React中Transition的作用
/**
* `Transaction` creates a black box that is able to wrap any method such that
* certain invariants are maintained before and after the method is invoked
* (Even if an exception is thrown while invoking the wrapped method). Whoever
* instantiates a transaction can provide enforcers of the invariants at
* creation time. The `Transaction` class itself will supply one additional
* automatic invariant for you - the invariant that any transaction instance
* should not be run while it is already being run. You would typically create a
* single instance of a `Transaction` for reuse multiple times, that potentially
* is used to wrap several different methods. Wrappers are extremely simple -
* they only require implementing two methods.
*
* <pre>
* wrappers (injected at creation time)
* + +
* | |
* +-----------------|--------|--------------+
* | v | |
* | +---------------+ | |
* | +--| wrapper1 |---|----+ |
* | | +---------------+ v | |
* | | +-------------+ | |
* | | +----| wrapper2 |--------+ |
* | | | +-------------+ | | |
* | | | | | |
* | v v v v | wrapper
* | +---+ +---+ +---------+ +---+ +---+ | invariants
* perform(anyMethod) | | | | | | | | | | | | maintained
* +----------------->|-|---|-|---|-->|anyMethod|---|---|-|---|-|-------->
* | | | | | | | | | | | |
* | | | | | | | | | | | |
* | | | | | | | | | | | |
* | +---+ +---+ +---------+ +---+ +---+ |
* | initialize close |
* +-----------------------------------------+
* </pre>
*
* Use cases:
* - Preserving the input selection ranges before/after reconciliation.
* Restoring selection even in the event of an unexpected error.
* - Deactivating events while rearranging the DOM, preventing blurs/focuses,
* while guaranteeing that afterwards, the event system is reactivated.
* - Flushing a queue of collected DOM mutations to the main UI thread after a
* reconciliation takes place in a worker thread.
* - Invoking any collected `componentDidUpdate` callbacks after rendering new
* content.
* - (Future use case): Wrapping particular flushes of the `ReactWorker` queue
* to preserve the `scrollTop` (an automatic scroll aware DOM).
* - (Future use case): Layout calculations before and after DOM updates.
*
* Transactional plugin API:
* - A module that has an `initialize` method that returns any precomputation.
* - and a `close` method that accepts the precomputation. `close` is invoked
* when the wrapped process is completed, or has failed.
*
* @param {Array<TransactionalWrapper>} transactionWrapper Wrapper modules
* that implement `initialize` and `close`.
* @return {Transaction} Single transaction for reuse in thread.
*
* @class Transaction
*/
简单的说就是，加入到某个transaction中之后，这个transaction会有前置和后置的wrapper，不如停止event，还原selection之类的。
