英文原文:https://gstreamer.freedesktop.org/documentation/plugin-development/basics/args.html
控制一个element行为的主要的且最重要的方式是通过GObject性质。GObject性质是在_class_init()函数里定义的。element可以选择实现一个_get_property()函数或一个_set_property()函数。这些函数将在一个应用改变或要求一个性质的值时被通知,然后填充对应性质的值或在内部对需求采取相应的方法改变对应性质的值。
你可能想保持一个事例对象的性质的值为当前配置的,此值为你在get和set函数中用到的。注意到GObject不会自动设置你的事例变量的值为默认值,你需要在你的element的_init()函数中自己设置。
/* properties */
enum {
PROP_0,
PROP_SILENT
/* FILL ME */
};
static void gst_my_filter_set_property (GObject *object,
guint prop_id,
const GValue *value,
GParamSpec *pspec);
static void gst_my_filter_get_property (GObject *object,
guint prop_id,
GValue *value,
GParamSpec *pspec);
static void
gst_my_filter_class_init (GstMyFilterClass *klass)
{
GObjectClass *object_class = G_OBJECT_CLASS (klass);
/* define virtual function pointers */
object_class->set_property = gst_my_filter_set_property;
object_class->get_property = gst_my_filter_get_property;
/* define properties */
g_object_class_install_property (object_class, PROP_SILENT,
g_param_spec_boolean ("silent", "Silent",
"Whether to be very verbose or not",
FALSE, G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
}
static void
gst_my_filter_set_property (GObject *object,
guint prop_id,
const GValue *value,
GParamSpec *pspec)
{
GstMyFilter *filter = GST_MY_FILTER (object);
switch (prop_id) {
case PROP_SILENT:
filter->silent = g_value_get_boolean (value);
g_print ("Silent argument was changed to %s\n",
filter->silent ? "true" : "false");
break;
default:
G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
break;
}
}
static void
gst_my_filter_get_property (GObject *object,
guint prop_id,
GValue *value,
GParamSpec *pspec)
{
GstMyFilter *filter = GST_MY_FILTER (object);
switch (prop_id) {
case PROP_SILENT:
g_value_set_boolean (value, filter->silent);
break;
default:
G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
break;
}
}
以上是一个非常简单的关于如何使用性质的例子。图形应用将使用这些性质并显示一个用户控制窗口用于改变这些性质的值。这意味着——使性质设置变得尽可能用户友好的——你应该尽可能准确的定义性质。不仅仅要定义有效性质的取值范围(如integers, floats等),还要使用具有描述性的strings来定义性质,并且,如果可能的话用enums和flags,而不是integers。GObject的文档很详细的描述了这些,但是在下面,我们将给出一个简短的例子来看这在哪里是有用的。注意到,在这里用integers的话,可能会使用户困惑,因为它在这里是没有任何意义的。此例子是源自于videotestsrc中的。
typedef enum {
GST_VIDEOTESTSRC_SMPTE,
GST_VIDEOTESTSRC_SNOW,
GST_VIDEOTESTSRC_BLACK
} GstVideotestsrcPattern;
[..]
#define GST_TYPE_VIDEOTESTSRC_PATTERN (gst_videotestsrc_pattern_get_type ())
static GType
gst_videotestsrc_pattern_get_type (void)
{
static GType videotestsrc_pattern_type = 0;
if (!videotestsrc_pattern_type) {
static GEnumValue pattern_types[] = {
{ GST_VIDEOTESTSRC_SMPTE, "SMPTE 100% color bars", "smpte" },
{ GST_VIDEOTESTSRC_SNOW, "Random (television snow)", "snow" },
{ GST_VIDEOTESTSRC_BLACK, "0% Black", "black" },
{ 0, NULL, NULL },
};
videotestsrc_pattern_type =
g_enum_register_static ("GstVideotestsrcPattern",
pattern_types);
}
return videotestsrc_pattern_type;
}
[..]
static void
gst_videotestsrc_class_init (GstvideotestsrcClass *klass)
{
[..]
g_object_class_install_property (G_OBJECT_CLASS (klass), PROP_PATTERN,
g_param_spec_enum ("pattern", "Pattern",
"Type of test pattern to generate",
GST_TYPE_VIDEOTESTSRC_PATTERN, GST_VIDEOTESTSRC_SMPTE,
G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
[..]
}