                                m_name(name), m_parent(parent){
        //add as child to parent
        if(parent) parent->addChild(this);

        //set the flag to enable selection
        m_flags = Qt::ItemIsSelectable;

        qDebug() << "Created Item "+name;
    };

    ~Item(){
        //delete children
        for (auto& child : m_children){
            delete child;
        }
    };

    //get the name
    const QString& name() const {return m_name;}

    //get the flags
    const Qt::ItemFlags& flags() const {return m_flags;}

    //gte the parent
    Item* parent() const {return m_parent;}

    //get the children
    const QList<Item*>& children() {return m_children;}

    //add a child
    void addChild(Item* item) {m_children.append(item);}

private:
    //name
    QString m_name;

    //flags
    Qt::ItemFlags m_flags;

    //parent
    Item* m_parent;

    //list og children
    QList<Item*> m_children;
};


#endif

场景.h:

#ifndef GRAPHICSSCENE_H
#define GRAPHICSSCENE_H

#include <QGraphicsScene>

//fwd dec
QT_FORWARD_DECLARE_CLASS(QItemSelection)
QT_FORWARD_DECLARE_CLASS(QGraphicsRectItem)

class Scene : public QGraphicsScene {
public:
    Scene(QObject* parent);
    virtual ~Scene(){}

public slots:
    //pass the selection to the squares
    void setSelection(const QItemSelection& selected, const QItemSelection& deselected);
};

#endif

场景.cpp:

#include "Scene.h"
#include "DerivedModel.h"
#include "Item.h"

#include <QItemSelectionModel>
#include <QGraphicsRectItem>
#include <QRect>
#include <QDebug>

Scene::Scene(QObject* parent) : QGraphicsScene(parent)
{
    //connect to the models selection change
    connect(DerivedModel::instance().selectionModel(), &QItemSelectionModel::selectionChanged,
            this, &Scene::setSelection);
}

void Scene::setSelection(const QItemSelection& selected, const QItemSelection& deselected){
    Q_UNUSED(deselected);
    //testing changes
    int A = selected.size();
    int B = selected.indexes().size();

    QModelIndex index = selected.value(0).topLeft();
    QString name = "";
    if(index.isValid()) name = static_cast<Item*>(index.internalPointer())->name();

    qDebug() << "Scene recieved item selection change";
    qDebug() << "Number of selected QItemRanges from source = "+QString::number(A); 
    qDebug() << "Number of selected INDEXES from source = "+QString::number(B); 
    qDebug() << "Manually accessd 1st index in 1st range returns item named: "+name;
}

小部件.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

//fwd dec
QT_FORWARD_DECLARE_CLASS(QPushButton)
QT_FORWARD_DECLARE_CLASS(QVBoxLayout)
QT_FORWARD_DECLARE_CLASS(QHBoxLayout)
QT_FORWARD_DECLARE_CLASS(QGraphicsView)

class DerivedModel;
class Scene;

class Widget : public QWidget{

public:
    Widget(QWidget* parent=nullptr);
    virtual ~Widget(){}

private slots:
    //button toggle alot
    void toggle(bool state);

private:
    //layout
    QVBoxLayout* m_mainVBLayout;

    //horizontal layout for the buttons
    QHBoxLayout* m_HBButtonsLayout;

    //GraphicsView widget for the scene
    QGraphicsView* m_graphicsView;

    //the graphics scene recieving the change event where the problem is
    Scene* m_scene;

    //push buttons
    QPushButton* m_button1;
    QPushButton* m_button2;

    //the DerivedModel
    DerivedModel* m_model;
};


#endif

小部件.cpp:

#include "Widget.h"
#include "DerivedModel.h"
#include "Scene.h"

#include <QPushButton>
#include <QVBoxLayout>
#include <QHBoxLayout>
#include <QGraphicsView>
#include <QDebug>
#include <QItemSelection>

Widget::Widget(QWidget* parent) : QWidget(parent)
                                  ,m_mainVBLayout(nullptr)
                                  ,m_HBButtonsLayout(nullptr)
                                  ,m_graphicsView(nullptr)
                                  ,m_scene(nullptr)
                                  ,m_button1(nullptr)
                                  ,m_button2(nullptr)
                                  ,m_model(nullptr)
{
    //init the DerivedModel
    m_model = new DerivedModel(this);

    //add two items to the model
    m_model->addItem("A");
    m_model->addItem("B");

    //create the main layout
    m_mainVBLayout = new QVBoxLayout(this);

    //create the buttons layout
    m_HBButtonsLayout = new QHBoxLayout;

    //add it to the main layout
    m_mainVBLayout->addLayout(m_HBButtonsLayout);

    //create the buttons
    m_button1 = new QPushButton("A", this);
    m_button2 = new QPushButton("B", this);

    //set them to be checkable
    m_button1->setCheckable(true);
    m_button2->setCheckable(true);

    //connect their signals
    connect(m_button1, &QPushButton::toggled, this, &Widget::toggle);
    connect(m_button2, &QPushButton::toggled, this, &Widget::toggle);

    //add them to the layout
    m_HBButtonsLayout->addWidget(m_button1);
    m_HBButtonsLayout->addWidget(m_button2);

    //create the graphics view
    m_graphicsView = new QGraphicsView(this);

    //create the scene
    m_scene = new Scene(this);
    m_scene->setSceneRect(QRect(0,0,50,25));

    //set its scene
    m_graphicsView->setScene(m_scene);

    //add the graphics view to the layout
    m_mainVBLayout->addWidget(m_graphicsView);
}   

void Widget::toggle(bool state){
    //get the sender
    QPushButton* button(nullptr);
    if(sender()==m_button1) button = m_button1;
    else button = m_button2;

    //get the name of the item related to the button to change
    QString name = button->text();

    //get the index based on the name
    //im using the instance of the DerivedModel because this is how I implement it in my project;
    QModelIndex itemIndex = DerivedModel::instance().indexFromName(name);

    //check if index is valid
    if(!itemIndex.isValid()){
        qDebug() << "Index for item "+name+" not valid!";
        return;
    }
    else 
        qDebug() << "Found Item :)";

    //create a QItemSelection as how it is in my project
    QItemSelection selection;

    //add the index to the selection
    selection.select(itemIndex, itemIndex);

    //check the state
    if(state){
        //add to the selection
        DerivedModel::instance().selectionModel()->select(selection, QItemSelectionModel::Select);
    }
    else{
        //remove from selection
        DerivedModel::instance().selectionModel()->select(selection, QItemSelectionModel::Deselect);
    }
}

Answer 1

好的，所以我找到了罪魁祸首:)

以防其他人遇到同样的问题，我的错误出在派生的 QAbstractItemModel 类的::flags() 函数中:在定义中我没有调用基类 QAbstractItemModel::flags(index) 函数，一旦我调用它而不是自己返回标志，一切顺利。

所以我认为只要你的项目有一个模型可以调用的 Qt::flags flags() 函数，你就不必重新实现 QAbstractItemModel::flags() 函数。

似乎模型正在通过 QModelIndex::flags() 函数查询标志。

感谢“Ezee”和“Kuba Ober”愿意提供帮助。