improvement: avoid duplicate map tile loading

Previously a map tile cache miss would cause 2x loading of the tile: once from
the remote tile server (which is then written to disk) and once from disk
during the default MKTileOverlay.loadTile function. Instead we now directly
implement loadTile so that we can avoid the duplicate loading and simply
return the fetched remote tile after it is cached, which leads to a
noticeable improvement in offline map performance.

Meshtastic/Helpers/Map/OfflineTileManager.swift

@@ -103,20 +103,26 @@ class OfflineTileManager: ObservableObject {
 			}
 		}
 	}
-	func getTileOverlay(for path: MKTileOverlayPath) -> URL {
-		let file = "\(UserDefaults.mapTileServer.id)-z\(path.z)x\(path.x)y\(path.y).png"
-		// Check is tile is already available
-		let tilesUrl = documentsDirectory.appendingPathComponent("tiles").appendingPathComponent(file)
-		if fileManager.fileExists(atPath: tilesUrl.path) {
-			return tilesUrl
-		} else {
-			if UserDefaults.enableOfflineMaps, UserDefaults.mapTileServer.zoomRange.contains(path.z) { // Get and persist newTile
-				return persistLocally(path: path)
-			} else { // Else display empty tile (transparent over Maps tiles)
-				return Bundle.main.url(forResource: "alpha", withExtension: "png")!
-			}
+
+	func loadAndCacheTileOverlay(for path: MKTileOverlayPath) throws -> Data {
+		guard UserDefaults.enableOfflineMaps, UserDefaults.mapTileServer.zoomRange.contains(path.z) else {
+			return try Data(contentsOf: Bundle.main.url(forResource: "alpha", withExtension: "png")!)
+		}
+
+		let tilesUrl = documentsDirectory
+			.appendingPathComponent("tiles")
+			.appendingPathComponent("\(UserDefaults.mapTileServer.id)-z\(path.z)x\(path.x)y\(path.y)")
+			.appendingPathExtension("png")
+
+		do {
+			return try Data(contentsOf: tilesUrl)
+		} catch let error as NSError where error.code == NSFileReadNoSuchFileError {
+			let data = try Data(contentsOf: overlay.url(forTilePath: path))
+			try data.write(to: tilesUrl)
+			return data
 		}
 	}
+
 	// MARK: Private methods
 	private func computeTileOverlayPaths(boundingBox box: MKMapRect, maxZ: Int = 17) -> [MKTileOverlayPath] {
 		var paths = [MKTileOverlayPath]()

Meshtastic/Helpers/Map/TileOverlay.swift

@@ -11,5 +11,7 @@ import MapKit
 typealias TileCoordinates = (x: Int, y: Int, z: Int)
 
 class TileOverlay: MKTileOverlay {
-	override func url(forTilePath path: MKTileOverlayPath) -> URL { OfflineTileManager.shared.getTileOverlay(for: path) }
+	override func loadTile(at path: MKTileOverlayPath) async throws -> Data {
+		return try OfflineTileManager.shared.loadAndCacheTileOverlay(for: path)
+	}
 }