请求协助：将MQTT通道sensor/X与sensor/Y数据合并为单个JSON对象

Got it, let's sort this out. The problem with your current flow is that it processes each MQTT message from sensor/X and sensor/Y separately—there's no mechanism to cache both values and merge them into one JSON object once both are available. Here's a step-by-step solution using a Function node:

Step 1: Adjust Your Node-RED Flow #

1. Remove the existing json node between your mqtt in and debug nodes.
2. Add a Function node in its place (connect mqtt in → Function → debug).

Step 2: Paste This JavaScript Code into the Function Node #

This code uses Node-RED's context storage to cache values from both topics, then outputs the combined JSON only when both X and Y values are present:

// Retrieve cached sensor values from context (initialize if empty)
let sensorCache = context.get('sensorCache') || { X: null, Y: null };

// Parse the incoming MQTT topic to identify if it's X or Y
const topicSegments = msg.topic.split('/');
const sensorType = topicSegments[1]; // Will be "X" or "Y"

// Update the cache with the current message's payload
if (sensorType === 'X') {
    sensorCache.X = msg.payload;
    // If your Python code sends JSON instead of raw numbers, use this instead:
    // sensorCache.X = msg.payload.value; // Adjust based on your actual payload structure
} else if (sensorType === 'Y') {
    sensorCache.Y = msg.payload;
    // Similarly, use msg.payload.value if sending JSON
}

// Save the updated cache back to context
context.set('sensorCache', sensorCache);

// Only output the combined JSON if both values are available
if (sensorCache.X !== null && sensorCache.Y !== null) {
    // Create the final combined payload
    msg.payload = {
        X: sensorCache.X,
        Y: sensorCache.Y
    };
    
    // Optional: Clear the cache after output (remove if you want to keep updating with new values)
    // context.set('sensorCache', { X: null, Y: null });
    
    return msg;
} else {
    // Don't output anything if one value is missing
    return null;
}

Step 3: Test the Flow #

1. Send a value to sensor/X from your Python code—no output will appear in the debug tab yet (since Y is missing).
2. Send a value to sensor/Y—the debug tab will now show your combined JSON object: { "X": [your X value], "Y": [your Y value] }.

Key Notes #

• Payload Structure: If your Python code sends JSON payloads (e.g., {"value": 42}) instead of raw numbers, adjust the code to reference msg.payload.value instead of msg.payload.
• Persistent Caching: By default, Node-RED uses in-memory context—if you restart Node-RED, the cached values will be lost. For persistent storage, configure context storage to use a file or database.
• Continuous Updates: If you want to output an updated combined object every time either X or Y changes (instead of only once), remove the optional cache-clearing line. This way, every new value will trigger an output with the latest combined data.

内容的提问来源于stack exchange，提问作者RogerLM