main.cpp

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// Copyright (C) 2022 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
#include <QCoreApplication>
#include <QLoggingCategory>
 
// in a header
Q_DECLARE_LOGGING_CATEGORY(driverUsb)
 
// in one source file
Q_LOGGING_CATEGORY(driverUsb, "driver.usb")
 
Q_LOGGING_CATEGORY(driverUsbEvents, "driver.usb.events", QtWarningMsg)
 
// Completely made up example, inspired by en.wikipedia.org/wiki/USB :)
struct UsbEntry {
    int id;
    int classtype;
};
 
QDebug operator<<(QDebug &debug, const UsbEntry &entry)
{
    QDebugStateSaver saver(debug);
    debug.nospace() << "" << entry.id << " (" << entry.classtype << ')';
    return debug;
}
 
QList<UsbEntry> usbEntries() {
    QList<UsbEntry> entries;
    return entries;
}
 
void myCategoryFilter(QLoggingCategory *);
 
static QLoggingCategory::CategoryFilter oldCategoryFilter = nullptr;
 
void myCategoryFilter(QLoggingCategory *category)
{
    // For a category set up after this filter is installed, we first set it up
    // with the old filter. This ensures that any driver.usb logging configured
    // by the user is kept, aside from the one level we override; and any new
    // categories we're not interested in get configured by the old filter.
    if (oldCategoryFilter)
        oldCategoryFilter(category);
 
    // Tweak driver.usb's logging, over-riding the default filter:
    if (qstrcmp(category->categoryName(), "driver.usb") == 0)
        category->setEnabled(QtDebugMsg, true);
}
 
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
 
    QLoggingCategory::setFilterRules(QStringLiteral("driver.usb.debug=true"));
 
oldCategoryFilter = QLoggingCategory::installFilter(myCategoryFilter);
 
    qSetMessagePattern("%{category} %{message}");
 
    // usbEntries() will only be called if driverUsb category is enabled
    qCDebug(driverUsb) << "devices: " << usbEntries();
 
    {
    QLoggingCategory category("driver.usb");
    qCDebug(category) << "a debug message";
    }
 
    QLoggingCategory category("driver.usb");
    qCInfo(category) << "an informational message";
 
    {
    QLoggingCategory category("driver.usb");
    qCWarning(category) << "a warning message";
    }
 
    {
    QLoggingCategory category("driver.usb");
    qCCritical(category) << "a critical message";
    }
 
    {
    QLoggingCategory category("driver.usb");
    qCDebug(category, "a debug message logged into category %s", category.categoryName());
    }
 
    {
    QLoggingCategory category("driver.usb");
    qCInfo(category, "an informational message logged into category %s", category.categoryName());
    }
 
    {
    QLoggingCategory category("driver.usb");
    qCWarning(category, "a warning message logged into category %s", category.categoryName());
    }
 
    {
    QLoggingCategory category("driver.usb");
    qCCritical(category, "a critical message logged into category %s", category.categoryName());
    }
 
    {
    QLoggingCategory category("driver.usb");
    qCFatal(category) << "a fatal message. Program will be terminated!";
    }
 
    {
    QLoggingCategory category("driver.usb");
    qCFatal(category, "a fatal message. Program will be terminated!");
    }
 
    return 0;
}