While the target variable is guaranteed to be only assigned once, the delegate may be called by multiple concurrent threads unless a synchronization object is passed in.

这保证了目标变量只会设置一次。不过，如果没有使用同步对象的话，委托可能会被几个并发线程调用多次。

youdao

While the target variable is guaranteed to be only assigned once, the delegate may be called by multiple concurrent threads unless a synchronization object is passed in.

这保证了目标变量只会设置一次。不过，如果没有使用同步对象的话，委托可能会被几个并发线程调用多次。

youdao