102 lines
3.8 KiB
Objective-C
102 lines
3.8 KiB
Objective-C
/**
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* Copyright (c) 2015-present, Facebook, Inc.
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* All rights reserved.
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*
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* This source code is licensed under the BSD-style license found in the
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* LICENSE file in the root directory of this source tree. An additional grant
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* of patent rights can be found in the PATENTS file in the same directory.
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*/
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#import <UIKit/UIKit.h>
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#import <React/RCTAssert.h>
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#import <React/RCTDefines.h>
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/**
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* Queues Problem Intro:
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* UIManager queue is a special queue because it has a special relationship with
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* the Main queue.
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*
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* This particular relationship comes from two key factors:
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* 1. UIManager initiates execution of many blocks on the Main queue;
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* 2. In some cases, we want to initiate (and wait for) some UIManager's work *synchronously* from
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* the Main queue.
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*
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* So, how can we meet these criteria?
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* "Pseudo UIManager queue" comes to rescue!
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*
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* "Pseudo UIManager queue" means the safe execution of typical UIManager's work
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* on the Main queue while the UIManager queue is explicitly blocked for preventing
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* simultaneous/concurrent memory access.
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*
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* So, how can we technically do this?
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* 1. `RCTAssertUIManagerQueue` is okay with execution on both actual UIManager and
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* Pseudo UIManager queues.
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* 2. Both `RCTExecuteOnUIManagerQueue` and `RCTUnsafeExecuteOnUIManagerQueueSync`
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* execute given block *synchronously* if they were called on actual UIManager
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* or Pseudo UIManager queues.
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* 3. `RCTExecuteOnMainQueue` executes given block *synchronously* if we already on
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* the Main queue.
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* 4. `RCTUnsafeExecuteOnUIManagerQueueSync` is smart enough to do the trick:
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* It detects calling on the Main queue and in this case, instead of doing
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* trivial *synchronous* dispatch, it does:
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* - Block the Main queue;
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* - Dispatch the special block on UIManager queue to block the queue and
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* concurrent memory access;
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* - Execute the given block on the Main queue;
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* - Unblock the UIManager queue.
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*
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* Imagine the analogy: We have two queues: the Main one and UIManager one.
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* And these queues are two lanes of railway that go in parallel. Then,
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* at some point, we merge UIManager lane with the Main lane, and all cars use
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* the unified the Main lane.
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* And then we split lanes again.
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*
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* This solution assumes that the code running on UIManager queue will never
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* *explicitly* block the Main queue via calling `RCTUnsafeExecuteOnMainQueueSync`.
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* Otherwise, it can cause a deadlock.
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*/
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/**
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* Returns UIManager queue.
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*/
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RCT_EXTERN dispatch_queue_t RCTGetUIManagerQueue(void);
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/**
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* Default name for the UIManager queue.
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*/
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RCT_EXTERN char *const RCTUIManagerQueueName;
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/**
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* Check if we are currently on UIManager queue.
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* Please do not use this unless you really know what you're doing.
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*/
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RCT_EXTERN BOOL RCTIsUIManagerQueue(void);
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/**
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* Check if we are currently on Pseudo UIManager queue.
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* Please do not use this unless you really know what you're doing.
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*/
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RCT_EXTERN BOOL RCTIsPseudoUIManagerQueue(void);
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/**
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* *Asynchronously* executes the specified block on the UIManager queue.
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* Unlike `dispatch_async()` this will execute the block immediately
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* if we're already on the UIManager queue.
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*/
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RCT_EXTERN void RCTExecuteOnUIManagerQueue(dispatch_block_t block);
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/**
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* *Synchorously* executes the specified block on the UIManager queue.
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* Unlike `dispatch_sync()` this will execute the block immediately
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* if we're already on the UIManager queue.
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* Please do not use this unless you really know what you're doing.
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*/
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RCT_EXTERN void RCTUnsafeExecuteOnUIManagerQueueSync(dispatch_block_t block);
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/**
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* Convenience macro for asserting that we're running on UIManager queue.
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*/
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#define RCTAssertUIManagerQueue() RCTAssert(RCTIsUIManagerQueue() || RCTIsPseudoUIManagerQueue(), \
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@"This function must be called on the UIManager queue")
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