Reading and writing a ByteArray

Any meaningful data stream is organized into a format that you can analyze to find the information that you want. A record in a simple employee file, for example, would probably include an ID number, a name, an address, a phone number, and so on. An MP3 audio file contains an ID3 tag that identifies the title, author, album, publishing date, and genre of the file that’s being downloaded. The format allows you to know the order in which to expect the data on the data stream. It allows you to read the byte stream intelligently.

The ByteArray class includes several methods that make it easier to read from and write to a data stream. Some of these methods include readBytes() and writeBytes(), readInt() and writeInt(), readFloat() and writeFloat(), readObject() and writeObject(), and readUTFBytes() and writeUTFBytes(). These methods enable you to read data from the data stream into variables of specific data types and write from specific data types directly to the binary data stream.

For example, the following code reads a simple array of strings and floating-point numbers and writes each element to a ByteArray. The organization of the array allows the code to call the appropriate ByteArray methods (writeUTFBytes() and writeFloat()) to write the data. The repeating data pattern makes it possible to read the array with a loop.

// The following example reads a simple Array (groceries), made up of strings 
// and floating-point numbers, and writes it to a ByteArray. 
 
import flash.utils.ByteArray; 
 
// define the grocery list Array 
var groceries:Array = ["milk", 4.50, "soup", 1.79, "eggs", 3.19, "bread" , 2.35] 
// define the ByteArray 
var bytes:ByteArray = new ByteArray(); 
// for each item in the array 
for (var i:int = 0; i < groceries.length; i++) { 
        bytes.writeUTFBytes(groceries[i++]); //write the string and position to the next item 
        bytes.writeFloat(groceries[i]);    // write the float 
        trace("bytes.position is: " + bytes.position);    //display the position in ByteArray 
} 
trace("bytes length is: " +  bytes.length);    // display the length 

The position property stores the current position of the pointer that indexes the ByteArray during reading or writing. The initial value of the position property is 0 (zero) as shown in the following code:

var bytes:ByteArray = new ByteArray(); 
trace("bytes.position is initially: " + bytes.position);     // 0 

When you read from or write to a ByteArray, the method that you use updates the position property to point to the location immediately following the last byte that was read or written. For example, the following code writes a string to a ByteArray and afterward the position property points to the byte immediately following the string in the ByteArray:

var bytes:ByteArray = new ByteArray(); 
trace("bytes.position is initially: " + bytes.position);     // 0 
bytes.writeUTFBytes("Hello World!"); 
trace("bytes.position is now: " + bytes.position);    // 12 

Likewise, a read operation increments the position property by the number of bytes read.

var bytes:ByteArray = new ByteArray(); 
 
trace("bytes.position is initially: " + bytes.position);     // 0 
bytes.writeUTFBytes("Hello World!"); 
trace("bytes.position is now: " + bytes.position);    // 12 
bytes.position = 0; 
trace("The first 6 bytes are: " + (bytes.readUTFBytes(6)));    //Hello  
trace("And the next 6 bytes are: " + (bytes.readUTFBytes(6)));    // World! 

Notice that you can set the position property to a specific location in the ByteArray to read or write at that offset.

The length and bytesAvailable properties tell you how long a ByteArray is and how many bytes remain in it from the current position to the end. The following example illustrates how you can use these properties. The example writes a String of text to the ByteArray and then reads the ByteArray one byte at a time until it encounters either the character “a” or the end (bytesAvailable <= 0).

var bytes:ByteArray = new ByteArray(); 
var text:String = "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Vivamus etc."; 
 
bytes.writeUTFBytes(text); // write the text to the ByteArray 
trace("The length of the ByteArray is: " + bytes.length);    // 70 
bytes.position = 0; // reset position 
while (bytes.bytesAvailable > 0 && (bytes.readUTFBytes(1) != 'a')) { 
    //read to letter a or end of bytes 
} 
if (bytes.position < bytes.bytesAvailable) { 
    trace("Found the letter a; position is: " + bytes.position);     // 23 
    trace("and the number of bytes available is: " + bytes.bytesAvailable);    // 47 
} 

Computers can differ in how they store multibyte numbers, that is, numbers that require more than 1 byte of memory to store them. An integer, for example, can take 4 bytes, or 32 bits, of memory. Some computers store the most significant byte of the number first, in the lowest memory address, and others store the least significant byte first. This attribute of a computer, or of byte ordering, is referred to as being either big endian (most significant byte first) or little endian (least significant byte first). For example, the number 0x31323334 would be stored as follows for big endian and little endian byte ordering, where a0 represents the lowest memory address of the 4 bytes and a3 represents the highest:

The endian property of the ByteArray class allows you to denote this byte order for multibyte numbers that you are processing. The acceptable values for this property are either "bigEndian" or "littleEndian" and the Endian class defines the constants BIG_ENDIAN and LITTLE_ENDIAN for setting the endian property with these strings.

The compress() method allows you to compress a ByteArray in accordance with a compression algorithm that you specify as a parameter. The uncompress() method allows you to uncompress a compressed ByteArray in accordance with a compression algorithm. After calling compress() and uncompress(), the length of the byte array is set to the new length and the position property is set to the end.

The CompressionAlgorithm class defines constants that you can use to specify the compression algorithm. AIR supports both the deflate and zlib algorithms. The deflate compression algorithm is used in several compression formats, such as zlib, gzip, and some zip implementations. The zlib compressed data format is described at http://www.ietf.org/rfc/rfc1950.txt and the deflate compression algorithm is described at http://www.ietf.org/rfc/rfc1951.txt.

