[PHP知识库]Qt MVC model 加载 大量数据的性能优化

Qt MVC model 加载 大量数据的性能优化

自定义 canFetchMore() 、fetchMore()

canFetchMore()函数的作用是:检查父节点是否有更多可用的数据，并相应地返回true或false。fetchMore()函数的作用是:根据指定的父对象获取数据。例如，可以在涉及增量数据的数据库查询中组合这两个函数，以填充QAbstractItemModel。我们重新实现canFetchMore()来指示是否有更多的数据需要获取，并根据需要fetchMore()来填充模型。
例如动态填充树模型，当树模型中的一个分支展开时，我们将重新实现。
如果fetchMore()的重新实现向模型中添加了行，则需要调用beginInsertRows()和endInsertRows()。同样，canFetchMore()和fetchMore()都必须被重新实现。

filelistmodel.h

#ifndef FILELISTMODEL_H
#define FILELISTMODEL_H

#include <QAbstractListModel>
#include <QList>
#include <QStringList>

//![0]
class FileListModel : public QAbstractListModel
{
    Q_OBJECT

public:
    FileListModel(QObject *parent = 0);

    int rowCount(const QModelIndex &parent = QModelIndex()) const override;
    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;


signals:
    ///通知外部,新获取了多少行
    void numberPopulated(int number);

public slots:
    ///设置模型数据
    void setDirPath(const QString &path);
    ///设置 fetchMore每次加载的最大数据行数
    void setPAGE_ITEM_COUNT(int value);

protected:
    bool canFetchMore(const QModelIndex &parent) const override;
    void fetchMore(const QModelIndex &parent) override;

private:
    QStringList fileList;
    int fileCount;

    ///fetchMore每次加载的最大数据行数
    int PAGE_ITEM_COUNT;
};
//![0]

#endif // FILELISTMODEL_H

filelistmodel.cpp

#include "filelistmodel.h"

#include <QApplication>
#include <QBrush>
#include <QDir>
#include <QPalette>




FileListModel::FileListModel(QObject *parent)
    : QAbstractListModel(parent)
    ,PAGE_ITEM_COUNT(5)
{
}

//![4]
int FileListModel::rowCount(const QModelIndex & /* parent */) const
{
    return fileCount;
}

QVariant FileListModel::data(const QModelIndex &index, int role) const
{
    if (!index.isValid())
        return QVariant();

    if (index.row() >= fileList.size() || index.row() < 0)
        return QVariant();

    if (role == Qt::DisplayRole) {
        return fileList.at(index.row());
    } else if (role == Qt::BackgroundRole) {
        int batch = (index.row() / 100) % 2;
        if (batch == 0)
            return qApp->palette().base();
        else
            return qApp->palette().alternateBase();
    }
    return QVariant();
}
//![4]

//![1]
bool FileListModel::canFetchMore(const QModelIndex & /* index */) const
{
    if (fileCount < fileList.size())
        return true;
    else
        return false;
}
//![1]

//![2]
void FileListModel::fetchMore(const QModelIndex & /* index */)
{
    int remainder = fileList.size() - fileCount;
    int itemsToFetch = qMin(PAGE_ITEM_COUNT, remainder);

    if (itemsToFetch <= 0)
        return;

    beginInsertRows(QModelIndex(), fileCount, fileCount+itemsToFetch-1);

    fileCount += itemsToFetch;

    endInsertRows();

    emit numberPopulated(itemsToFetch);
}


void FileListModel::setPAGE_ITEM_COUNT(int value)
{
    PAGE_ITEM_COUNT = value;
}
//![2]

//![0]
void FileListModel::setDirPath(const QString &path)
{
    QDir dir(path);

    beginResetModel();
    fileList = dir.entryList();
    fileCount = 0;
    endResetModel();
}
//![0]

使用

    FileListModel *model = new FileListModel(this);
    model->setDirPath(QLibraryInfo::location(QLibraryInfo::PrefixPath));

    //一次fetchMore最大加载数量
    QLabel *lb = new QLabel(tr("Fetch Max Count:"));
    QSpinBox *sbMaxCount = new QSpinBox();

    QLabel *label = new QLabel(tr("&Directory:"));
    QLineEdit *lineEdit = new QLineEdit;
    label->setBuddy(lineEdit);

    QListView *view = new QListView;
    view->setModel(model);

    logViewer = new QTextBrowser;
    logViewer->setSizePolicy(QSizePolicy(QSizePolicy::Preferred, QSizePolicy::Preferred));

    connect(sbMaxCount, QOverload<int>::of(&QSpinBox::valueChanged),
            model, &FileListModel::setPAGE_ITEM_COUNT);
    sbMaxCount->setValue(3);

    connect(lineEdit, &QLineEdit::textChanged,
            model, &FileListModel::setDirPath);
    connect(lineEdit, &QLineEdit::textChanged,
            logViewer, &QTextEdit::clear);
    connect(model, &FileListModel::numberPopulated,
            this, [&](int number)
    {
        logViewer->append(tr("%1 items added.").arg(number));
    });

    QGridLayout *layout = new QGridLayout;
    layout->addWidget(lb,0,0);
    layout->addWidget(sbMaxCount,0,1);
    layout->addWidget(label, 1, 0);
    layout->addWidget(lineEdit, 1, 1);
    layout->addWidget(view, 2, 0, 1, 2);
    layout->addWidget(logViewer, 3, 0, 1, 2);

    setLayout(layout);
    setWindowTitle(tr("Fetch More Example"));

说明

当滑动滚动条，模型还没完全加载数据，继续加载显示

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