[avcustomedit] Some timing and simplification fixes (#349)

Running this sample on device with the interpreter has spotted a few
issues in the sample.

* Timing issue: Use `DispatchGroup.Notify` (like sample) instead of
  `Waiting` (it's not the same) and remove the `InvokeOnMainThread`
  since we're already running on that thread. Also uncomment some
  code that, likely, did not work because `Notify` was not used;

* Timing issue: `composition` can be `null` but it triggers an
  `ArgumentNullException` (because the API does not _officially_ accept
   `nil`).

* Enhancement: Set initial capacity of `List<T>` - just like it's
  done in the original sample;

* Simplification: There's no point in checking for `null` values inside
  `List<T>` since that would have triggered an `ArgumentNullException`;

* Simplification" There's no point in converting `CMTimeRange` into
  `NSValue` (and back into `CMTimeRange`). In contrast to `NSArray`
  we can add non-`NSObject` (including value type like struct) inside
  `List<T>`;

* Cleanup: remove ugly `goto` and use `_` for unused `out NSError`

This will fix https://github.com/xamarin/xamarin-macios/issues/5364

AVCustomEdit/AVCustomEdit/SimpleEditor.cs

@@ -13,13 +13,19 @@ namespace AVCustomEdit
 
         public List<AVAsset> Clips { get; set; } // array of AVURLAssets
 
-        public List<NSValue> ClipTimeRanges { get; set; } // array of CMTimeRanges stored in NSValues.
+        public List<CMTimeRange> ClipTimeRanges { get; set; }
 
         public TransitionType TransitionType { get; set; }
 
         public CMTime TransitionDuration { get; set; }
 
-        public AVPlayerItem PlayerItem =>  new AVPlayerItem(this.composition) { VideoComposition = this.videoComposition };
+        public AVPlayerItem PlayerItem {
+            get {
+                if (composition == null)
+                    return null;
+                return new AVPlayerItem(this.composition) { VideoComposition = this.videoComposition };
+            }
+        }
 
         private void BuildTransitionComposition(AVMutableComposition mutableComposition, AVMutableVideoComposition mutableVideoComposition)
         {
@@ -30,12 +36,9 @@ namespace AVCustomEdit
             var transitionDuration = this.TransitionDuration;
             foreach (var clipTimeRange in this.ClipTimeRanges)
             {
-                if (clipTimeRange != null)
-                {
-                    var halfClipDuration = clipTimeRange.CMTimeRangeValue.Duration;
-                    halfClipDuration.TimeScale *= 2; // You can halve a rational by doubling its denominator.
-                    transitionDuration = CMTime.GetMinimum(transitionDuration, halfClipDuration);
-                }
+                var halfClipDuration = clipTimeRange.Duration;
+                halfClipDuration.TimeScale *= 2; // You can halve a rational by doubling its denominator.
+                transitionDuration = CMTime.GetMinimum(transitionDuration, halfClipDuration);
             }
 
             // Add two video tracks and two audio tracks.
@@ -55,16 +58,13 @@ namespace AVCustomEdit
             {
                 int alternatingIndex = i % 2; // alternating targets: 0, 1, 0, 1, ...
                 var asset = this.Clips[i];
-                var clipTimeRange = this.ClipTimeRanges[i];
-
-                var timeRangeInAsset = clipTimeRange != null ? clipTimeRange.CMTimeRangeValue
-                                                             : new CMTimeRange { Start = CMTime.Zero, Duration = asset.Duration };
+                var timeRangeInAsset = this.ClipTimeRanges[i];
 
                 var clipVideoTrack = asset.TracksWithMediaType(AVMediaType.Video)[0];
-                compositionVideoTracks[alternatingIndex].InsertTimeRange(timeRangeInAsset, clipVideoTrack, nextClipStartTime, out NSError error);
+                compositionVideoTracks[alternatingIndex].InsertTimeRange(timeRangeInAsset, clipVideoTrack, nextClipStartTime, out _);
 
                 var clipAudioTrack = asset.TracksWithMediaType(AVMediaType.Audio)[0];
-                compositionAudioTracks[alternatingIndex].InsertTimeRange(timeRangeInAsset, clipAudioTrack, nextClipStartTime, out error);
+                compositionAudioTracks[alternatingIndex].InsertTimeRange(timeRangeInAsset, clipAudioTrack, nextClipStartTime, out _);
 
                 // Remember the time range in which this clip should pass through.
                 // First clip ends with a transition.

