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10.将 Appsrc 链接到 Playbin

丁毅桂2026/1/28大约 5 分钟gstreamernotesappsrcplaybin

GStreamer学习笔记：10.将 Appsrc 链接到 Playbin

本示例尝试将 appsrc 作为数据源链接到 playbin，试图实现向 playbin 注入自定义数据。这对于需要将内存数据或实时生成的数据通过 playbin 播放的场景非常有用。

核心概念

1. 使用 Appsrc 协议

Playbin 支持多种 URI 协议，其中 appsrc:// 是一个特殊协议，用于使用 appsrc 作为数据源。

data.pipeline = gst_parse_launch("playbin uri=appsrc://", NULL);

当 playbin 检测到 URI 为 appsrc:// 时，它会在内部创建一个 appsrc 元素。

2. Source-Setup 信号

当 playbin 创建了 appsrc 元素后，它会发出 source-setup 信号，通知应用程序来配置这个 appsrc。

g_signal_connect(data.pipeline, "source-setup", G_CALLBACK(source_setup), &data);

3. 配置 Appsrc

在 source_setup 回调函数中配置 appsrc：

static void source_setup(GstElement *pipeline, GstElement *source, CustomData *data)
{
    GstAudioInfo info;
    GstCaps *audio_caps;

    g_print("Source has been created. Configuring.\n");
    data->app_source = source; // 由 playbin 创建元素 appsrc

    // 配置 appsrc
    // info = { int16, 44.1khz, 单通道 }
    gst_audio_info_set_format(&info, GST_AUDIO_FORMAT_S16, SAMPLE_RATE, 1, NULL);
    audio_caps = gst_audio_info_to_caps(&info);

    g_object_set(source, "caps", audio_caps, "format", GST_FORMAT_TIME, NULL);

    g_signal_connect(source, "need-data", G_CALLBACK(start_feed), data);
    g_signal_connect(source, "enough-data", G_CALLBACK(stop_feed), data);

    gst_caps_unref(audio_caps);
}

4. 数据生成与推送

与之前的 appsrc-appsink 示例类似，我们需要实现数据生成和推送：

推送数据

static gboolean push_data(CustomData *data)
{
    GstBuffer *buffer;
    GstFlowReturn ret;
    GstMapInfo map;
    gint16 *raw;
    gint num_samples = CHUNK_SIZE / 2;
    gfloat freq;

    buffer = gst_buffer_new_and_alloc(CHUNK_SIZE);

    GST_BUFFER_TIMESTAMP(buffer) = gst_util_uint64_scale(data->num_samples, GST_SECOND, SAMPLE_RATE);
    GST_BUFFER_DURATION(buffer) = gst_util_uint64_scale(num_samples, GST_SECOND, SAMPLE_RATE);

    gst_buffer_map(buffer, &map, GST_MAP_WRITE);
    raw = (gint16 *)map.data;

    data->c += data->d;
    data->d -= data->c / 1000;
    freq = 1100 + 1000 * data->d;
    for (int i = 0; i < num_samples; i++)
    {
        data->a += data->b;
        data->b -= data->a / freq;
        raw[i] = (gint16)(500 * data->a);
    }
    gst_buffer_unmap(buffer, &map);
    data->num_samples += num_samples;

    g_signal_emit_by_name(data->app_source, "push-buffer", buffer, &ret);
    gst_buffer_unref(buffer);

    return ret == GST_FLOW_OK;
}

信号回调

static void start_feed(GstElement *source, guint size, CustomData *data)
{
    if (data->sourceid == 0)
    {
        g_print("Start feeding\n");
        data->sourceid = g_idle_add((GSourceFunc)push_data, data);
    }
}

static void stop_feed(GstElement *source, CustomData *data)
{
    if (data->sourceid != 0)
    {
        g_print("Stop feeding\n");
        g_source_remove(data->sourceid);
        data->sourceid = 0;
    }
}

5. 错误处理

static void error_cb(GstBus *bus, GstMessage *msg, CustomData *data)
{
    GError *err;
    gchar *debug_info;

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

    g_main_loop_quit(data->main_loop);
}

6. Bus 信号监听

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

7. GLib 主循环

data.main_loop = g_main_loop_new(NULL, FALSE);
g_main_loop_run(data.main_loop);

Pipeline 结构

appsrc (由 playbin 内部创建)
  -> [内部自动构建的播放 pipeline]

当使用 playbin uri=appsrc:// 时，playbin 会：

  1. 创建 appsrc 元素作为数据源
  2. 发出 source-setup 信号
  3. 在 source-setup 回调中配置 appsrc
  4. 自动构建后续的解码和播放 pipeline

