QProcess 在没有 waitForFinished() 时不发出信号

Question

在下面的代码中，省略waitForFinished()使 QProcess 停止发出它的信号。它到底有什么问题？这是一个Qt错误吗？(5.7)。请注意，此代码与 QtConcurrent 并行运行。但这不应该改变任何事情，不是吗？Afaik 在其他线程中发送信号很好，尽管它们会排队。

QProcess *process = new QProcess;
process->setReadChannel(QProcess::StandardOutput);

connect(process, &QProcess::readyReadStandardOutput, [](){
    qDebug()<< "readyReadStandardOutput";
});

connect(process, &QProcess::stateChanged, [](QProcess::ProcessState state){
    qDebug()<< "stateChanged"<< state;
});

connect(process, static_cast<void(QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished),
    [=](){
    qDebug()<< "finsished";
});

connect(process, static_cast<void(QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished),
    [this, process](int exitCode, QProcess::ExitStatus exitStatus){
    qDebug()<< "finsished";
    if (exitStatus == QProcess::NormalExit && exitCode == 0){
        while (process->canReadLine()) {
           QString line = QString::fromLocal8Bit(process->readLine());
           QRegularExpression regex("\"(.*)\" {(.*)}");
           QRegularExpressionMatch match = regex.match(line);
           names_.push_back(match.captured(1));
           uuids_.push_back(match.captured(2));
        }
    }
    process->deleteLater();
});
process->start("VBoxManage",  {"list", "vms"});
process->waitForFinished(); // This line changes everything
qDebug()<< "leftWaitForFinished";

Answer 1

您没有在QProcess实例所在的线程中运行事件循环。任何QObject在没有事件循环线程只是部分功能-定时器将不会运行，排队的呼叫将无法送达，等于是你不能做到这一点。使用QObjects withQtConcurrent::run需要小心。

至少，只要进程存在，您就应该有一个临时事件循环 - 在这种情况下，您应该QProcess按值持有，因为deleteLater在事件循环退出后不会执行。

QProcess process;
...
QEventLoop loop;
connect(process, &QProcess::finished, &loop, &QEventLoop::quit);
loop.exec();

Otherwise, you need to keep the process in a more durable thread, and keep that thread handle (QThread is but a handle!) in a thread that has an event loop that can dispose of it when it's done.

// This can be run from a lambda that runs in an arbitrary thread

auto thread = new QThread;
auto process = new QProcess;

...

connect(process, static_cast<void(QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished),
    [this, process](int exitCode, QProcess::ExitStatus exitStatus){
    ...
    process->deleteLater();
    process->thread()->quit();
});

process->start("VBoxManage",  {"list", "vms"});
process->moveToThread(thread);

// Move the thread **handle** to the main thread
thread->moveToThread(qApp->thread());
connect(thread, &QThread::finished, thread, &QObject::deleteLater);
thread->start();

Alas, this is very silly since you're creating temporary threads and that's expensive and wasteful. You should have one additional worker thread where you take care of all low-overhead work such as QProcess interaction. That thread should always be running, and you can move all QProcess and similar object instances to it, from concurrent lambdas etc.