[ios, osx] Allow creating invalid offline packs

Removed NS_UNAVAILABLE attribute from -[MGLOfflinePack init]. Packs initialized with this initializer are doomed to invalidity.

Fixes #4421.

platform/darwin/include/MGLOfflinePack.h

@@ -83,6 +83,10 @@ typedef struct MGLOfflinePackProgress {
 /**
  An `MGLOfflinePack` represents a collection of resources necessary for viewing
  a region offline to a local database.
+ 
+ To create an instance of `MGLOfflinePack`, use the
+ `+[MGLOfflineStorage addPackForRegion:withContext:completionHandler:]` method.
+ A pack created using `-[MGLOfflinePack init]` is immediately invalid.
  */
 @interface MGLOfflinePack : NSObject
 
@@ -123,8 +127,6 @@ typedef struct MGLOfflinePackProgress {
  */
 @property (nonatomic, readonly) MGLOfflinePackProgress progress;
 
-- (instancetype)init NS_UNAVAILABLE;
-
 /**
  Resumes downloading if the pack is inactive.
  

platform/darwin/src/MGLOfflinePack.mm

@@ -48,11 +48,11 @@ private:
 @implementation MGLOfflinePack
 
 - (instancetype)init {
-    [NSException raise:@"Method unavailable"
-                format:
-     @"-[MGLOfflinePack init] is unavailable. "
-     @"Use +[MGLOfflineStorage addPackForRegion:withContext:completionHandler:] instead."];
-    return nil;
+    if (self = [super init]) {
+        _state = MGLOfflinePackStateInvalid;
+        NSLog(@"%s called; did you mean to call +[MGLOfflineStorage addPackForRegion:withContext:completionHandler:] instead?", __PRETTY_FUNCTION__);
+    }
+    return self;
 }
 
 - (instancetype)initWithMBGLRegion:(mbgl::OfflineRegion *)region {
@@ -123,7 +123,7 @@ private:
 }
 
 - (void)requestProgress {
-    MGLAssertOfflinePackIsValid();
+    NSAssert(_state != MGLOfflinePackStateInvalid, @"Cannot request progress from an invalid offline pack.");
     
     mbgl::DefaultFileSource *mbglFileSource = [[MGLOfflineStorage sharedOfflineStorage] mbglFileSource];
     

platform/darwin/src/MGLOfflineStorage.mm

@@ -178,6 +178,11 @@ NSString * const MGLOfflinePackMaximumCountUserInfoKey = @"MaximumCount";
 - (void)_removePack:(MGLOfflinePack *)pack withCompletionHandler:(MGLOfflinePackRemovalCompletionHandler)completion {
     mbgl::OfflineRegion *mbglOfflineRegion = pack.mbglOfflineRegion;
     [pack invalidate];
+    if (!mbglOfflineRegion) {
+        completion(nil);
+        return;
+    }
+    
     self.mbglFileSource->deleteOfflineRegion(std::move(*mbglOfflineRegion), [&, completion](std::exception_ptr exception) {
         NSError *error;
         if (exception) {