AVCustomEdit/AVCustomEdit/ViewController.cs

@@ -19,7 +19,7 @@ namespace AVCustomEdit
         private SimpleEditor editor;
 
         private List<AVAsset> clips;
-        private List<NSValue> clipTimeRanges;
+        private List<CMTimeRange> clipTimeRanges;
 
         private AVPlayer player;
         private AVPlayerItem playerItem;
@@ -44,8 +44,8 @@ namespace AVCustomEdit
             base.ViewDidLoad();
 
             this.editor = new SimpleEditor();
-            this.clips = new List<AVAsset>();
-            this.clipTimeRanges = new List<NSValue>();
+            this.clips = new List<AVAsset>(2);
+            this.clipTimeRanges = new List<CMTimeRange>(2);
 
             // Defaults for the transition settings.
             this.transitionType = TransitionType.DiagonalWipeTransition;
@@ -115,8 +115,9 @@ namespace AVCustomEdit
             this.LoadAsset(asset2, assetKeysToLoadAndTest, dispatchGroup);
 
             // Wait until both assets are loaded
-            dispatchGroup.Wait(DispatchTime.Forever);
-            base.InvokeOnMainThread(() => this.SynchronizeWithEditor());
+            dispatchGroup.Notify(DispatchQueue.MainQueue, () => {
+                SynchronizeWithEditor();
+            });
         }
 
         private void LoadAsset(AVAsset asset, string[] assetKeysToLoad, DispatchGroup dispatchGroup)
@@ -124,28 +125,32 @@ namespace AVCustomEdit
             dispatchGroup.Enter();
             asset.LoadValuesAsynchronously(assetKeysToLoad, () =>
             {
+                bool add_asset = true;
                 // First test whether the values of each of the keys we need have been successfully loaded.
                 foreach (var key in assetKeysToLoad)
                 {
                     if (asset.StatusOfValue(key, out NSError error) == AVKeyValueStatus.Failed)
                     {
                         Console.WriteLine($"Key value loading failed for key:{key} with error: {error?.LocalizedDescription ?? ""}");
-                        goto bail;
+                        add_asset = false;
+                        break;
                     }
                 }
 
                 if (!asset.Composable)
                 {
                     Console.WriteLine("Asset is not composable");
-                    goto bail;
+                    add_asset = false;
                 }
 
-                this.clips.Add(asset);
-                // This code assumes that both assets are atleast 5 seconds long.
-                var value = NSValue.FromCMTimeRange(new CMTimeRange { Start = CMTime.FromSeconds(0, 1), Duration = CMTime.FromSeconds(5, 1) });
-                this.clipTimeRanges.Add(value);
+                if (add_asset)
+                {
+                    this.clips.Add(asset);
+                    // This code assumes that both assets are atleast 5 seconds long.
+                    var value = new CMTimeRange { Start = CMTime.FromSeconds(0, 1), Duration = CMTime.FromSeconds(5, 1) };
+                    this.clipTimeRanges.Add(value);
+                }
 
-            bail:
                 dispatchGroup.Leave();
             });
         }
@@ -185,8 +190,8 @@ namespace AVCustomEdit
         private void SynchronizeWithEditor()
         {
             // Clips
-            this.SynchronizeEditorClipsWithOurClips();
-            this.SynchronizeEditorClipTimeRangesWithOurClipTimeRanges();
+            this.editor.Clips = new List<AVAsset> (clips);
+            this.editor.ClipTimeRanges = new List<CMTimeRange> (clipTimeRanges);
 
             // Transitions
             if (this.isTransitionsEnabled)
@@ -200,36 +205,8 @@ namespace AVCustomEdit
             }
 
             // Build AVComposition and AVVideoComposition objects for playback
-            //this.editor.BuildCompositionObjectsForPlayback(true);
-            //this.SynchronizePlayerWithEditor();
-        }
-
-        private void SynchronizeEditorClipsWithOurClips()
-        {
-            var validClips = new List<AVAsset>();
-            foreach (var asset in this.clips)
-            {
-                if (asset != null)
-                {
-                    validClips.Add(asset);
-                }
-            }
-
-            this.editor.Clips = validClips;
-        }
-
-        private void SynchronizeEditorClipTimeRangesWithOurClipTimeRanges()
-        {
-            var validClipTimeRanges = new List<NSValue>();
-            foreach (var timeRange in this.clipTimeRanges)
-            {
-                if (timeRange != null)
-                {
-                    validClipTimeRanges.Add(timeRange);
-                }
-            }
-
-            this.editor.ClipTimeRanges = validClipTimeRanges;
+           this.editor.BuildCompositionObjectsForPlayback(true);
+           this.SynchronizePlayerWithEditor();
         }
 
         #endregion