Add optional callbacks to xas_object

Changes:

    * Implement xas_object_init() to initialise an object call table
      to all null values

    * Rework dispatch methods to fail gracefully if no pointer value is
      provided for a given callback entry

Additional changes:

    * Add callbacks for changing bank and frequency

1 files changed, 54 insertions, 1 deletions

diff --git a/src/object.c b/src/object.c
index 7aaaa6e..4526bc5 100644
--- a/src/object.c
+++ b/src/object.c
@@ -1,21 +1,74 @@
+#include <string.h>
+#include <errno.h>
+
 #include <xas/object.h>
 
+void xas_object_init(xas_object *object) {
+    memset(object, '\0', sizeof(xas_object));
+}
+
 int xas_object_start(xas_object *object) {
+    if (object->start == NULL) {
+        errno = ENOSYS;
+
+        return -1;
+    }
+
     return object->start(object);
 }
 
 int xas_object_stop(xas_object *object) {
+    if (object->stop == NULL) {
+        errno = ENOSYS;
+
+        return -1;
+    }
+
     return object->stop(object);
 }
 
 int xas_object_set_gain(xas_object *object, float gain) {
+    if (object->set_gain == NULL) {
+        errno = ENOSYS;
+
+        return -1;
+    }
+
     return object->set_gain(object, gain);
 }
 
+int xas_object_set_bank(xas_object *object, size_t bank) {
+    if (object->set_bank == NULL) {
+        errno = ENOSYS;
+
+        return -1;
+    }
+
+    return object->set_bank(object, bank);
+}
+
+int xas_object_set_freq(xas_object *object, size_t freq) {
+    if (object->set_freq == NULL) {
+        errno = ENOSYS;
+
+        return -1;
+    }
+
+    return object->set_freq(object, freq);
+}
+
 xas_audio_stream *xas_object_stream_new(xas_object *object) {
+    if (object->stream_new == NULL) {
+        errno = ENOSYS;
+
+        return NULL;
+    }
+
     return object->stream_new(object);
 }
 
 void xas_object_destroy(xas_object *object) {
-    object->destroy(object);
+    if (object->destroy) {
+        object->destroy(object);
+    }
 }