The following example compresses a ByteArray called bytes using the deflate algorithm:

bytes.compress(CompressionAlgorithm.DEFLATE);

The following example uncompresses a compressed ByteArray using the deflate algorithm:

bytes.uncompress(CompressionAlgorithm.DEFLATE);

The readObject() and writeObject() methods read an object from and write an object to a ByteArray, encoded in serialized Action Message Format (AMF). AMF is a proprietary message protocol created by Adobe and used by various ActionScript 3.0 classes, including Netstream, NetConnection, NetStream, LocalConnection, and Shared Objects.

A one-byte type marker describes the type of the encoded data that follows. AMF uses the following 13 data types:

value-type = undefined-marker | null-marker | false-marker | true-marker | integer-type |  
    double-type | string-type | xml-doc-type | date-type | array-type | object-type | 
    xml-type | byte-array-type

The encoded data follows the type marker unless the marker represents a single possible value, such as null or true or false, in which case nothing else is encoded.

There are two versions of AMF: AMF0 and AMF3. AMF 0 supports sending complex objects by reference and allows endpoints to restore object relationships. AMF 3 improves AMF 0 by sending object traits and strings by reference, in addition to object references, and by supporting new data types that were introduced in ActionScript 3.0. The ByteArray.objectEcoding property specifies the version of AMF that is used to encode the object data. The flash.net.ObjectEncoding class defines constants for specifying the AMF version: ObjectEncoding.AMF0 and ObjectEncoding.AMF3.

The following example calls writeObject() to write an XML object to a ByteArray, which it then compresses using the Deflate algorithm and writes to the order file on the desktop. The example uses a label to display the message “Wrote order file to desktop!” in the AIR window when it is finished.

<?xml version="1.0" encoding="utf-8"?> 
<mx:WindowedApplication xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute"  
    creationComplete="init();"> 
<mx:Script> 
    <![CDATA[ 

import flash.filesystem.*; 
import flash.utils.ByteArray; 

import mx.controls.Label; 
private function init():void { 

    var bytes:ByteArray = new ByteArray(); 
    var myLabel:Label = new Label(); 
    myLabel.move(150, 150); 
    myLabel.width = 200; 
    addChild(myLabel); 
 
    var myXML:XML =  
     <order> 
         <item id='1'> 
             <menuName>burger</menuName> 
             <price>3.95</price> 
         </item> 
         <item id='2'> 
             <menuName>fries</menuName> 
             <price>1.45</price> 
         </item> 
       </order> 
 
    // Write XML object to ByteArray 
    bytes.writeObject(myXML); 
    bytes.position = 0;        //reset position to beginning 
    bytes.compress(CompressionAlgorithm.DEFLATE);    // compress ByteArray 
    outFile("order", bytes); 
    myLabel.text = "Wrote order file to desktop!"; 

} // end of init() 

function outFile(fileName:String, data:ByteArray):void { 
    var outFile:File = File.desktopDirectory; // dest folder is desktop 
    outFile = outFile.resolvePath(fileName);  // name of file to write 
    var outStream:FileStream = new FileStream(); 
    // open output file stream in WRITE mode 
    outStream.open(outFile, FileMode.WRITE); 
    // write out the file 
    outStream.writeBytes(data, 0, data.length); 
    // close it 
    outStream.close(); 
} 

    ]]> 
</mx:Script>             
</mx:WindowedApplication>

The readObject() method reads an object in serialized AMF from a ByteArray and stores it in an object of the specified type. The following example reads the order file from the desktop into a ByteArray (inBytes), uncompresses it, and calls readObject() to store it in the XML object orderXML. The example uses a for each() loop construct to add each node to a text area for display. The example also displays the value of the objectEncoding property along with a header for the contents of the order file.

<?xml version="1.0" encoding="utf-8"?> 
<mx:WindowedApplication xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" 
    creationComplete="init();"> 
<mx:Script> 
        <![CDATA[ 

import flash.filesystem.*; 
import flash.utils.ByteArray; 

import mx.controls.TextArea; 
 
private function init():void { 

    var inBytes:ByteArray = new ByteArray(); 
    // define text area for displaying XML content 
    var myTxt:TextArea = new TextArea(); 
    myTxt.width = 550; 
    myTxt.height = 400; 
    addChild(myTxt); 
    //display objectEncoding and file heading 
    myTxt.text = "Object encoding is: " + inBytes.objectEncoding + "\n\n" + "order file: \n\n"; 
    readFile("order", inBytes); 
 
    inBytes.position = 0; // reset position to beginning 
    inBytes.uncompress(CompressionAlgorithm.DEFLATE); 
    inBytes.position = 0;    //reset position to beginning 
    // read XML Object 
    var orderXML:XML = inBytes.readObject(); 
 
    //for each node in orderXML 
    for each(var child:XML in orderXML) { 
        // append child node to text area 
        myTxt.text += child + "\n"; 
    } 

} // end of init() 

// read specified file into byte array 
function readFile(fileName:String, data:ByteArray) { 
    var inFile:File = File.desktopDirectory; // source folder is desktop 
    inFile = inFile.resolvePath(fileName);  // name of file to read 
    var inStream:FileStream = new FileStream(); 
    inStream.open(inFile, FileMode.READ); 
    inStream.readBytes(data, 0, data.length); 
    inStream.close(); 
} 

        ]]> 
    </mx:Script> 
     
</mx:WindowedApplication>