来源：https://raw.githubusercontent.com/benjamintanweihao/elixir-cheatsheets/master/GenServer_CheatSheet.pdf

Client

```elixir
def start_link(opts \\ []) do
 GenServer.start_link(__MODULE__, :ok, opts)
end
```
Returns
```elixir
{:ok, pid}
```

Callback
```
def init(:ok) do
 state = init_state()
 {:ok, state}         # 这里的 :ok 对应初始化的 :ok
end
```

回调可以返回的值
```
{:ok, state}
{ok, state, 5_000}
{:ok, state, :hibernate}
{:stop, reason*}
RETURN VALUES
:ignore
```