Develop Reference

When to call SELinux.restorecon()?

The Android open source code contains code such as this. Can anyone explain why and under which circumstances it is necessary to do SELinux.restorecon(File)?
// If Encrypted file systems is enabled or disabled, this call will return the
// correct directory.
mBaseStateDir = new File(Environment.getSecureDataDirectory(), "backup");
mBaseStateDir.mkdirs();
if (!SELinux.restorecon(mBaseStateDir)) {
Slog.e(TAG, "SELinux restorecon failed on " + mBaseStateDir);
}
Can we do it too often?

SELinux restorecon is used to restore the default security contexts on some files/dir.
The thing (which is not obvious) is that we also use it to apply context changes, and/or to ensure that the file contexts are as you expect.
Usually, we define/override default security contexts using semanage-fcontext, and then run restorecon to relabel files according to these new defaults.
Here the code tries to relabel mBaseStateDir but fails to do so. It could be that :
there is no policy supporting the default context associated to that directory (given it's path)
mBaseStateDir is mislabeled (context type is not apllicable)
user/process running that code doesn't have the sufficient permission to relabel the directory (even it just created it).

How to clear cache in osmdroid?

The code below is not working.
mapView.getTileProvider().clearTileCache();
Is there another way to do it?

I found a solution to my question. After going through the source in order to find out why the cache is not cleared after executing
mapView.getTileProvider().clearTileCache();
i found two functions in:
org.osmdroid.tileprovider.modules.SqlTileWriter.java
Functions:
/* purges and deletes everything from the cache database */
public boolean purgeCache(){..}
and
/* purges and deletes all tiles from the given tile source name from the cache database */
public boolean purgeCache(String mTileSourceName){..}
Example to delete cache for specific tile source:
SqlTileWriter sqlTileWriter = new SqlTileWriter();
boolean isCleared = sqlTileWriter.purgeCache(mapView.getTileProvider().getTileSource().name());

Appcelerator reset app to initial state

I am working on an app with Appcelerator.
Appcelerator uses nearly the same kind of require.js like node.js does.
Now I want to implement a feature that logges out the current user and does not leave any trace.
The most simple way would be to restart the app, but Appcelerator and especially Apple does not support this.
So i have to open the login window and clean all the data that leaves a trace to the old user.
The easiest way would be to dereference one of the main nodes in the require chain leaving all the data dereferenced and garbage collected.
I know there is a way (as mentioned here) to do that in node:
/**
* Removes a module from the cache
*/
function purgeCache(moduleName) {
// Traverse the cache looking for the files
// loaded by the specified module name
searchCache(moduleName, function (mod) {
delete require.cache[mod.id];
});
// Remove cached paths to the module.
// Thanks to #bentael for pointing this out.
Object.keys(module.constructor._pathCache).forEach(function(cacheKey) {
if (cacheKey.indexOf(moduleName)>0) {
delete module.constructor._pathCache[cacheKey];
}
});
};
/**
* Traverses the cache to search for all the cached
* files of the specified module name
*/
function searchCache(moduleName, callback) {
// Resolve the module identified by the specified name
var mod = require.resolve(moduleName);
// Check if the module has been resolved and found within
// the cache
if (mod && ((mod = require.cache[mod]) !== undefined)) {
// Recursively go over the results
(function traverse(mod) {
// Go over each of the module's children and
// traverse them
mod.children.forEach(function (child) {
traverse(child);
});
// Call the specified callback providing the
// found cached module
callback(mod);
}(mod));
}
};
So I tried to read out the require-cache in Appcelerator with:console.log(require, "-" ,require.cache); with an output like: <KrollCallback: 0x79f6fe50> - <null>
So now my questions:
Is there a way to reach the require-cache in Appcelerator?
Do you know a way to clean up a big Appcelerator-App?
Since it is possible to wirte native Modules for Appcelerator:
Do you know a way to clean up a big Android App?
Do you know a way to clean up a big iOS App?
Thank you very much

Android Volley - Some disc cached images dont get displayed?

