Client发送请求
```
def sync_op(pid, args) do
 GenServer.call(pid, {:sync_op, args})  # :sync_op 是我们指定操作的名称
end
```

Callback
```
def handle_call({:sync_op, args}, from, state) do
 new_state = f(state, args)
 {:reply, new_state}
end
```

回调返回值示例
```
{:reply, reply, new_state}
{:reply, reply, new_state, 5_000}
{:reply, reply, new_state, :hibernate}
{:noreply, new_state}
{:noreply, new_state, 5_000}
{:noreply, new_state, :hibernate}
{:stop, reason*, reply, new_state}
{:stop, reason*, new_state}
```