How to write a file to Android filesystem using Adobe AIR

The other night I ended up banging my head against the wall wondering why I couldn't write a file (or even create a directory for that matter) to my Samsung Galaxy S filesystem using a Flex application running on Adobe AIR.


The code was simple enough and apparently the code should have worked on my mobile without any additional changes.  However, when things didn't work I was left stumped.


The code:

 private function saveFile() : void {  
      var myFile:File = File.documentsDirectory.resolvePath("Test Folder/test.txt");  
      var fs:FileStream = new FileStream();  
      fs.open(myFile,FileMode.WRITE);  
      fs.writeUTFBytes("blah blah");  
      fs.close();  
  }  

Using the code above nothing was written to the filesystem.  WTF?? I tried to figure out the paths of the special directories to see if was trying to write the file in a weird directory somewhere.  


The debug code below:

 debugText += File.applicationDirectory.nativePath + "\n";  
 debugText += File.applicationStorageDirectory.nativePath + "\n";  
 debugText += File.documentsDirectory.nativePath + "\n";  
 debugText += File.userDirectory.nativePath + "\n";  

resulted in the following output:

    /data/data/air.TestApp.debug/TestApp.debug/Local Store
   /mnt/sdcard
   /mnt/sdcard
   /

So it should have correctly written to the internal SD card on my Galaxy S but there wasn't a file or directory in sight.  The next thing I tried to do was create a directory using the code below, while checking for any errors.

 var file:File = File.documentsDirectory.resolvePath("Test Folder");  
 try {  
      file.createDirectory();  
 } catch (e:Error) {  
      debugText += e.message;  
 }  

This resulted in "error #3001" which means "File or directory access denied."
http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/runtimeErrors.html

Why was access being denied since the "/mnt/sdcard/" folder had read and write access? The answer lied in the security permissions for an android application as described on the android developer guide.
http://developer.android.com/guide/topics/security/security.html

By default, an android application doesn't have permission to write to external storage so in order to write to my Samsung Galaxy S's internal SD card ("/mnt/sdcard/") I needed to set the WRITE_EXTERNAL_STORAGE permission to my applications XML descriptor file ("TestApp-app.xml").

 <android>  
   <manifestAdditions><![CDATA[  
      <manifest>  
           <!-- See the Adobe AIR documentation for more information about setting    Google Android permissions -->  
           ...  
         ...  
           <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>  
         ...  
         ...  
      </manifest>  
 ]]></manifestAdditions>  
  </android>  

After adding this line everything worked like a charm.  Such relief.