Core IO operations

Artikkeli
09/25/2024

In addition to providing tools to read spatial data files, the spatial IO module exposes core underlying libraries to read and write XML and delimited data fast and efficiently.

The atlas.io.core namespace contains two low-level classes that can quickly read and write CSV and XML data. These base classes power the spatial data readers and writers in the Spatial IO module. Feel free to use them to add more reading and writing support for CSV or XML files.

Read delimited files

The atlas.io.core.CsvReader class reads strings that contain delimited data sets. This class provides two methods for reading data:

The read function reads the full data set and return a two-dimensional array of strings representing all cells of the delimited data set.
The getNextRow function reads each line of text in a delimited data set and returns an array of string representing all cells in that line of data set. The user can process the row and dispose any unneeded memory from that row before processing the next row. So, function is more memory efficient.

By default, the reader uses the comma character as the delimiter. However, the delimiter can be changed to any single character or set to 'auto'. When set to 'auto', the reader analyzes the first line of text in the string. Then, it selects the most common character from the following table to use as the delimiter.

Delimiter	Character
Comma	`,`
Tab	`\t`
Pipe	`\|`

This reader also supports text qualifiers that are used to handle cells that contain the delimiter character. The quote ('"') character is the default text qualifier, but it can be changed to any single character.

Write delimited files

The atlas.io.core.CsvWriter writes an array of objects as a delimited string. Any single character can be used as a delimiter or a text qualifier. The default delimiter is comma (',') and the default text qualifier is the quote ('"') character.

Follow the steps to use this class:

Create an instance of the class and optionally set a custom delimiter or text qualifier.
Write data to the class using the write function or the writeRow function. For the write function, pass a two-dimensional array of objects representing multiple rows and cells. To use the writeRow function, pass an array of objects representing a row of data with multiple columns.
Call the toString function to retrieve the delimited string.
Optionally, call the clear method to make the writer reusable and reduce its resource allocation, or call the delete method to dispose of the writer instance.

Note

The number of columns written will be constrained to the number of cells in the first row of the data passed to the writer.

Read XML files

The atlas.io.core.SimpleXmlReader class is faster at parsing XML files than DOMParser. However, the atlas.io.core.SimpleXmlReader class requires XML files to be well formatted. XML files that aren't well formatted, for example missing closing tags, may result in an error.

The following code demonstrates how to use the SimpleXmlReader class to parse an XML string into a JSON object and serialize it into a desired format.

//Create an instance of the SimpleXmlReader and parse an XML string into a JSON object.
var xmlDoc = new atlas.io.core.SimpleXmlReader().parse(xmlStringToParse);

//Verify that the root XML tag name of the document is the file type your code is designed to parse.
if (xmlDoc && xmlDoc.root && xmlDoc.root.tagName && xmlDoc.root.tagName === '<Your desired root XML tag name>') {

    var node = xmlDoc.root;

    //Loop through the child node tree to navigate through the parsed XML object.
    for (var i = 0, len = node.childNodes.length; i < len; i++) {
        childNode = node.childNodes[i];

        switch (childNode.tagName) {
            //Look for tag names, parse and serialized as desired.
        }
    }
}

Write XML files

The atlas.io.core.SimpleXmlWriter class writes well-formatted XML in a memory efficient way.

The following code demonstrates how to use the SimpleXmlWriter class to generate a well-formatted XML string.

//Create an instance of the SimpleXmlWriter class.
var writer = new atlas.io.core.SimpleXmlWriter();

//Start writing the document. All write functions return a reference to the writer, making it easy to chain the function calls to reduce the code size.
writer.writeStartDocument(true)
    //Specify the root XML tag name, in this case 'root'
    .writeStartElement('root', {
        //Attributes to add to the root XML tag.
        'version': '1.0',
        'xmlns': 'http://www.example.com',
         //Example of a namespace.
        'xmlns:abc': 'http://www.example.com/abc'
    });

//Start writing an element that has the namespace abc and add other XML elements as children.
writer.writeStartElement('abc:parent');

//Write a simple XML element like <title>Azure Maps is awesome!</title>
writer.writeElement('title', 'Azure Maps is awesome!');

//Close the element that we have been writing children to.
writer.writeEndElement();

//Finish writing the document by closing the root tag and the document.
writer.writeEndElement().writeEndDocument();

//Get the generated XML string from the writer.
var xmlString = writer.toString();

The generated XML from the above code would look like the following.

<?xml version="1.0" encoding="UTF-8"?>
<root version="1.0" xmlns="http://www.example.com" xmlns:abc="http://www.example.com/abc">
    <abc:parent>
        <title>Azure Maps is awesome!</title>
    </abc:parent>
</root>