Added ResolvingAudioSource

From the PR ryanheise#800

just_audio/lib/just_audio.dart

@@ -2832,6 +2832,89 @@ abstract class StreamAudioSource extends IndexedAudioSource {
       id: _id, uri: _uri.toString(), headers: null, tag: tag);
 }
 
+typedef ResolveSoundUrl = Future<Uri?> Function(String uniquidId);
+
+//An [AudioSource] likes [UriAudioSource] but resolve http url in time.
+class ResolvingAudioSource extends StreamAudioSource {
+  final String uniqueId;
+  final ResolveSoundUrl resolveSoundUrl;
+  final Map<String, String>? headers;
+
+  var _hasRequestedSoundUrl = false;
+  final _soundUrlCompleter = Completer<Uri?>();
+
+  Future<Uri?> get _soundUrl => _soundUrlCompleter.future;
+
+  HttpClient? _httpClient;
+
+  HttpClient get httpClient => _httpClient ?? (_httpClient = HttpClient());
+
+  ResolvingAudioSource(
+      {required this.uniqueId,
+      required this.resolveSoundUrl,
+      this.headers,
+      dynamic tag})
+      : super(tag: tag);
+
+  @override
+  Future<StreamAudioResponse> request([int? start, int? end]) async {
+    if (!_hasRequestedSoundUrl) {
+      _hasRequestedSoundUrl = true;
+      final soundUrl = await resolveSoundUrl(uniqueId);
+      _soundUrlCompleter.complete(soundUrl);
+    }
+    final soundUrl = await _soundUrl;
+    if (soundUrl == null) {
+      return StreamAudioResponse(
+          sourceLength: null,
+          contentLength: null,
+          offset: null,
+          stream: const Stream.empty(),
+          contentType: '');
+    }
+    final request = await httpClient.getUrl(soundUrl);
+    for (var entry in headers?.entries ?? <MapEntry<String, String>>[]) {
+      request.headers.set(entry.key, entry.value);
+    }
+    if (_player?._userAgent != null) {
+      request.headers.set(HttpHeaders.userAgentHeader, _player!._userAgent!);
+    }
+    if (start != null || end != null) {
+      request.headers
+          .set(HttpHeaders.rangeHeader, 'bytes=${start ?? ""}-${end ?? ""}');
+    }
+    final response = await request.close();
+    final acceptRangesHeader =
+        response.headers.value(HttpHeaders.acceptRangesHeader);
+    final contentRange = response.headers.value(HttpHeaders.contentRangeHeader);
+    int? offset;
+    if (contentRange != null) {
+      int offsetEnd = contentRange.indexOf('-');
+      if (offsetEnd >= 6) {
+        offset = int.tryParse(contentRange.substring(6, offsetEnd));
+      }
+    }
+    final contentLength =
+        response.headers.value(HttpHeaders.contentLengthHeader);
+    final contentType = response.headers.value(HttpHeaders.contentTypeHeader);
+    return StreamAudioResponse(
+        rangeRequestsSupported:
+            acceptRangesHeader != null && acceptRangesHeader != 'none',
+        sourceLength: null,
+        contentLength:
+            contentLength == null ? null : int.tryParse(contentLength),
+        offset: offset,
+        stream: response.asBroadcastStream(),
+        contentType: contentType ?? "");
+  }
+
+  @override
+  AudioSourceMessage _toMessage() {
+    return ProgressiveAudioSourceMessage(
+        id: _id, uri: _uri.toString(), headers: headers, tag: tag);
+  }
+}
+
 /// The response for a [StreamAudioSource]. This API is experimental.
 @experimental
 class StreamAudioResponse {