audiolevelsiodevice.cpp Example File

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#include "audiolevelsiodevice.h"
#include <QtCore/QDebug>

using namespace QtDataVisualization;

static const int resolution = 8;
static const int rowSize = 800;
static const int rowCount = 7; // Must be odd number
static const int middleRow = rowCount / 2;

AudioLevelsIODevice::AudioLevelsIODevice(QBarDataProxy *proxy, QObject *parent)
    : QIODevice(parent),
      m_array(new QBarDataArray)
    // We reuse the existing array for maximum performance, so construct the array here
    for (int i = 0; i < rowCount; i++)
        m_array->append(new QBarDataRow(rowSize));

    qDebug() << "Total of" << (rowSize * rowCount) << "items in the array.";

// Implementation required for this pure virtual function
qint64 AudioLevelsIODevice::readData(char *data, qint64 maxSize)
    return -1;

qint64 AudioLevelsIODevice::writeData(const char *data, qint64 maxSize)
    // The amount of new data available.
    int newDataSize = maxSize / resolution;

    // If we get more data than array size, we need to adjust the start index for new data.
    int newDataStartIndex = qMax(0, (newDataSize - rowSize));

    // Move the old data ahead in the rows (only do first half of rows + middle one now).
    // If the amount of new data was larger than row size, skip copying.
    if (!newDataStartIndex) {
        for (int i = 0; i <= middleRow; i++) {
            QBarDataItem *srcPos = m_array->at(i)->data();
            QBarDataItem *dstPos = srcPos + newDataSize;
            memmove((void *)dstPos, (void *)srcPos, (rowSize - newDataSize) * sizeof(QBarDataItem));

    // Insert data in reverse order, so that newest data is always at the front of the row.
    int index = 0;
    for (int i = newDataSize - 1; i >= newDataStartIndex; i--) {
        // Add 0.01 to the value to avoid gaps in the graph (i.e. zero height bars).
        // Also, scale to 0...100
        float value = float(quint8(data[resolution * i]) - 128) / 1.28f + 0.01f;
        // Insert a fractional value into front half of the rows.
        for (int j = 1; j <= middleRow; j++) {
            float fractionalValue = value / float(j + 1);
            (*m_array->at(middleRow - j))[index].setValue(fractionalValue);

    // Copy the front half of rows to the back half for symmetry.
    index = 0;
    for (int i = rowCount - 1; i > middleRow; i--) {
        QBarDataItem *srcPos = m_array->at(index++)->data();
        QBarDataItem *dstPos = m_array->at(i)->data();
        memcpy((void *)dstPos, (void *)srcPos, rowSize * sizeof(QBarDataItem));

    // Reset the proxy array now that data has been updated to trigger a redraw.

    return maxSize;

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