File IO: Difference between revisions
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fstream.init(file, 1, 0, false); | fstream.init(file, 1, 0, false); | ||
sstream.init(fstream); | sstream.init(fstream); | ||
var bytes = sstream.available(); | var bytes = sstream.available(); | ||
data += sstream.read(bytes); | data += sstream.read(bytes); | ||
sstream.close(); | sstream.close(); |
Revision as of 03:08, 22 January 2005
This page is part of the extension development documentation project.
Ask your questions in MozillaZine Forums. Also try browsing example code.
Note: development documentation is in process of being moved to Mozilla Development Center (MDC).
This article describes local file input/output in Javascript.
You access the filesystem using Mozilla XPCOM components. The list of components used for local IO is available at XulPlanet.com.
Available libraries
There are a few JavaScript wrappers for IO XPCOM components. See JSLib and MonkeeSage's IO module. MonkeeSage's module is much smaller and very easy to use (simple examples are included in the module).
Creating a file object ("opening" files)
var file = Components.classes["@mozilla.org/file/local;1"]. createInstance(Components.interfaces.nsILocalFile); file.initWithPath("/home");
Note: the path passed to initWithPath() should be in 'native' form (eg. "C:\\Windows"). If you need to use file:// URIs as initializers, see below.
Also note, that initWithPath() / initWithFile() functions don't throw an exception if specified file does not exist. An exception is thrown when methods that require the file existance are called, e.g. isDirectory(), moveTo() etc.
Getting special files
// get profile directory var file = Components.classes["@mozilla.org/file/directory_service;1"]. getService(Components.interfaces.nsIProperties). get("ProfD", Components.interfaces.nsIFile);
Here are some strings you can put in place of "ProfD" (stolen from MonkeeSage's IO module comments)
String | Meaning |
---|---|
ProfD | profile directory |
DefProfRt | user (e.g., /root/.mozilla) |
UChrm | %profile%/chrome |
DefRt | %installation%/defaults |
PrfDef | %installation%/defaults/pref |
ProfDefNoLoc | %installation%/defaults/profile |
APlugns | %installation%/plugins |
AChrom | %installation%/chrome |
ComsD | %installation%/components |
CurProcD | installation (usually) |
Home | OS root (e.g., /root) |
TmpD | OS tmp (e.g., /tmp) |
Look in the Source for other strings available: [1] [2].
Getting an nsILocalFile from user using nsIFilePicker
See Dev : nsIFilePicker.
nsIFile and path strings
You can use nsIFile::path to get platform-specific path string, eg. C:\Windows\System32 or /usr/share. If you want to get a file:// URL of a file or an nsIFile from file:// URL, you need to use nsIFileProtocolHandler:
// file is nsIFile var ios = Components.classes["@mozilla.org/network/io-service;1"]. getService(Components.interfaces.nsIIOService); var fileHandler = ios.getProtocolHandler("file"). QueryInterface(Components.interfaces.nsIFileProtocolHandler); var URL = fileHandler.getURLSpecFromFile(file);
To load from file://, http://, chrome://, resource:// and other URLs directly, use nsIChannel. Until a knowledge base article on nsIChannel is written, here's an example for you.
Note: do not use nsILocalFile::persistentDescriptor to get the file path!
Also note that generally you don't need to use nsIFile::path. Use nsIFile directly wherever possible. An example below shows how you should save a path in user prefs.
Storing nsILocalFile in preferences
The following two snippets show the right way to store a file path in user preferences (more about preferences in Mozilla):
Absolute path (nsILocalFile)
To store arbitrary path in user preferences, use this code.
// |file| is nsILocalFile // 1. Write path to prefs var prefs = Components.classes["@mozilla.org/preferences-service;1"]. getService(Components.interfaces.nsIPrefService). getBranch("extensions.myext."); prefs.setComplexValue("filename", Components.interfaces.nsILocalFile, file); // 2. Read path from prefs var file = prefs.getComplexValue("filename", Components.interfaces.nsILocalFile);
Relative path (nsIRelativeFilePref)
To store paths relative to one of the predefined folders listed above, for example file relative to profile folder, use the following code:
// 1. Write to prefs var relFile = Components.classes["@mozilla.org/pref-relativefile;1"] .createInstance(Components.interfaces.nsIRelativeFilePref); relFile.relativeToKey = "ProfD"; // or any other string listed above relFile.file = file; // |file| is nsILocalFile prefs.setComplexValue("filename", Components.interfaces.nsIRelativeFilePref, relFile); // 2. Read from prefs var value = prefs.getComplexValue("filename", Components.interfaces.nsIRelativeFilePref); // |value.file| is the file.
