VLCMediaDiscoverer: use libdispatch instead of NSThread

(cherry picked from commit 01976dbd462b0b238ac736a20f458d62449b1158)

Sources/VLCMediaDiscoverer.m

@@ -2,7 +2,7 @@
  * VLCMediaDiscoverer.m: VLCKit.framework VLCMediaDiscoverer implementation
  *****************************************************************************
  * Copyright (C) 2007 Pierre d'Herbemont
- * Copyright (C) 2014-2015 Felix Paul Kühne
+ * Copyright (C) 2014-2017 Felix Paul Kühne
  * Copyright (C) 2007, 2015 VLC authors and VideoLAN
  * $Id$
  *
@@ -44,6 +44,7 @@ NSString *const VLCMediaDiscovererCategory = @"VLCMediaDiscovererCategory";
     libvlc_media_discoverer_t *_mdis;
 
     VLCLibrary *_privateLibrary;
+    dispatch_queue_t _libVLCBackgroundQueue;
 }
 @end
 
@@ -92,6 +93,7 @@ NSString *const VLCMediaDiscovererCategory = @"VLCMediaDiscovererCategory";
     if (self = [super init]) {
         _localizedName = nil;
         _discoveredMedia = nil;
+        _libVLCBackgroundQueue = dispatch_queue_create("libvlcQueue", DISPATCH_QUEUE_SERIAL_WITH_AUTORELEASE_POOL);
 
         if (libraryInstance != nil) {
             _privateLibrary = libraryInstance;
@@ -113,14 +115,16 @@ NSString *const VLCMediaDiscovererCategory = @"VLCMediaDiscovererCategory";
 
 - (void)dealloc
 {
+    [[VLCEventManager sharedManager] cancelCallToObject:self];
+
     if (_mdis) {
-        if (libvlc_media_discoverer_is_running(_mdis))
-            libvlc_media_discoverer_stop(_mdis);
-        libvlc_media_discoverer_release(_mdis);
+        dispatch_sync(_libVLCBackgroundQueue, ^{
+            if (libvlc_media_discoverer_is_running(_mdis))
+                libvlc_media_discoverer_stop(_mdis);
+            libvlc_media_discoverer_release(_mdis);
+        });
     }
 
-    [[VLCEventManager sharedManager] cancelCallToObject:self];
-
     libvlc_release(_privateLibrary.instance);
 }
 
@@ -143,12 +147,13 @@ NSString *const VLCMediaDiscovererCategory = @"VLCMediaDiscovererCategory";
 
 - (void)stopDiscoverer
 {
-    if ([NSThread isMainThread]) {
-        [self performSelectorInBackground:@selector(stopDiscoverer) withObject:nil];
+    if (![self isRunning]) {
         return;
     }
 
-    libvlc_media_discoverer_stop(_mdis);
+    dispatch_async(_libVLCBackgroundQueue, ^{
+        libvlc_media_discoverer_stop(_mdis);
+    });
 }
 
 - (VLCMediaList *)discoveredMedia