qt5base-lts/examples/tools/contiguouscache/randomlistmodel.cpp

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#include "randomlistmodel.h"
#include <stdlib.h>

static const int bufferSize(500);
static const int lookAhead(100);
static const int halfLookAhead(lookAhead/2);

RandomListModel::RandomListModel(QObject *parent)
: QAbstractListModel(parent), m_rows(bufferSize), m_count(10000)
{
}

RandomListModel::~RandomListModel()
{
}

int RandomListModel::rowCount(const QModelIndex &) const
{
    return m_count;
}

//! [0]
QVariant RandomListModel::data(const QModelIndex &index, int role) const
{
    if (role != Qt::DisplayRole)
        return QVariant();

    int row = index.row();

    if (row > m_rows.lastIndex()) {
        if (row - m_rows.lastIndex() > lookAhead)
            cacheRows(row-halfLookAhead, qMin(m_count, row+halfLookAhead));
        else while (row > m_rows.lastIndex())
            m_rows.append(fetchRow(m_rows.lastIndex()+1));
    } else if (row < m_rows.firstIndex()) {
        if (m_rows.firstIndex() - row > lookAhead)
            cacheRows(qMax(0, row-halfLookAhead), row+halfLookAhead);
        else while (row < m_rows.firstIndex())
            m_rows.prepend(fetchRow(m_rows.firstIndex()-1));
    }

    return m_rows.at(row);
}

void RandomListModel::cacheRows(int from, int to) const
{
    for (int i = from; i <= to; ++i)
        m_rows.insert(i, fetchRow(i));
}
//![0]

//![1]
QString RandomListModel::fetchRow(int position) const
{
    return QString::number(rand() % ++position);
}
//![1]