Get a file in given directory
Assume, file is an nsIFile pointing to some directory (f.e. a user profile directory). You can use this code: file.append("myfile.txt"); - to make file point to myfile.txt inside that directory.
Notes: avoid using dir.path+"\\"+"myfile.txt", as it is not cross-platform code. Using something like ((path.search(/\\/) != -1) ? path + "\\" : path + "/") + "myfile.txt"; is possible, but the nsIFile::append() is much easier to read and is guaranteed to work on all platforms Mozilla itself works.
Enumerating files in given directory
The snippet below makes an array of |nsIFile|s corresponding to sub-files/subdirectories of given directory. You can tell files from folders by calling nsIFile::isDirectory() and nsIFile::isFile() methods on each |entry|.
// file is the given directory (nsIFile) var entries = file.directoryEntries; var array = []; while(entries.hasMoreElements()) { var entry = entries.getNext(); entry.QueryInterface(Components.interfaces.nsIFile); array.push(entry); }
Reading from a file
Simple
// |file| is nsIFile var data = ""; var fstream = Components.classes["@mozilla.org/network/file-input-stream;1"] .createInstance(Components.interfaces.nsIFileInputStream); var sstream = Components.classes["@mozilla.org/scriptableinputstream;1"] .createInstance(Components.interfaces.nsIScriptableInputStream); fstream.init(file, 1, 0, false); sstream.init(fstream); var bytes = sstream.available(); data += sstream.read(bytes); sstream.close(); fstream.close(); alert(data);
Line by line
// open an input stream from file var istream = Components.classes["@mozilla.org/network/file-input-stream;1"] .createInstance(Components.interfaces.nsIFileInputStream); istream.init(file, 0x01, 0444, 0); istream.QueryInterface(Components.interfaces.nsILineInputStream); // read lines into array var line = {}, lines = [], hasmore; do { hasmore = istream.readLine(line); lines.push(line.value); } while(hasmore); istream.close(); // do something with read data alert(lines);
Asynchronously
This will allow you to read a file without locking up the UI thread.
// |file| is nsIFile var ios = Components.classes["@mozilla.org/network/io-service;1"] .getService(Components.interfaces.nsIIOService); var fileURI = ios.newFileURI(file); var channel = ios.newChannelFromURI(fileURI); var observer = { onStreamComplete : function(aLoader, aContext, aStatus, aLength, aResult) { alert(aResult); } }; var sl = Components.classes["@mozilla.org/network/stream-loader;1"] .createInstance(Components.interfaces.nsIStreamLoader); sl.init(channel, observer, null);
Writing to a file
// file is nsIFile, data is a string var foStream = Components.classes["@mozilla.org/network/file-output-stream;1"] .createInstance(Components.interfaces.nsIFileOutputStream); // use 0x02 | 0x10 to open file for appending. stream.init(file, 0x02 | 0x08 | 0x20, 0664, 0); // write, create, truncate stream.write(data, data.length); stream.close();
Flags parameter to the nsIFileOutputStream::init() function (For more information refer to prio.h file).
flags: The file status flags. It is a bitwise OR of the following bit flags (only one of the first three flags below may be used):
Name | Value | Description |
---|---|---|
PR_RDONLY | 0x01 | Open for reading only. |
PR_WRONLY | 0x02 | Open for writing only. |
PR_RDWR | 0x04 | Open for reading and writing. |
PR_CREATE_FILE | 0x08 |
If the file does not exist, the file is created. If the file exists, this flag has no effect. |
PR_APPEND | 0x10 |
The file pointer is set to the end of the file prior to each write. |
PR_TRUNCATE | 0x20 |
If the file exists, its length is truncated to 0. |
PR_SYNC | 0x40 |
If set, each write will wait for both the file data and file status to be physically updated. |
PR_EXCL | 0x80 |
With PR_CREATE_FILE, if the file does not exist, the file is created. If the file already exists, no action and NULL is returned. |