c++ - 必须在QObject::connect之前调用g_signal_connect？

Question

我正在创建一个结合了GStreamer和Qt的应用程序。看来，如果我在使用g_signal_connect将回调函数注册到GStreamer总线上的事件之前使用QObject::connect将信号连接到插槽，则永远不会调用g_signal_connect回调函数。如果我颠倒顺序。这是预期的吗？

例:

main.cpp

#include <QApplication>

#include <QPushButton>

#include "acquisitiontype.h"

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);


    AcquisitionType acquisition("224.1.1.1", 5004);

    QPushButton* button = new QPushButton("click me");
    QObject::connect(button, SIGNAL(clicked()), &app, SLOT(quit()));
    button->show();


    return app.exec();
}

Acquisitiontype.cpp

#include "acquisitiontype.h"


void AcquisitionType::udp_source_timeout_callback(GstBus* bus, GstMessage* message, gstreamer_data* user_data) {
    (void) bus;
    (void) user_data;
    const GstStructure* st = gst_message_get_structure(message);


    if (GST_MESSAGE_TYPE(message) == GST_MESSAGE_ELEMENT) {
        if (gst_structure_has_name(st, "GstUDPSrcTimeout")) {
            printf("callback called\n");
        }
    }
}

void AcquisitionType::bus_error_callback(GstBus* bus, GstMessage* message, gstreamer_data* user_data) {
    (void) bus;
    (void) user_data;
    GError* err;
    gchar* debug_info;


    gst_message_parse_error(message, &err, &debug_info);
    g_printerr("Error received from element %s: %s\n", GST_OBJECT_NAME(message->src), err->message);
    g_printerr("Debugging information: %s\n", debug_info ? debug_info : "none");
    g_clear_error(&err);
    g_free(debug_info);

    exit(-1);
}

AcquisitionType::AcquisitionType(char const* address, gint port) {
    GstStateChangeReturn ret;
    GstBus* bus;


    gst_init(NULL, NULL);


    data.udp_source = gst_element_factory_make("udpsrc", "udp_source");
    g_object_set(G_OBJECT(data.udp_source),
        "address", address,
        "port", port,
        "caps", gst_caps_new_empty_simple("application/x-rtp"),
        "timeout", 1000000000,
        NULL);


    data.sink = gst_element_factory_make("fakesink", "sink");


    data.pipeline = gst_pipeline_new("pipeline");


    if (
        !data.pipeline ||
        !data.udp_source ||
        !data.sink
        )
        {
            g_printerr("Not all elements could be created.\n");
            exit(-1);
        }


    gst_bin_add_many(
        GST_BIN(data.pipeline),
        data.udp_source,
        data.sink,
        NULL);


    if (gst_element_link_many(
        data.udp_source,
        data.sink,
        NULL) != TRUE)
        {
            g_printerr("Elements could not be linked.\n");
            gst_object_unref(data.pipeline);
            exit(-1);
        }


    bus = gst_element_get_bus(data.pipeline);
    gst_bus_add_signal_watch(bus);
    g_signal_connect(G_OBJECT(bus), "message::error", (GCallback) bus_error_callback, &data);
    g_signal_connect(G_OBJECT(bus), "message::element", (GCallback) udp_source_timeout_callback, &data);
    gst_object_unref(bus);


    ret = gst_element_set_state(data.pipeline, GST_STATE_PLAYING);
    if (ret == GST_STATE_CHANGE_FAILURE) {
        g_printerr("Unable to set the pipeline to the playing state.\n");
        gst_object_unref(data.pipeline);
        exit(-1);
    }
}

AcquisitionType::~AcquisitionType() {
    GstBus* bus;


    gst_element_set_state(data.pipeline, GST_STATE_NULL);

    bus = gst_element_get_bus(data.pipeline);
    gst_bus_remove_signal_watch(bus);
    gst_object_unref(bus);

    gst_object_unref(data.pipeline);
}

采集类型

#include <gst/gst.h>

#include <QObject>

class AcquisitionType;

struct gstreamer_data {
    GstElement* pipeline;
    GstElement* udp_source;
    GstElement* sink;
};


class AcquisitionType : public QObject
{
    Q_OBJECT

public:
    AcquisitionType(char const* address, gint port);
    ~AcquisitionType();

private:
    static void bus_error_callback(GstBus* bus, GstMessage* message, gstreamer_data* user_data);
    static void udp_source_timeout_callback(GstBus* bus, GstMessage* message, gstreamer_data* user_data);

    gstreamer_data data;
};

如果按原样运行，则调用回调。如果将 AcquisitionType acquisition("224.1.1.1", 5004);移到 button->show()之后，则不会。

Answer 1

看来我需要将"timeout", 1000000000,更改为"timeout", G_GUINT64_CONSTANT(1000000000),。