NmeaParser/concepts/CustomMessages.html
2025-01-16 03:51:54 +00:00

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<h1 id="creating-custom-nmea-messages">Creating custom NMEA messages</h1>
<p>Custom NMEA messages can be registered for parsing as well.
To create a new message, add the NmeaMessageType attribute to the class, and declare the 5-character message type.</p>
<p><em>Note: You can use <code>--</code> as the first two characters to make it independent of the Talker Type.</em></p>
<p>Next ensure you have a constructor that takes the <code>TypeName</code> string parameter first, and a second <code>string[]</code> parameter that will contain all the message values:</p>
<p>Example:</p>
<pre><code class="lang-cs">[NmeaMessageType(&quot;PTEST&quot;)]
public class CustomMessage : NmeaMessage
{
public CustomMessage(string type, string[] parameters) : base(type, parameters)
{
Value = parameters[0];
}
public string Value { get; }
}
</code></pre>
<p>Next register this with the NMEA Parser using either:</p>
<pre><code class="lang-cs">NmeaMessage.RegisterAssembly(typeof(CustomMessage).Assembly); //Registers all types in the provided assembly
NmeaMessage.RegisterMessage(typeof(CustomMessage).GetTypeInfo()); //Registers a single NMEA message
</code></pre>
<p>Note that these methods will throw if the NMEA type has already been registered (there's an overload where you can declare the <code>replace</code> parameter to <code>true</code> to overwrite already registered messages.</p>
<p>Next you should be able to test this method using the Parse method:</p>
<pre><code class="lang-cs">var input = &quot;$PTEST,TEST*7C&quot;;
var msg = NmeaMessage.Parse(input);
</code></pre>
<h1 id="creating-a-multi-sentence-message">Creating a multi-sentence message</h1>
<p>A NMEA message cannot exceed 82 characters, so often messages are split into multiple sentences. To create a custom multi message, either implement <code>IMultiSentenceMessage</code> or simply subclass <code>NmeaMultiSentenceMessage</code>.</p>
<pre><code class="lang-cs">[NmeaMessageType(&quot;PTST2&quot;)]
private class CustomMultiMessage : NmeaMultiSentenceMessage, IMultiSentenceMessage
{
public CustomMultiMessage(string type, string[] parameters) : base(type, parameters)
{
}
public string Id { get; private set; }
public List&lt;string&gt; Values { get; } = new List&lt;string&gt;();
// Set index in the message where the total count is:
protected override int MessageCountIndex =&gt; 0;
// Set index in the message where the message number is:
protected override int MessageNumberIndex =&gt; 1;
protected override bool ParseSentences(Talker talkerType, string[] message)
{
// Ensure this message matches the previous message.
// Use any indicator to detect message difference, so you can error out and avoid
// appending the wrong message
if (Id == null)
Id = message[2]; //First time it's not set
else if (Id != message[2])
return false;
Values.AddRange(message.Skip(3));
return true;
}
}
</code></pre>
<p>Note that the message is parsed in the <code>ParseSentences</code> method, and not the constructor. Also note that the talkerType is parsed to you, because multi-sentence messages allows a mix of talker types, if you use the <code>--</code> prefex in the NMEA type.</p>
<p>Next we can parse the two messages and have the second one be appended to the first one:</p>
<pre><code class="lang-cs">NmeaMessage.RegisterNmeaMessage(typeof(CustomMultiMessage).GetTypeInfo());
var input1 = &quot;$PTST2,2,1,123,A,B,C,D*2A&quot;;
var input2 = &quot;$PTST2,2,2,123,E,F,G,H*21&quot;;
var msg1 = NmeaMessage.Parse(input1);
var msg2 = NmeaMessage.Parse(input2, msg1 as IMultiSentenceMessage);
</code></pre>
<p>If msg1 and msg2 aren't the same instance, it means the message couldn't be added to the previous message, and a new message was generated.</p>
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