[ios, macos] Fix MGLMapSnapshotter concurrency bugs (issue #11827).

- Biggest change: when we apply the watermark on a background thread, don't capture self (turn most of the related instance methods into class methods)
- Don't call mbglMapSnapshotter->snapshot from a user-provided queue, since it's an asynchronous call anyway and starting it on the user's queue requires capturing self.

# Conflicts:
#	platform/darwin/src/MGLMapSnapshotter.mm

1 files changed, 1 insertions, 6 deletions

diff --git a/platform/darwin/src/MGLMapSnapshotter.mm b/platform/darwin/src/MGLMapSnapshotter.mm
index 2a2bef8fb8..73db8ea151 100644
--- a/platform/darwin/src/MGLMapSnapshotter.mm
+++ b/platform/darwin/src/MGLMapSnapshotter.mm
@@ -108,11 +108,6 @@ const CGFloat MGLSnapshotterMinimumPixelSize = 64;
 
 - (void)startWithQueue:(dispatch_queue_t)queue completionHandler:(MGLMapSnapshotCompletionHandler)completion
 {
-    if (!mbgl::Scheduler::GetCurrent()) {
-        [NSException raise:NSInvalidArgumentException
-                    format:@"startWithQueue:completionHandler: must be called from a thread with an active run loop."];
-    }
-
     if ([self isLoading]) {
         [NSException raise:NSInternalInconsistencyException
                     format:@"Already started this snapshotter."];
@@ -126,7 +121,7 @@ const CGFloat MGLSnapshotterMinimumPixelSize = 64;
     _snapshotCallback = std::make_unique<mbgl::Actor<mbgl::MapSnapshotter::Callback>>(*mbgl::Scheduler::GetCurrent(), [=](std::exception_ptr mbglError, mbgl::PremultipliedImage image, mbgl::MapSnapshotter::Attributions attributions, mbgl::MapSnapshotter::PointForFn pointForFn) {
         __typeof__(self) strongSelf = weakSelf;
         // If self had died, _snapshotCallback would have been destroyed and this block would not be executed
-        NSCAssert(strongSelf, @"Snapshot callback executed after being destroyed.");
+        assert(strongSelf);
 
         strongSelf.loading = false;