# 关注重连事件

Because reconnect is primarily under the covers, many libraries provide an event listener you can use to be notified of reconnect events. This event can be especially important for applications sending a lot of messages.

{% tabs %}
{% tab title="Go" %}

```go
// Connection event handlers are invoked asynchronously
// and the state of the connection may have changed when
// the callback is invoked.
nc, err := nats.Connect("demo.nats.io",
    nats.DisconnectErrHandler(func(nc *nats.Conn, err error) {
        // handle disconnect error event
    }),
    nats.ReconnectHandler(func(nc *nats.Conn) {
        // handle reconnect event
    }))
if err != nil {
    log.Fatal(err)
}
defer nc.Close()

// Do something with the connection
```

{% endtab %}

{% tab title="Java" %}

```java
Options options = new Options.Builder().
                            server("nats://demo.nats.io:4222").
                            connectionListener((conn, type) -> {
                                if (type == Events.RECONNECTED) {
                                    // handle reconnected
                                } else if (type == Events.DISCONNECTED) {
                                    // handle disconnected, wait for reconnect
                                }
                            }).
                            build();
Connection nc = Nats.connect(options);

// Do something with the connection

nc.close();
```

{% endtab %}

{% tab title="JavaScript" %}

```javascript
const nc = await connect({
    maxReconnectAttempts: 10,
    servers: ["demo.nats.io"],
  });

  (async () => {
    for await (const s of nc.status()) {
      switch (s.type) {
        case Status.Reconnect:
          t.log(`client reconnected - ${s.data}`);
          break;
        default:
      }
    }
  })().then();
});
```

{% endtab %}

{% tab title="Python" %}

```python
nc = NATS()

async def disconnected_cb():
   print("Got disconnected!")

async def reconnected_cb():
   # See who we are connected to on reconnect.
   print("Got reconnected to {url}".format(url=nc.connected_url.netloc))

await nc.connect(
   servers=["nats://demo.nats.io:4222"],
   reconnect_time_wait=10,
   reconnected_cb=reconnected_cb,
   disconnected_cb=disconnected_cb,
   )

# Do something with the connection.
```

{% endtab %}

{% tab title="Ruby" %}

```ruby
require 'nats/client'

NATS.start(servers: ["nats://127.0.0.1:1222", "nats://127.0.0.1:1223", "nats://127.0.0.1:1224"]) do |nc|
   # Do something with the connection
   nc.on_reconnect do
    puts "Got reconnected to #{nc.connected_server}"
  end

  nc.on_disconnect do |reason|
    puts "Got disconnected! #{reason}"
  end
end
```

{% endtab %}

{% tab title="C" %}

```c
static void
disconnectedCB(natsConnection *conn, void *closure)
{
    // Handle disconnect error event
}

static void
reconnectedCB(natsConnection *conn, void *closure)
{
    // Handle reconnect event
}

(...)

natsConnection      *conn      = NULL;
natsOptions         *opts      = NULL;
natsStatus          s          = NATS_OK;

s = natsOptions_Create(&opts);

// Connection event handlers are invoked asynchronously
// and the state of the connection may have changed when
// the callback is invoked.
if (s == NATS_OK)
    s = natsOptions_SetDisconnectedCB(opts, disconnectedCB, NULL);
if (s == NATS_OK)
    s = natsOptions_SetReconnectedCB(opts, reconnectedCB, NULL);

if (s == NATS_OK)
    s = natsConnection_Connect(&conn, opts);

(...)

// Destroy objects that were created
natsConnection_Destroy(conn);
natsOptions_Destroy(opts);
```

{% endtab %}
{% endtabs %}


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