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Summary: Queues Problem Intro: UIManager queue is special queue because it has special relationship with the Main queue. This particular relationship comes from two key factors: 1. UIManager initiates execution of many blocks on the Main queue; 2. In some cases, we want to initiate (and wait for) some UIManager's work *synchronously* from the Main queue. So, how can we meet these criteria? "Pseudo UIManager queue" comes to rescue! "Pseudo UIManager queue" means safe execution of typical UIManager's work on the Main queue while the UIManager queue is explicitly blocked for preventing simultaneous/concurrent memory access. So, how can we technically do this? 1. `RCTAssertUIManagerQueue` is okay with execution on both actual UIManager and Pseudo UIManager queues. 2. Both `RCTExecuteOnUIManagerQueue` and `RCTUnsafeExecuteOnUIManagerQueueSync` execute given block *synchronously* if they were called on actual UIManager or Pseudo UIManager queues. 3. `RCTExecuteOnMainQueue` executes given block *synchronously* if we already on the Main queue. 4. `RCTUnsafeExecuteOnUIManagerQueueSync` is smart enough to do the trick: It detects calling on the Main queue and in this case, instead of doing trivial *synchronous* dispatch, it does: - Block the Main queue; - Dispatch the special block on UIManager queue to block the queue and concurrent memory access; - Execute the given block on the Main queue; - Unblock the UIManager queue. Imagine the analogy: We have two queues: the Main one and UIManager one. And these queues are two lanes of railway go in parallel. Then, at some point, we merge UIManager lane with the Main lane, and all cars use the unified the Main lane. And then we split lanes again. This solution assumes that the code running on UIManager queue will never *explicitly* block the Main queue via calling `RCTUnsafeExecuteOnMainQueueSync`. Otherwise, it can cause a deadlock. Reviewed By: mmmulani Differential Revision: D5935464 fbshipit-source-id: 6a60ff236280d825b4e2b101f06222266097b97f
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