Versione delle 17:46, 21 lug 2020

Mostriamo un esempio di esecuzione di un file Midi con un'applicazione Gambas a riga di comando:

Library "libgstreamer-1.0"

Private Enum GST_STATE_VOID_PENDING = 0, GST_STATE_NULL, GST_STATE_READY, GST_STATE_PAUSED, GST_STATE_PLAYING
Private Const GST_FORMAT_TIME As Integer = 3

' gst_init (int *argc, char **argv[])
' Initializes the GStreamer library, setting up internal path lists, registering built-in elements, and loading standard plugins.
Private Extern gst_init(argc As Pointer, argv As Pointer)

' GstElement * gst_element_factory_make(const gchar *factoryname, Const gchar * name)
' Create a new element of the type defined by the given element factory.
Private Extern gst_element_factory_make(factoryname As String, name As String) As Pointer

' gchar * g_filename_to_uri (const gchar *filename, const gchar *hostname, GError **error)
' Converts an absolute filename to an escaped ASCII-encoded URI.
Private Extern g_filename_to_uri(filename As String, hostname As String, GError As Pointer) As String

' void g_object_set(gpointer object, const gchar *first_property_name, ...)
' Sets properties on an object.
Private Extern g_object_set(gobject As Pointer, property_name As String, value_1 As String, value_2 As String, value_3 As Float)

' GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)
' Sets the state of the element.
Private Extern gst_element_set_state(gstelement As Pointer, state As Integer) As Integer

' gboolean gst_element_query_duration (GstElement *element, GstFormat format, gint64 *duration)
' Queries an element for the total stream duration in nanoseconds.
Private Extern gst_element_query_duration(gstelement As Pointer, gstformat As Pointer, duration As Pointer) As Boolean

' void gst_object_unref(gpointer object)
' Decrements the reference count on object .
Private Extern gst_object_unref(gobject As Pointer)


Public Sub Main()

 Dim elem As Pointer
 Dim filemidi as string
 Dim durata As Long
 Dim dur, tempus As Date
 Dim dd As Integer
 Dim volume As Float

 gst_init(0, 0)
 
 elem = gst_element_factory_make("playbin", "riproduzione")

 filemidi = "/percorso/del/file.mid"

' Imposta il percorso del file Midi da eseguire:
 g_object_set(elem, "uri", g_filename_to_uri(filemidi, Null, 0), Null, 0.0)
  
' Imposta il valore del volume (da 0 a 10) di esecuzione:
 volume = 7.0
 g_object_set(elem, "volume", Null, Null, volume)
  
' Avvia la riproduzione del file Midi:
 gst_element_set_state(elem, GST_STATE_PLAYING)
 
 Repeat
   gst_element_query_duration(elem, GST_FORMAT_TIME, VarPtr(durata))
   dur = Date(0, 0, 0, 0, 0, 0, durata / 1000000)
 Until dur > 0
 
 tempus = Now
 
 Repeat
   dd = DateDiff(tempus, Now, gb.Millisecond)
   Write "\r\e[0mDurata: " & dur & "      Pos. \e[31m" & CStr(Date(0, 0, 0, 0, 0, 0, dd))
   Wait 0.001
 Until dd > durata / 1000000
 
 
' Va in chiusura e libera la memoria precedentemente allocata:
 gst_object_unref(elem)
 Print "\nEsecuzione terminata."
 Quit
  
End

Va fatto notare che le due proprietà ("uri" e "volume") del plugin playbin potrebbero essere invocate con un'unica funzione g_object_set(). In tal caso della funzione esterna di GStreamer andrebbe dichiarata sostanzialmente come segue con 6 argomenti (il 2° ed il 3° relativi alla propietà "uri", mentre il 4°, il 5° ed il 6° relativi alla proprietà "volume"):

Private Extern g_object_set(gobject As Pointer, property_1 As String, value As String, Property_2 As String, value1 As String, value2 As Float)

e nel codice sarebbe così utilizzata:

g_object_set(elem, "uri", g_filename_to_uri(fileaudio, Null, 0), "volume", Null, volume)

Riferimenti

• http://docs.gstreamer.com/display/GstSDK/gst-launch
• http://gstreamer.freedesktop.org/
• http://gstreamer.com/