Every file is a set of bytes on a disk. In ActionScript, the
data from a file can always be represented as a ByteArray. For example,
the following code reads the data from a file into a ByteArray object
named
bytes
:
var myFile:File = File.documentsDirectory.resolvePath("AIR Test/test.txt");
var myFileStream:FileStream = new FileStream();
myFileStream.addEventListener(Event.COMPLETE, completeHandler);
myFileStream.openAsync(myFile, FileMode.READ);
var bytes:ByteArray = new ByteArray();
function completeHandler(event:Event):void
{
myFileStream.readBytes(bytes, 0, myFileStream.bytesAvailable);
}
Similarly, the following code writes data from a ByteArray named
bytes
to
a file:
var myFile:File = File.documentsDirectory.resolvePath("AIR Test/test.txt");
var myFileStream:FileStream = new FileStream();
myFileStream.open(myFile, FileMode.WRITE);
myFileStream.writeBytes(bytes, 0, bytes.length);
However, often you do not want to store the data in an ActionScript ByteArray
object. And often the data file is in a specified file format.
For example, the data in the file may be in a text file format,
and you may want to represent such data in a String object.
For this reason, the FileStream class includes read and write
methods for reading and writing data to and from types other than
ByteArray objects. For example, the
readMultiByte()
method
lets you read data from a file and store it to a string, as in the
following code:
var myFile:File = File.documentsDirectory.resolvePath("AIR Test/test.txt");
var myFileStream:FileStream = new FileStream();
myFileStream.addEventListener(Event.COMPLETE, completed);
myFileStream.openAsync(myFile, FileMode.READ);
var str:String = "";
function completeHandler(event:Event):void
{
str = myFileStream.readMultiByte(myFileStream.bytesAvailable, "iso-8859-1");
}
The second parameter of the
readMultiByte()
method
specifies the text format that ActionScript uses to interpret the
data ("iso-8859-1" in the example). Adobe AIR supports common character
set encodings (see
Supported character sets
).
The FileStream class also includes the
readUTFBytes()
method,
which reads data from the read buffer into a string using the UTF-8
character set. Since characters in the UTF-8 character set are of
variable length, do not use
readUTFBytes()
in a
method that responds to the
progress
event, since
the data at the end of the read buffer may represent an incomplete
character. (This is also true when using the
readMultiByte()
method
with a variable-length character encoding.) For this reason, read
the entire set of data when the FileStream object dispatches the
complete
event.
There are also similar write methods,
writeMultiByte()
and
writeUTFBytes()
,
for working with String objects and text files.
The
readUTF()
and the
writeUTF()
methods
(not to be confused with
readUTFBytes()
and
writeUTFBytes()
)
also read and write the text data to a file, but they assume that
the text data is preceded by data specifying the length of the text
data, which is not a common practice in standard text files.
Some UTF-encoded text files begin with a "UTF-BOM" (byte order
mark) character that defines the endianness as well as the encoding
format (such as UTF-16 or UTF-32).
For an example of reading and writing to a text file, see
Example: Reading an XML file into an XML object
.
The
readObject()
and
writeObject()
are
convenient ways to store and retrieve data for complex ActionScript
objects. The data is encoded in AMF (ActionScript Message Format).
Adobe AIR, Flash Player, Flash Media Server, and Flex Data Services
include APIs for working with data in this format.
There are some other read and write methods (such as
readDouble()
and
writeDouble()
).
However, if you use these, make sure that the file format matches
the formats of the data defined by these methods.
File formats are often more complex than simple text formats.
For example, an MP3 file includes compressed data that can only
be interpreted with the decompression and decoding algorithms specific
to MP3 files. MP3 files also may include ID3 tags that contain meta
tag information about the file (such as the title and artist for
a song). There are multiple versions of the ID3 format, but the simplest
(ID3 version 1) is discussed in the
Example: Reading and writing data with random access
section.
Other files formats (for images, databases, application documents,
and so on) have different structures, and to work with their data
in ActionScript, you must understand how the data is structured.
