Request and Sometimes pads

到目前为止，我们只处理了总是可用的pad。然而，也有一些pad仅在某些情况下创建，或者仅在应用程序请求pad时创建。第一个有时被称为a;第二个被称为请求pad。pad的可用性(always, sometimes or request)可以在pad的模板中看到。本章将讨论它们各自在什么时候有用，如何创建它们，以及何时处理它们。

Sometimes pads

“有时”pad是在特定条件下创建的pad，但不是在所有情况下创建的pad。这在很大程度上取决于流的内容:demuxers通常会解析流的首部，决定哪些基本流(视频、音频、字幕等)嵌入到系统流中，然后为每个基本流创建一个pad。根据自己的选择，它还可以为每个元素实例创建这些元素的多个实例。唯一的限制是每个新创建的pad都应该有一个唯一的名称。有时，当流数据被销毁时(即从暂停状态切换到就绪状态时)，补码也会被销毁。你不应该在EOS上处理pad，因为有人可能会重新激活管道并寻求回到流结束点之前。在EOS之后流应该仍然有效，至少在流数据被处理之前。在任何情况下，元素都是该pad的所有者。

下面的示例代码将解析一个文本文件，其中第一行是一个数字(n)。接下来的行都以数字(0到n-1)开始，这是应该发送数据的源pad的编号。

3
0: foo
1: bar
0: boo
2: bye

解析此文件并创建动态“sometimes”pads的代码如下所示:

typedef struct _GstMyFilter {
[..]gboolean firstrun;GList *srcpadlist;
} GstMyFilter;static GstStaticPadTemplate src_factory =
GST_STATIC_PAD_TEMPLATE ("src_%u",GST_PAD_SRC,GST_PAD_SOMETIMES,GST_STATIC_CAPS ("ANY")
);static void
gst_my_filter_class_init (GstMyFilterClass *klass)
{GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
[..]gst_element_class_add_pad_template (element_class,gst_static_pad_template_get (&src_factory));
[..]
}static void
gst_my_filter_init (GstMyFilter *filter)
{
[..]filter->firstrun = TRUE;filter->srcpadlist = NULL;
}/** Get one line of data - without newline.*/static GstBuffer *
gst_my_filter_getline (GstMyFilter *filter)
{guint8 *data;gint n, num;/* max. line length is 512 characters - for safety */for (n = 0; n < 512; n++) {num = gst_bytestream_peek_bytes (filter->bs, &data, n + 1);if (num != n + 1)return NULL;/* newline? */if (data[n] == '\n') {GstBuffer *buf = gst_buffer_new_allocate (NULL, n + 1, NULL);gst_bytestream_peek_bytes (filter->bs, &data, n);gst_buffer_fill (buf, 0, data, n);gst_buffer_memset (buf, n, '\0', 1);gst_bytestream_flush_fast (filter->bs, n + 1);return buf;}}
}static void
gst_my_filter_loopfunc (GstElement *element)
{GstMyFilter *filter = GST_MY_FILTER (element);GstBuffer *buf;GstPad *pad;GstMapInfo map;gint num, n;/* parse header */if (filter->firstrun) {gchar *padname;guint8 id;if (!(buf = gst_my_filter_getline (filter))) {gst_element_error (element, STREAM, READ, (NULL),("Stream contains no header"));return;}gst_buffer_extract (buf, 0, &id, 1);num = atoi (id);gst_buffer_unref (buf);/* for each of the streams, create a pad */for (n = 0; n < num; n++) {padname = g_strdup_printf ("src_%u", n);pad = gst_pad_new_from_static_template (src_factory, padname);g_free (padname);/* here, you would set _event () and _query () functions *//* need to activate the pad before adding */gst_pad_set_active (pad, TRUE);gst_element_add_pad (element, pad);filter->srcpadlist = g_list_append (filter->srcpadlist, pad);}}/* and now, simply parse each line and push over */if (!(buf = gst_my_filter_getline (filter))) {GstEvent *event = gst_event_new (GST_EVENT_EOS);GList *padlist;for (padlist = srcpadlist;padlist != NULL; padlist = g_list_next (padlist)) {pad = GST_PAD (padlist->data);gst_pad_push_event (pad, gst_event_ref (event));}gst_event_unref (event);/* pause the task here */return;}/* parse stream number and go beyond the ':' in the data */gst_buffer_map (buf, &map, GST_MAP_READ);num = atoi (map.data[0]);if (num >= 0 && num < g_list_length (filter->srcpadlist)) {pad = GST_PAD (g_list_nth_data (filter->srcpadlist, num);/* magic buffer parsing foo */for (n = 0; map.data[n] != ':' &&map.data[n] != '\0'; n++) ;if (map.data[n] != '\0') {GstBuffer *sub;/* create region copy that starts right past the space. The reason* that we don't just forward the data pointer is because the* pointer is no longer the start of an allocated block of memory,* but just a pointer to a position somewhere in the middle of it.* That cannot be freed upon disposal, so we'd either crash or have* a memleak. Creating a region copy is a simple way to solve that. */sub = gst_buffer_copy_region (buf, GST_BUFFER_COPY_ALL,n + 1, map.size - n - 1);gst_pad_push (pad, sub);}}gst_buffer_unmap (buf, &map);gst_buffer_unref (buf);
}

请注意，我们在每个地方都进行了大量检查，以确保文件中的内容是有效的。这样做有两个目的:首先，文件可能是错误的，在这种情况下，我们可以防止崩溃。第二个也是最重要的原因是——在极端情况下——该文件可能被恶意使用，导致插件中出现未定义的行为，这可能会导致安全问题。总是假定文件可能被用来做坏事。

Request pads

“请求”填充与有时填充类似，不同之处在于，请求是根据元素外部的内容而不是元素内部的内容创建的。这个概念经常用于muxers，对于将要放置在输出系统流中的每个基本流，将要求一个sink pad。它还可以用于具有可变数量输入或输出填充的元素中，例如tee(多输出)或input-selector(多输入)元素。

要实现请求填充，需要提供一个带有GST_PAD_REQUEST的padtemplate，并在GstElement中实现request_new_pad虚拟方法。为了进行清理，你需要实现release_pad虚拟方法。

static GstPad * gst_my_filter_request_new_pad   (GstElement     *element,GstPadTemplate *templ,const gchar    *name,const GstCaps  *caps);static void gst_my_filter_release_pad (GstElement *element,GstPad *pad);static GstStaticPadTemplate sink_factory =
GST_STATIC_PAD_TEMPLATE ("sink_%u",GST_PAD_SINK,GST_PAD_REQUEST,GST_STATIC_CAPS ("ANY")
);static void
gst_my_filter_class_init (GstMyFilterClass *klass)
{GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
[..]gst_element_class_add_pad_template (klass,gst_static_pad_template_get (&sink_factory));
[..]element_class->request_new_pad = gst_my_filter_request_new_pad;element_class->release_pad = gst_my_filter_release_pad;
}static GstPad *
gst_my_filter_request_new_pad (GstElement     *element,GstPadTemplate *templ,const gchar    *name,const GstCaps  *caps)
{GstPad *pad;GstMyFilterInputContext *context;context = g_new0 (GstMyFilterInputContext, 1);pad = gst_pad_new_from_template (templ, name);gst_pad_set_element_private (pad, context);/* normally, you would set _chain () and _event () functions here */gst_element_add_pad (element, pad);return pad;
}static void
gst_my_filter_release_pad (GstElement *element,GstPad *pad)
{GstMyFilterInputContext *context;context = gst_pad_get_element_private (pad);g_free (context);gst_element_remove_pad (element, pad);
}