如何在Eclipse Milo中正确使用多维数组？OPC UA客户端开发咨询

Hey Chris, let's break down your questions about working with multi-dimensional arrays in Eclipse Milo step by step:

1. Do you have to create an ExtensionObject to decode a matrix? #

Not necessarily—it depends on how the multi-dimensional array is structured and exposed by the OPC UA server:

• If the server uses standard OPC UA built-in array types (like Double[][], Integer[][], etc.), Milo's Variant can handle these directly. You don't need an ExtensionObject here; you can cast the value from the Variant to the corresponding multi-dimensional array type once you confirm its type.
• If the matrix is a custom structured type (defined in the server's address space as a custom ArrayType or complex object), it will likely be wrapped in an ExtensionObject. In this case, you'll need to decode the ExtensionObject using the appropriate type definition (either generated from the server's nodeset or manually defined) to access the underlying matrix data.

2. Is there a property that stores array dimensions, and how to access it? #

Absolutely! OPC UA defines a Dimensions property for array types, and Milo provides straightforward ways to access this info:

For standard built-in multi-dimensional arrays #

When you fetch a DataValue from the server, extract the Variant and check if it's an array. Use variant.getArrayDimensions() to get the dimensions as an int[]:

import org.eclipse.milo.opcua.stack.core.types.builtin.DataValue;
import org.eclipse.milo.opcua.stack.core.types.builtin.Variant;
import java.util.Arrays;

// Assume you've retrieved the DataValue from the server
DataValue dataValue = ...;
Variant variant = dataValue.getValue();

if (variant.isArray()) {
    int[] dimensions = variant.getArrayDimensions();
    if (dimensions != null) {
        System.out.println("Matrix dimensions: " + Arrays.toString(dimensions));
    }

    // Cast to the correct multi-dimensional array type
    if (variant.getValue() instanceof double[][]) {
        double[][] matrix = (double[][]) variant.getValue();
        // Work with your matrix here
    }
}

For custom types wrapped in ExtensionObject #

If the matrix is inside an ExtensionObject, first decode it into your custom type (you'll need the type defined in your client code, either via nodeset generation or manual implementation). Then access the dimensions property built into your custom type:

import org.eclipse.milo.opcua.stack.core.types.builtin.ExtensionObject;

if (variant.getValue() instanceof ExtensionObject) {
    ExtensionObject extensionObject = (ExtensionObject) variant.getValue();
    // Decode into your custom matrix type
    CustomMatrixType matrix = extensionObject.decodeAs(CustomMatrixType.class);
    
    // Access the dimensions property (naming depends on your type definition)
    int[] dimensions = matrix.getDimensions();
    System.out.println("Custom matrix dimensions: " + Arrays.toString(dimensions));
}

Just remember: if you're working with a custom type, make sure you've properly imported or generated the corresponding Java classes in your project—Milo needs these to decode the ExtensionObject correctly.

内容的提问来源于stack exchange，提问作者Chris18191