qt - Qt postEvent() 和事件过滤器

Question

我想在事件循环中实际处理之前 过滤一些发送给我的工作人员QThread的不需要的事件。事件过滤器何时实际过滤事件：在调用线程中还是在事件循环中？ 我认为答案是“在事件循环中”，但我在 qt 文档中找不到该确切问题的答案。QCoreApplication::postEvent(...)

postEvent()QThread

Answer 1

worker 和它的事件过滤器都需要在同一个线程中。事件由线程的事件循环拾取，并在传递给它们的接收器之前通过过滤器QObject（当过滤器允许时）。这是一个演示此行为的示例：

#include <QtCore>

//a thread that can be destroyed at any time
//see http://stackoverflow.com/a/25230470
class SafeThread : public QThread{
    using QThread::run;
public:
    explicit SafeThread(QObject *parent = nullptr):QThread(parent){}
    ~SafeThread(){ quit(); wait(); }
};

struct MyEvent : public QEvent {
    static const QEvent::Type myType = static_cast<QEvent::Type>(2000);
    MyEvent():QEvent(myType){}
};

//worker QObject that handles MyEvent by printing a message
class Worker : public QObject {
public:
    explicit Worker(QObject *parent = nullptr):QObject(parent){}
    bool event(QEvent *event) {
        if(event->type() == MyEvent::myType) {
            qInfo() << "event in thread: " << QThread::currentThreadId();
            return true;
        }
        return QObject::event(event);
    }
};

class EventFilter : public QObject {
public:
    explicit EventFilter(QObject *parent = nullptr):QObject(parent){}
    bool eventFilter(QObject *watched, QEvent *event) {
        if(event->type() == MyEvent::myType) {
            qInfo() << "filter in thread: " << QThread::currentThreadId();
            return false; //do not filter the event
        }
        return QObject::eventFilter(watched, event);
    }
};


int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);
    //create worker and thread
    Worker worker;
    EventFilter filter;
    SafeThread thread;
    filter.moveToThread(&thread);
    worker.moveToThread(&thread);
    worker.installEventFilter(&filter);
    thread.start();

    qInfo() << "calling postEvent from thread: " << QThread::currentThreadId();
    QCoreApplication::postEvent(&worker, new MyEvent());

    QTimer::singleShot(1000, &a, &QCoreApplication::quit);
    return a.exec();
}

输出应类似于：

calling postEvent from thread:  0xAAAA
filter in thread:  0xBBBB
event in thread:  0xBBBB

Answer 2

过滤必须在事件传递时执行，因为过滤器期望目标对象存在并且具有可访问状态。这种存在和状态只能在事件被传递的过程中得到保证。使用 's 方法是不安全QObject的，从其他线程中保存一些选择，因此通常在发布事件时，不可能安全地访问目标对象！. 只有当对象的生命周期得到保证，并且使用线程安全的方法来访问状态，并且以不会导致死锁的方式完成时，才有可能这样做。