使用 QDialog::nativeEvent 时 QLabel 重绘（连接/断开 USB 设备）

我编写了一个工作测试程序来监控 USB 设备的连接/断开连接。就我而言，这是一个读卡器，系统将其识别为 USB 输入设备和 HID USB。但是有一个难以理解的时刻 - 当我的插槽被触发（参见下面的代码）Dialog::SlotArrived并且Dialog::SlotRemoved文本仅在第一次更改时。但是，如果您将鼠标指针移到 thisQLabel上，则会重新绘制它。找到了出路——QLabel::setText如果放置后QLabel::repaint，那么渲染是正常的。下面是项目的正文。你能告诉我我哪里写错了吗...

使用 Qt 5.14.1，С++17，编译器版本 19.16.27035 来自 studio 2017

UsbTestWatcher.pro

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17

# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    dialog.cpp

HEADERS += \
    dialog.h

LIBS += -lUser32

# Fuck windows-1251 !!!
QMAKE_EXTRA_TARGETS += before_build makefilehook
makefilehook.target = $(MAKEFILE)
makefilehook.depends = .beforebuild
PRE_TARGETDEPS += .beforebuild
before_build.target = .beforebuild
before_build.depends = FORCE
before_build.commands = chcp 1251

主文件

#include <QApplication>
#include "dialog.h"

int main(int argc, char *argv[]) {
    QApplication a(argc, argv);
    Dialog w;
    w.show();
    return a.exec();
} 

对话框.h

#ifndef DIALOG_H
#define DIALOG_H
#include <QtWidgets>
#include <dbt.h>

class Dialog : public QDialog {
        Q_OBJECT
        QLabel *L;
        bool nativeEvent(const QByteArray& eventType, void *message, long *result);
    public:
        Dialog(QWidget *parent = nullptr);
        ~Dialog() {}
    public slots:
        void SlotRegister();
        void SlotArrived(QString iName);
        void SlotRemoved(QString iName);
    signals:
        void SigUsbArrived(QString iName);
        void SigUsbRemoved(QString iName);
};
#endif // DIALOG_H

对话框.cpp

#include "dialog.h"

//
// Можно расширить своими классами USB-устройств, тех которые нужно отслеживать
//

static const GUID GUID_DEVINTERFACE_LIST[] = {
    // USB Raw Device Interface Class GUID
    {   0xa5dcbf10, 0x6530, 0x11d2, {0x90, 0x1f, 0x00, 0xc0, 0x4f, 0xb9, 0x51, 0xed}},
    // Disk Device Interface Class GUID
    {   0x53f56307, 0xb6bf, 0x11d0, {0x94, 0xf2, 0x00, 0xa0, 0xc9, 0x1e, 0xfb, 0x8b}},
    // Human Interface Device Class GUID
    {   0x4d1e55b2, 0xf16f, 0x11Cf, {0x88, 0xcb, 0x00, 0x11, 0x11, 0x00, 0x00, 0x30}},
    // FTDI_D2XX_Device Class GUID
    {   0x219d0508, 0x57a8, 0x4ff5, {0x97, 0xa1, 0xbd, 0x86, 0x58, 0x7c, 0x6c, 0x7e}},
    // FTDI_VCP_Device Class GUID
    {   0x86e0d1e0, 0x8089, 0x11d0, {0x9c, 0xe4, 0x08, 0x00, 0x3e, 0x30, 0x1f, 0x73}}
};

Dialog::Dialog(QWidget *parent)
    : QDialog(parent) {
    L = new QLabel("Можно читать список usb-устройств при старте ...");
    QVBoxLayout *H = new QVBoxLayout();
    H->addWidget(L);
    setLayout(H);
    setSizePolicy(QSizePolicy::MinimumExpanding, QSizePolicy::MinimumExpanding);
    connect(this, &Dialog::SigUsbArrived, this, &Dialog::SlotArrived);
    connect(this, &Dialog::SigUsbRemoved, this, &Dialog::SlotRemoved);
    QTimer::singleShot(100, this, &Dialog::SlotRegister);
}

void Dialog::SlotRegister() {
    DEV_BROADCAST_DEVICEINTERFACE NotificationFilter;
    ZeroMemory(&NotificationFilter, sizeof(NotificationFilter));
    NotificationFilter.dbcc_size = sizeof(DEV_BROADCAST_DEVICEINTERFACE);
    NotificationFilter.dbcc_devicetype = DBT_DEVTYP_DEVICEINTERFACE;
    HWND hw = (HWND) this->effectiveWinId();
    for (size_t i = 0; i < sizeof(GUID_DEVINTERFACE_LIST) / sizeof(GUID); i++) {
        NotificationFilter.dbcc_classguid = GUID_DEVINTERFACE_LIST[i];
        HDEVNOTIFY hDevNotify = RegisterDeviceNotification(hw, &NotificationFilter,
                                                           DEVICE_NOTIFY_WINDOW_HANDLE);
        if (!hDevNotify) {
            QMessageBox::critical(this, "Ошибка", "Ошибка регистрации уведомителя!");
            emit close();
        }
    }
}

void Dialog::SlotArrived(QString iName) {
    L->setText(QString("Подключено: %1").arg(iName));
    L->repaint();
    qDebug() << QString("Подключено: %1").arg(iName);
}

void Dialog::SlotRemoved(QString iName) {
    L->setText(QString("Отключено: %1").arg(iName));
    L->repaint();
    qDebug() << QString("Отключено: %1").arg(iName);
}

bool Dialog::nativeEvent([[maybe_unused]] const QByteArray& eventType, void *message,
                                                 [[maybe_unused]] long *result) {
    MSG *msg = static_cast< MSG * >(message);
    int msgType = msg->message;
    if (msgType == WM_DEVICECHANGE) {
        if (msg->wParam == DBT_DEVICEARRIVAL || msg->wParam == DBT_DEVICEREMOVECOMPLETE) {
            PDEV_BROADCAST_HDR lpdb = (PDEV_BROADCAST_HDR)msg->lParam;
            PDEV_BROADCAST_DEVICEINTERFACE lpdbv = (PDEV_BROADCAST_DEVICEINTERFACE) lpdb;
            if (lpdb->dbch_devicetype == DBT_DEVTYP_DEVICEINTERFACE) {
                QString path = QString::fromWCharArray(lpdbv->dbcc_name);
                if (msg->wParam == DBT_DEVICEARRIVAL)
                    emit SigUsbArrived(path);
                else
                    emit SigUsbRemoved(path);
                return true;
            }
        }
    }
    return false;
}