25 lines
1.1 KiB
Markdown
25 lines
1.1 KiB
Markdown
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---
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tag: docs
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type: other
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title: Subscription Callback
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---
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If the client is used to subscribe to topics, a callback function must be provided in the constructor. This function is called when new messages arrive at the client.
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The callback function has the following signature:
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void callback(const char[] topic, byte* payload, unsigned int length)
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##### Parameters
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- <span class="methodparams">topic</span> <span class="methodparamstype">const char[]</span> - the topic the message arrived on
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- <span class="methodparams">payload</span> <span class="methodparamstype">byte[]</span> - the message payload
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- <span class="methodparams">length</span> <span class="methodparamstype">unsigned int</span> - the length of the message payload
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Internally, the client uses the same buffer for both inbound and outbound
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messages. After the callback function returns, or if a call to either `publish`
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or subscribe is made from within the callback function, the `topic`
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and `payload` values passed to the function will be overwritten. The application
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should create its own copy of the values if they are required after the callback returns.
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