Moved EventChannel creation in the downloadOfflineRegion method (#205)

Here i splitted the downloadOfflineRegion method on the native side both
on IOS and android. This is to avoid edge case when the download is so
fast that the event channel doesnt link to the native side. When this
happen, the callback `onEvent` is never called. This is an inconsitency
that can create infinite loadings.

android/src/main/java/com/mapbox/mapboxgl/GlobalMethodHandler.java

@@ -28,6 +28,7 @@ class GlobalMethodHandler implements MethodChannel.MethodCallHandler {
   @NonNull private final BinaryMessenger messenger;
   @Nullable private PluginRegistry.Registrar registrar;
   @Nullable private FlutterPlugin.FlutterAssets flutterAssets;
+  @Nullable private OfflineChannelHandlerImpl downloadOfflineRegionChannelHandler;
 
   GlobalMethodHandler(@NonNull PluginRegistry.Registrar registrar) {
     this.registrar = registrar;
@@ -93,19 +94,29 @@ public void onMethodCall(MethodCall methodCall, MethodChannel.Result result) {
         Map<String, String> headers = (Map<String, String>) methodCall.argument("headers");
         MapboxHttpRequestUtil.setHttpHeaders(headers, result);
         break;
+      case "downloadOfflineRegion#setup":
+        String channelName = methodCall.argument("channelName");
+        // Prepare args
+        downloadOfflineRegionChannelHandler = new OfflineChannelHandlerImpl(messenger, channelName);
+        result.success(null);
+        break;
       case "downloadOfflineRegion":
         // Get args from caller
         Map<String, Object> definitionMap = (Map<String, Object>) methodCall.argument("definition");
         Map<String, Object> metadataMap = (Map<String, Object>) methodCall.argument("metadata");
-        String channelName = methodCall.argument("channelName");
 
-        // Prepare args
-        OfflineChannelHandlerImpl channelHandler =
-            new OfflineChannelHandlerImpl(messenger, channelName);
+        if (downloadOfflineRegionChannelHandler == null) {
+          result.error(
+                  "downloadOfflineRegion#setup NOT CALLED",
+                  "The setup has not been called, please call downloadOfflineRegion#setup before",
+                  null);
+          break;
+        }
 
         // Start downloading
         OfflineManagerUtils.downloadRegion(
-            result, context, definitionMap, metadataMap, channelHandler);
+            result, context, definitionMap, metadataMap, downloadOfflineRegionChannelHandler);
+        downloadOfflineRegionChannelHandler = null;
         break;
       case "getListOfRegions":
         OfflineManagerUtils.regionsList(result, context);

android/src/main/java/com/mapbox/mapboxgl/OfflineManagerUtils.java

@@ -76,9 +76,6 @@ public void onCreate(OfflineRegion offlineRegion) {
                 result.success(new Gson().toJson(regionData));
 
                 _offlineRegion = offlineRegion;
-                // Start downloading region
-                _offlineRegion.setDownloadState(OfflineRegion.STATE_ACTIVE);
-                channelHandler.onStart();
                 // Observe downloading state
                 OfflineRegion.OfflineRegionObserver observer =
                     new OfflineRegion.OfflineRegionObserver() {
@@ -140,7 +137,12 @@ public void mapboxTileCountLimitExceeded(long limit) {
                         deleteRegion(null, context, _offlineRegion.getID());
                       }
                     };
+
                 _offlineRegion.setObserver(observer);
+
+                // Start downloading region
+                _offlineRegion.setDownloadState(OfflineRegion.STATE_ACTIVE);
+                channelHandler.onStart();
               }
 
               /**

ios/Classes/SwiftMapboxGlFlutterPlugin.swift

@@ -4,6 +4,8 @@ import Mapbox
 import UIKit
 
 public class SwiftMapboxGlFlutterPlugin: NSObject, FlutterPlugin {
+    static var downloadOfflineRegionChannelHandler: OfflineChannelHandler? = nil
+
     public static func register(with registrar: FlutterPluginRegistrar) {
         let instance = MapboxMapFactory(withRegistrar: registrar)
         registrar.register(instance, withId: "plugins.flutter.io/mapbox_gl")
@@ -35,33 +37,55 @@ public class SwiftMapboxGlFlutterPlugin: NSObject, FlutterPlugin {
                 guard let arguments = methodCall.arguments as? [String: String] else { return }
                 let tilesdb = arguments["tilesdb"]
                 installOfflineMapTiles(registrar: registrar, tilesdb: tilesdb!)
+                result(nil)
+            case "downloadOfflineRegion#setup":
+                guard let args = methodCall.arguments as? [String: Any],
+                      let channelName = args["channelName"] as? String
+                else {
+                    print(
+                        "downloadOfflineRegion#setup unexpected arguments: \(String(describing: methodCall.arguments))"
+                    )
+                    result(nil)
+                    return
+                }
+
+                downloadOfflineRegionChannelHandler = OfflineChannelHandler(
+                    messenger: registrar.messenger(),
+                    channelName: channelName
+                )
+
                 result(nil)
             case "downloadOfflineRegion":
                 // Get download region arguments from caller
                 guard let args = methodCall.arguments as? [String: Any],
                       let definitionDictionary = args["definition"] as? [String: Any],
                       let metadata = args["metadata"] as? [String: Any],
-                      let defintion = OfflineRegionDefinition.fromDictionary(definitionDictionary),
-                      let channelName = args["channelName"] as? String
+                      let defintion = OfflineRegionDefinition.fromDictionary(definitionDictionary)
                 else {
                     print(
                         "downloadOfflineRegion unexpected arguments: \(String(describing: methodCall.arguments))"
                     )
                     result(nil)
                     return
                 }
-                // Prepare channel
-                let channelHandler = OfflineChannelHandler(
-                    messenger: registrar.messenger(),
-                    channelName: channelName
-                )
+
+                if (downloadOfflineRegionChannelHandler == nil) {
+                    result(FlutterError(
+                        code: "downloadOfflineRegion#setup NOT CALLED",
+                        message: "The setup has not been called, please call downloadOfflineRegion#setup before",
+                        details: nil
+                    ))
+                    return
+                }
+
                 OfflineManagerUtils.downloadRegion(
                     definition: defintion,
                     metadata: metadata,
                     result: result,
                     registrar: registrar,
-                    channelHandler: channelHandler
+                    channelHandler: downloadOfflineRegionChannelHandler!
                 )
+                downloadOfflineRegionChannelHandler = nil;
             case "setOfflineTileCountLimit":
                 guard let arguments = methodCall.arguments as? [String: Any],
                       let limit = arguments["limit"] as? UInt64

lib/src/global.dart

@@ -92,11 +92,9 @@ Future<OfflineRegion> downloadOfflineRegion(
   String channelName =
       'downloadOfflineRegion_${DateTime.now().microsecondsSinceEpoch}';
 
-  final result = await _globalChannel
-      .invokeMethod('downloadOfflineRegion', <String, dynamic>{
+  await _globalChannel
+      .invokeMethod('downloadOfflineRegion#setup', <String, dynamic>{
     'channelName': channelName,
-    'definition': definition.toMap(),
-    'metadata': metadata,
   });
 
   if (onEvent != null) {
@@ -144,5 +142,11 @@ Future<OfflineRegion> downloadOfflineRegion(
     });
   }
 
+  final result = await _globalChannel
+      .invokeMethod('downloadOfflineRegion', <String, dynamic>{
+    'definition': definition.toMap(),
+    'metadata': metadata,
+  });
+
   return OfflineRegion.fromMap(json.decode(result));
 }