a thread switching problem about using ucontext.h in a user-level thread implementation

Question

void Mythread::threadSchedule(int n)
{
    if(threadMap_.size() > 1)
    {
        int lastId = current_;  
        if(current_ != (--threadMap_.end())->first)
        {
            auto it = threadMap_.find(current_);
            current_ = (++it)->first;
        }
        else
        {
            current_ = threadMap_.begin()->first;
        }

        swapcontext(threadMap_[lastId].get(),threadMap_[current_].get());
    }
}

I recently saw a user-level thread implemented with ucontext, I just don't understand how the threads switch. In the code above, after this swapcontext done its job, the context of threadMap_[lastId].get()rather should be inside the threadSchedule? But the context is in the thread with lastID break point. It's just so confused that "current context" which swapcontext(ucontext_t *oucp, ucontext_t *ucp) saved to the oucp is not the context inside the function threadSchedule(int n) now but the old thread's context. The whole code is in https://github.com/Miaoshuai/Coroutine