与手动构建 Pipeline 的区别

手动构建 Pipeline

  • 需要手动创建所有元素（解码器、转换器、播放器）
  • 完全控制 pipeline 的每个环节
  • 代码更复杂，但更灵活

使用 Playbin + Appsrc

  • playbin 自动处理解码和播放
  • 只需要关注数据生成和注入
  • 代码更简洁，但定制性较低

完整代码

#include <gst/gst.h>
#include <gst/audio/audio.h>
#include <string.h>

#define CHUNK_SIZE 1024
#define SAMPLE_RATE 44100

typedef struct _CustomData
{
    GstElement *pipeline;
    GstElement *app_source;
    guint64 num_samples;
    gfloat a, b, c, d;
    guint sourceid;
    GMainLoop *main_loop;
} CustomData;

static gboolean push_data(CustomData *data);
static void start_feed(GstElement *source, guint size, CustomData *data);
static void stop_feed(GstElement *source, CustomData *data);
static void error_cb(GstBus *bus, GstMessage *msg, CustomData *data);
static void source_setup(GstElement *pipeline, GstElement *source, CustomData *data);

int main(int argc, char *argv[])
{
    CustomData data;
    GstBus *bus;

    memset(&data, 0, sizeof(data));
    data.b = 1;
    data.d = 1;

    gst_init(&argc, &argv);

    data.pipeline = gst_parse_launch("playbin uri=appsrc://", NULL);
    g_signal_connect(data.pipeline, "source-setup", G_CALLBACK(source_setup), &data);

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

    gst_element_set_state(data.pipeline, GST_STATE_PLAYING);

    data.main_loop = g_main_loop_new(NULL, FALSE);
    g_main_loop_run(data.main_loop);

    gst_element_set_state(data.pipeline, GST_STATE_NULL);
    gst_object_unref(data.pipeline);
    return 0;
}

static void source_setup(GstElement *pipeline, GstElement *source, CustomData *data)
{
    GstAudioInfo info;
    GstCaps *audio_caps;

    g_print("Source has been created. Configuring.\n");
    data->app_source = source;

    gst_audio_info_set_format(&info, GST_AUDIO_FORMAT_S16, SAMPLE_RATE, 1, NULL);
    audio_caps = gst_audio_info_to_caps(&info);
    g_object_set(source, "caps", audio_caps, "format", GST_FORMAT_TIME, NULL);
    g_signal_connect(source, "need-data", G_CALLBACK(start_feed), data);
    g_signal_connect(source, "enough-data", G_CALLBACK(stop_feed), data);
    gst_caps_unref(audio_caps);
}

static gboolean push_data(CustomData *data)
{
    GstBuffer *buffer;
    GstFlowReturn ret;
    GstMapInfo map;
    gint16 *raw;
    gint num_samples = CHUNK_SIZE / 2;
    gfloat freq;

    buffer = gst_buffer_new_and_alloc(CHUNK_SIZE);
    GST_BUFFER_TIMESTAMP(buffer) = gst_util_uint64_scale(data->num_samples, GST_SECOND, SAMPLE_RATE);
    GST_BUFFER_DURATION(buffer) = gst_util_uint64_scale(num_samples, GST_SECOND, SAMPLE_RATE);

    gst_buffer_map(buffer, &map, GST_MAP_WRITE);
    raw = (gint16 *)map.data;

    data->c += data->d;
    data->d -= data->c / 1000;
    freq = 1100 + 1000 * data->d;
    for (int i = 0; i < num_samples; i++)
    {
        data->a += data->b;
        data->b -= data->a / freq;
        raw[i] = (gint16)(500 * data->a);
    }
    gst_buffer_unmap(buffer, &map);
    data->num_samples += num_samples;

    g_signal_emit_by_name(data->app_source, "push-buffer", buffer, &ret);
    gst_buffer_unref(buffer);

    return ret == GST_FLOW_OK;
}

static void start_feed(GstElement *source, guint size, CustomData *data)
{
    if (data->sourceid == 0)
    {
        g_print("Start feeding\n");
        data->sourceid = g_idle_add((GSourceFunc)push_data, data);
    }
}

static void stop_feed(GstElement *source, CustomData *data)
{
    if (data->sourceid != 0)
    {
        g_print("Stop feeding\n");
        g_source_remove(data->sourceid);
        data->sourceid = 0;
    }
}

static void error_cb(GstBus *bus, GstMessage *msg, CustomData *data)
{
    GError *err;
    gchar *debug_info;

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

    g_main_loop_quit(data->main_loop);
}

编译和运行

gcc main.c -o main.out $(pkg-config --cflags --libs gstreamer-1.0)
./main.out

应用场景

将 appsrc 链接到 playbin 的典型应用场景包括：

  1. 内存数据播放：从内存中读取音频数据并播放
  2. 实时音频生成：实时合成音频并播放（如合成器、音效处理）
  3. 网络流处理：接收网络数据并播放
  4. 音频处理：对音频进行实时处理后再播放
  5. 游戏音频：将游戏音频引擎的输出通过 playbin 播放

总结

本示例展示了：

  1. Appsrc 协议：使用 appsrc:// URI 让 playbin 创建 appsrc
  2. Source-Setup 信号：在 playbin 创建 appsrc 后配置它
  3. 数据注入：向 playbin 注入自定义生成的数据
  4. 简化流程：利用 playbin 自动处理解码和播放，简化代码

这种方法结合了 appsrc 的灵活性和 playbin 的便捷性，是构建自定义媒体播放应用的有效方式。

贡献者: YiguiDing
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