I've run into this weird error, where some images get cached as usual and some don't, any idea why?
Both images do get displayed and memory cached just fine, but when offline some display error image.
For example, this works fine:
http://cs4381.vk.me/u73742951/a_58a41ac2.jpg
However, this does not: http://upload.wikimedia.org/wikipedia/commons/thumb/d/d7/Android_robot.svg/220px-Android_robot.svg.png
Both work fine displaying and memcaching but the second doesn't get displayed from disk cache, although I think I see it being saved, as app says it has 12kB cache in the system settings
Edit
I checked out a clean copy of Volley and it does the same thing. Its definatelly a bug...
From what Ive found out its that images do get cached, but Bitmap cachedBitmap = mCache.getBitmap(cacheKey); always returns null, so the cache says it doesnt have the bitmaps and then proceedes to download it again, and fail when offline, weird

The reason you're not getting any hits is because the default behavior in Volley for disk caching is dependent on the HTTP headers of the element you're requesting (in your case, an image).
Check the volley logs and see if you get the "cache-hit-expired" message - that means that the image was cached but it's TTL is expired as far as the default disk cache is concerned.
If you want the default settings to work, the images must have a Cache-Control header like max-age=??? where the question marks indicate enough seconds from the time it was downloaded.
If you want to change the default behavior, I'm not sure, but I think you have to edit the code a bit.
Look at the CacheDispatcher class in the Volley source.
Hope that helps.

A quick and dirty way:
private static class NoExpireDiskBasedCache extends DiskBasedCache
{
public NoExpireDiskBasedCache(File rootDirectory, int maxCacheSizeInBytes)
{
super(rootDirectory, maxCacheSizeInBytes);
}
public NoExpireDiskBasedCache(File rootDirectory)
{
super(rootDirectory);
}
#Override
public synchronized void put(String key, Entry entry)
{
if (entry != null)
{
entry.etag = null;
entry.softTtl = Long.MAX_VALUE;
entry.ttl = Long.MAX_VALUE;
}
super.put(key, entry);
}
}

Android local variable get's lost when using camera intent

I'm dealing with a random problem which related to camera usage. Before I call camera intent - I generate UUID to store file with this name. I store this UUID in private variable like so:
private String requestedFileName;
When camera done - I'm processing this file, looks something like this:
public void onPictureTaken(int index)
{
//First of all - remember picture in database for reference.
FileData.InsertFile(mContext, UUID.fromString(requestedFileName));
//Reduce taken picture if needed, otherwise let it be original.
if (Preferences.getImageSize(mContext) > 0)
{
Imaging.scaleImageFile(mContext, requestedFileName, Preferences.getImageSize(mContext));
}
I see users report issue exception that boils down to requestedFileName == null when onPictureTaken called
Caused by: java.lang.NullPointerException
at java.util.UUID.fromString(UUID.java:210)
at com.idatt.views.FourImagesView.onPictureTaken(FourImagesView.java:151)
at com.idatt.views.TrailerUnitView.onPictureTaken(TrailerUnitView.java:233)
Everything works good on my phone (Nexus S) and in emulator. But users report this exception and I'm not sure why this is happening..

I've seen this happen on the Nexus phones, and some others. If you use DDMS to watch what is going on, I bet you'll see that your process is actually being terminated and then restarted. Thus your local state is being lost. You need to persist it, since Android can basically kill your process and restart it whenever it wants if you switch to a new task (and most of the camera capture intents set the NEWTASK flag).
If your class is an Activity you can use onSaveInstanceState() to store your filename, then read it back out of the Bundle you get in onCreate().
If you are not an Activity you can use the SharedPreferences store as a temporary place to save the filename:
private static void saveTempFileName(Context context, String filename) {
SharedPreferences settings = context.getSharedPreferences("whatever", 0);
SharedPreferences.Editor editor = settings.edit();
editor.putString("com.yourstuff.whatever", filename);
editor.commit();
}

As #jeffamaphone noted you are probably having issues with app configuration changes. Application configuration change happens when something happens that affects the runtime environment of your app. Most notably this are: orientation change or keyboard hide/show.
Try this: start your app, invoke the Camera app (via your app action), change orientation, return to your app (via appropriate action). Does this sequence produce the error? Then you have issues with configuration change - when orientation changes usually (depending on your app settings) Android system restarts (kills and creates new instance) your Activity, which probably creates all new Views (without UUID set).
See handling configuration changes.