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FlutterEngineGroup keeps an array of all live FlutterEngine instances it has created such that after the first engine, it can spawn further engines using the first. Previously we manually managed this array by adding engines to it upon creation, then having [FlutterEngine dealloc] emit a notification such that FlutterEngineGroup can listen for it, and remove instances from the array upon reception of the notification. Instead, we now use an NSPointerArray of of weak pointers such that pointers are automatically nilled out by ARC after the last strong reference is collected. This eliminates the need for the manual notification during dealloc. Unfortunately, NSPointerArray is "clever" and assumes that if the array has not been mutated to store a nil pointer since its last compact call, it must not contain a nil pointer and can thus skip compaction. Under ARC, weak pointers are automatically nilled out when the last strong reference is released, so the above heuristic is no longer valid. We work around the issue by storing a nil pointer before calling compact. See http://www.openradar.me/15396578 for the radar tracking this bug. I'm not thrilled with the fact that we're replacing one sort of TODO with another, but the code is much simpler and avoids relying on a trip through the notification center, which seems objectively better. Issue: https://github.com/flutter/flutter/issues/155943 Issue: https://github.com/flutter/flutter/issues/137801 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style