Developing a Podcast Mobile App
A podcast app—specific niche within audio streaming. Content from open RSS feeds, long playback (30–90 minutes), needs variable speed, chapters, transcript sync, reliable offline mode. Ready SDK practically non-existent—assembled from components.
RSS and Feed Parsing
Podcasts published via RSS 2.0 with iTunes/Podcast namespace. Each episode—<item> element with <enclosure> (MP3/AAC link), <itunes:duration>, <itunes:image>, <itunes:episode>, <content:encoded> for show notes.
Parse on client—XMLParser (iOS, SAX-based) or XmlPullParser (Android). For regular feed updates, background task preferred: BGAppRefreshTask (iOS) min 15-minute interval, PeriodicWorkRequest in WorkManager (Android). Apple Podcast Index and Listen Notes—aggregators with REST API if needing thousands of podcasts without self-parsing.
// iOS — load and parse RSS feed
class PodcastFeedParser: NSObject, XMLParserDelegate {
private var currentElement = ""
private var currentEpisode: EpisodeBuilder?
func parser(_ parser: XMLParser, didStartElement elementName: String,
namespaceURI: String?, qualifiedName: String?,
attributes: [String: String] = [:]) {
currentElement = elementName
if elementName == "item" { currentEpisode = EpisodeBuilder() }
if elementName == "enclosure" {
currentEpisode?.audioURL = attributes["url"]
currentEpisode?.fileSize = Int(attributes["length"] ?? "0")
}
}
}
Variable Speed Playback
Speed playback (0.5x — 3.0x) without pitch shift—mandatory for podcasts. iOS: AVPlayer.rate changes speed. AVAudioSession with AVAudioTimePitchAlgorithm.timeDomain on AVPlayerItem—removes pitch shift when speeding. .spectral—best quality, CPU heavy.
Android: ExoPlayer Media3 with PlaybackParameters(speed = 1.5f, pitch = 1.0f). Pitch = 1.0 means fixed pitch regardless of speed.
Note: at speed > 2x, some speakers' voices become hard to understand due to pitch correction algorithm. Limitation of algorithm, not implementation bug.
Podcast Chapters
Chapters—navigation within long episode. Two formats: MP3 ID3 chapter frames (CHAP tag) and Podlove Simple Chapters (RSS XML). Apple Podcasts supports both.
Read ID3 chapters on iOS: AVMetadataItem via AVAsset.loadMetadata(for: .id3Metadata), search AVMetadataID3MetadataKeyChapterTOC and AVMetadataID3MetadataKeyChapter. Android—ID3 parsing via ExoPlayer Metadata output or libraries (jaudiotagger).
UI: timeline with markers, chapter list with tap-to-jump via AVPlayer.seek(to: CMTime) / player.seekTo(positionMs).
Transcript and Text Sync
Podcast Namespace <podcast:transcript>—type application/srt or application/json (WebVTT-like). During playback—highlight current line by AVPlayer.currentTime.
No transcript—generation via Whisper (OpenAI API or local via whisper.cpp). iPhone with Neural Engine—whisper.cpp with CoreML small model runs at 2–3x realtime. Server generation on demand—economically better cloud API.
Offline and Download Management
Download episode: URLSessionDownloadTask with URLSessionConfiguration.background. Progress via URLSessionDownloadDelegate.urlSession(_:downloadTask:didWriteData:). File saved in documentDirectory with episode-ID name.
Storage management—important UX: show downloaded size, suggest delete listened episodes, configure auto-download new.
Lock Screen and CarPlay
Lock Screen: MPNowPlayingInfoCenter with MPNowPlayingInfoPropertyPlaybackRate, MPNowPlayingInfoPropertyDefaultPlaybackRate, MPNowPlayingInfoPropertyChapterCount, MPNowPlayingInfoPropertyCurrentChapterIndex. Chapters displayed in iOS Control Center.
CarPlay: CPTemplateApplicationScene + CPListTemplate for catalog. CPNowPlayingTemplate—playback screen. Requires entitlement (com.apple.developer.carplay-audio).
Recommendations and Discovery
Podchaser API, Podcast Index API—aggregators with metadata and ratings. For listen-history recommendations—topic modeling via LDA or sentence embeddings (podcast descriptions → cosine similarity). At start, simple: "listened X → others who listened X also listened Y."
1 week — 3 months depending on scope: simple aggregator with player to full platform with custom hosting and analytics. Cost calculated individually.