Merge pull request #2790 from emqx/auto-subs

chore: add placeholder for auto-subscribe

en_US/messaging/mqtt-auto-subscription.md

@@ -18,7 +18,7 @@ Before EMQX 5.0, this feature is called **Proxy Subscription**.
 
 3. In the pop-up dialog box, type the test topic `a/1` in the **Topic** text box. Leave other settings as default.
 
-   - **Topic**: Type the topic that is automatically subscribed to for the client.
+   - **Topic**: Type the topic that is automatically subscribed to for the client. You can dynamically build the topic using placeholders. For details, see [Placeholders](#placeholders).
 
    - **QoS**: Specify the quality of service of the topic. Options: `0`, `1`, and `2`.
 
@@ -96,3 +96,13 @@ Basic publishing and subscribing operations using [MQTTX CLI](./publish-and-subs
 
    <img src="./assets/auto-sub-emqx_c.png" alt="auto-sub-emqx_c" style="zoom:60%;" />
 
+## Placeholders
+
+Auto Subscribe supports placeholders to dynamically build topics. The format of the placeholder is `${}`. The variables supported by the placeholder are:
+
+- `${clientid}`: Client ID.
+- `${username}`: Client username.
+- `${host}`: IP address when the client connects to EMQX.
+
+For example, when the client ID is `emqx_c` and the configured topic is `a/${clientid}`, the client will automatically subscribe to the topic `a/emqx_c` after connecting to EMQX.
+

zh_CN/messaging/mqtt-auto-subscription.md

@@ -18,7 +18,7 @@
 
 3. 在弹出的对话框中，在**主题**文本框中输入测试主题 `a/1`。其他设置保持默认值。
 
-   - **主题**: 输入客户端自动订阅的主题。
+   - **主题**: 输入客户端自动订阅的主题。您可以使用占位符动态构建主题，详见[占位符](#占位符)。
    - **QoS**: 指定主题的服务质量。选项：`0`、`1` 和 `2`。
    - **No local**: 选项：`False` 或 `True`。
    - **保留发布**: 指定是否保留使用指定主题发送的消息。选项：`False` 或 `True`。
@@ -92,5 +92,13 @@
 
    <img src="./assets/auto-sub-emqx_c.png" alt="auto-sub-emqx_c" style="zoom:60%;" />
 
+## 占位符
 
+自动订阅支持占位符来动态的构建主题。占位符的格式为 `${}`。占位符支持的变量有：
+
+- `${clientid}`: 客户端 ID。
+- `${username}`: 客户端用户名。
+- `${host}`: 客户端连接到 EMQX 时的 IP 地址。
+
+例如，当客户端 ID 为 `emqx_c`，配置主题为 `a/${clientid}` 时，客户端连接到 EMQX 后将自动订阅主题 `a/emqx_c`。