[Chromium]怎样安全的使用PostTask

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PostTask參数决策树

官方的解释总是最权威。有疑问看这里或者直接看代码中的说明: bind_helpers.h.　

传值方式	描写叙述
this 或对象指针	假设对象本身是一个RefCountedThreadSafe, 没有问题．假设是个裸指针，应当尽量避免，除非你能够保证它的线程安全．
base::Unretained	注意:使用这个的前提是有其他同步机制保障对象的生命周期．假设有其他同步机制保障对象的生命周期。能够使用Unretained()传递非引用计数的对象．假设是一个非引用计数的对象，能够使用Unretained()封装起来．
base::Owned	假设是暂时对象，或者操心任务运行完毕后对象可能出现泄露，能够使用Owned, 表示由Task 持有对象的全部权，在结束时析构它．
base::Passed	假设要运行Task须要传入scoped指针，就能够使用它转换，它也能够避免拷贝，而是相似move语义．
base::ConstRef	相似常量引用，不希望bind过程出现拷贝，就能够使用它．
base::IgnoreResult	假设Task要调用的方法带有返回值。而你又不关心返回值就能够使用IgnoreResult来传入对象指针．

假设这样一直将以引用计数来使用对象岂不最为简单。为什么要避免引用计数?

Reference-counted objects make it difficult to understand ownership and destruction order, especially when multiple threads are involved. There is almost always another way to design your object hierarchy to avoid refcounting. Avoiding refcounting in multithreaded situations is usually easier if you restrict each class to operating on just one thread, and use PostTask() and the like to proxy calls to the correct thread. base::Bind(), WeakPtr, and other tools make it possible to automatically cancel calls to such an object when it dies. Note that too much of our existing code uses refcounting, so just because you see existing code doing it does not mean it’s the right solution. (Bonus points if you’re able to clean up such cases.)