VEX-5938: Update resource loader to handle encrypted local files (#12)

Adds offline decryption key and uses it to decrypt content during offline playback

Jira: VEX-5938
https://jira.tenkasu.net/browse/VEX-5938

- Update to accept scheme for key required to play offline playback
- Uses provided scheme to intercept call from player and return the key
- Fixes player item observer removal pattern

### Reviews
- Major reviewer (domain expert): @armadilio3

ios/VideoCaching/RCTVideoCachingHandler.swift

@@ -1,52 +1,50 @@
 import Foundation
 import AVFoundation
 import DVAssetLoaderDelegate
+import Promises
 
 class RCTVideoCachingHandler: NSObject, DVAssetLoaderDelegatesDelegate {
     
     private var _videoCache:RCTVideoCache! = RCTVideoCache.sharedInstance()
-    private var _playerItemPrepareText: (AVAsset?, NSDictionary?, (AVPlayerItem?)->Void) -> Void
+    var playerItemPrepareText: ((AVAsset?, NSDictionary?) -> AVPlayerItem)?
     
-    init(_ playerItemPrepareText: @escaping (AVAsset?, NSDictionary?, (AVPlayerItem?)->Void) -> Void) {
-        _playerItemPrepareText = playerItemPrepareText
+    override init() {
+        super.init()
     }
     
-    func playerItemForSourceUsingCache(shouldCache:Bool, textTracks:[AnyObject]?, uri:String, assetOptions:NSMutableDictionary, handler:@escaping (AVPlayerItem?)->Void) -> Bool {
-        if shouldCache && ((textTracks == nil) || (textTracks!.count == 0)) {
+    func shouldCache(source: VideoSource, textTracks:[TextTrack]?) -> Bool {
+        if source.isNetwork && source.shouldCache && ((textTracks == nil) || (textTracks!.count == 0)) {
             /* The DVURLAsset created by cache doesn't have a tracksWithMediaType property, so trying
              * to bring in the text track code will crash. I suspect this is because the asset hasn't fully loaded.
              * Until this is fixed, we need to bypass caching when text tracks are specified.
              */
-            DebugLog("Caching is not supported for uri '\(uri)' because text tracks are not compatible with the cache. Checkout https://github.com/react-native-community/react-native-video/blob/master/docs/caching.md")
-            playerItemForSourceUsingCache(uri: uri, assetOptions:assetOptions, withCallback:handler)
+            DebugLog("Caching is not supported for uri '\(source.uri)' because text tracks are not compatible with the cache. Checkout https://github.com/react-native-community/react-native-video/blob/master/docs/caching.md")
             return true
         }
         return false
     }
     
-    func playerItemForSourceUsingCache(uri:String!, assetOptions options:NSDictionary!, withCallback handler: @escaping (AVPlayerItem?)->Void) {
+    func playerItemForSourceUsingCache(uri:String!, assetOptions options:NSDictionary!) -> Promise<AVPlayerItem?> {
         let url = URL(string: uri)
-        _videoCache.getItemForUri(uri, withCallback:{ [weak self] (videoCacheStatus:RCTVideoCacheStatus,cachedAsset:AVAsset?) in
-            guard let self = self else { return }
+        return getItemForUri(uri)
+        .then{ [weak self] (videoCacheStatus:RCTVideoCacheStatus,cachedAsset:AVAsset?) -> AVPlayerItem in
+            guard let self = self, let playerItemPrepareText = self.playerItemPrepareText else {throw  NSError(domain: "", code: 0, userInfo: nil)}
             switch (videoCacheStatus) {
             case .missingFileExtension:
                 DebugLog("Could not generate cache key for uri '\(uri)'. It is currently not supported to cache urls that do not include a file extension. The video file will not be cached. Checkout https://github.com/react-native-community/react-native-video/blob/master/docs/caching.md")
                 let asset:AVURLAsset! = AVURLAsset(url: url!, options:options as! [String : Any])
-                self._playerItemPrepareText(asset, options, handler)
-                return
+                return playerItemPrepareText(asset, options)
                 
             case .unsupportedFileExtension:
                 DebugLog("Could not generate cache key for uri '\(uri)'. The file extension of that uri is currently not supported. The video file will not be cached. Checkout https://github.com/react-native-community/react-native-video/blob/master/docs/caching.md")
                 let asset:AVURLAsset! = AVURLAsset(url: url!, options:options as! [String : Any])
-                self._playerItemPrepareText(asset, options, handler)
-                return
+                return playerItemPrepareText(asset, options)
                 
             default:
                 if let cachedAsset = cachedAsset {
                     DebugLog("Playing back uri '\(uri)' from cache")
                     // See note in playerItemForSource about not being able to support text tracks & caching
-                    handler(AVPlayerItem(asset: cachedAsset))
-                    return
+                    return AVPlayerItem(asset: cachedAsset)
                 }
             }
             
@@ -65,8 +63,16 @@ class RCTVideoCachingHandler: NSObject, DVAssetLoaderDelegatesDelegate {
              asset?.resourceLoader.setDelegate(resourceLoaderDelegate, queue: DispatchQueue.main)
              */
             
-            handler(AVPlayerItem(asset: asset))
-        })
+            return AVPlayerItem(asset: asset)
+        }
+    }
+
+    func getItemForUri(_ uri:String) ->  Promise<(videoCacheStatus:RCTVideoCacheStatus,cachedAsset:AVAsset?)> {
+        return Promise<(videoCacheStatus:RCTVideoCacheStatus,cachedAsset:AVAsset?)> { fulfill, reject in
+            self._videoCache.getItemForUri(uri, withCallback:{ (videoCacheStatus:RCTVideoCacheStatus,cachedAsset:AVAsset?) in
+                fulfill((videoCacheStatus, cachedAsset))
+            })
+        }
     }
     
     // MARK: - DVAssetLoaderDelegate