Develop Reference

Is it possible that system remove data from ExternalCacheDir of application?

My Android app uses ExternalCacheDir to save some files downloaded from my server which are used to show to user inside app (images, videos etc.).
In Google Play Store there's 3 comments with low grade that says after turning off application or device files are no longer available in app. All 3 users/devices with Android 7.0. But I've tested app in few Android 7.0 phones and few with other versions and nothing like this happend. Also there are many high grades and other people doesn't have that problem.
Is it possible that system at some point removes them? E.g. in case of low disk (external storage) space?
I'm using ExternalCacheDir not just ExternalStorageDir to hide files a little bit from user (files are also saved with shadowed random names).

Is it possible that system at some point removes them?
Yes, where by "system" it could be the OS itself or, more likely, stuff added into the device by the manufacturer. Also, users can remove them and other apps can remove them.
I'm using ExternalCacheDir not just ExternalStorageDir to hide files a little bit from user (files are also saved with shadowed random names).
Anything on external storage is visible to the user and to other apps.
If you specifically do not want users to have access to the files, use internal storage.

How does the multi-user feature work in terms of paths on Android?

background
Starting with version 4.2 , Android supports multi-user (link here and here).
Each user has its apps and their private data is visible just for the user.
The question
How does the encapsulation of the data per user work in terms of paths and accessing files?
I mean, what will be the paths per each user for:
the private, internal storage.
the emulated external storage (built in external storage)
the "real" external storage (sd cards)
?
I guess users can see the data stored on the sd cards that belong to other users, but what about the emulated external storage? And can they also write other users' files or just read them?
Does each user get its own special path automatically? or should the developer handle this?
If the developer needs to handle it, what should be used as the ID of the user?
The documentation says:
No matter which of these APIs you use to save data for a given user,
the data will not be accessible while running as a different user.
But that's all assuming you use the APIs for your own path. Could apps somehow bypass this by going to other paths?
What can an app query about each installation of itself on the same device? Can they get the size of apps of other users? Can they even get the list of apps of other users?
Does multiple installation of the same app also take multiple size?

what will be the paths per each user
If you care, you're doing it wrong. Use the Android SDK APIs for determining base directories, and work from there. So, for example:
getFilesDir() will return the right location for internal storage for the current user
getExternalFilesDir() and the methods on Environment will return the right locations for external storage for the current user
I guess users can see the data stored on the sd cards that belong to other users
That is outside the bounds of the Android SDK, generally.
but what about the emulated external storage?
Each user gets their own space.
And can they also write other users' files or just read them?
Neither, barring bugs in the device.
Does each user get its own special path automatically?
Yes, if you are using the Android SDK APIs for determining base directories.
Could apps somehow bypass this by going to other paths?
No, because they will have neither read nor write access, barring bugs.
What can an app query about each installation of itself on the same device?
AFAIK, nothing. From the app's standpoint, the fact that there are several installations on one device is indistinguishable from being installed on several devices.
Can they get the size of apps of other users?
I do not know what you mean by "size of apps", sorry.
Can they even get the list of apps of other users?
That's a fine question. I have not experimented with PackageManager to see what it exposes when used by apps run by secondary users. In theory, it should only report things that are available to the current user, particularly given Android's restricted profiles.
Does multiple installation of the same app also take multiple size?
The APK and the pieces of it that are unpacked (e.g., DEX files) are shared, as I understand it.

How to prevent Android app installation if SDCard absent?

My code logic needs an SD card installed in the device. I have added a check for this case in the application's splash screen, but would like to inform users before they download/install this app. Is there a way to achieve this ?
Thanks !

There is no way to do this before the app installs, as the only way to limit such things is by using the <uses-feature> tag. However, that tag has no options for storage requirements. The best warning you can give is to prominently include it in your app description.
On the other hand, every device I've ever heard of an encountered has some form of external storage, be it a SD Card or inbuilt memory mounted as external storage. What you're doing by using the Splash Screen to check for the external storage is the best way to do this, as there is no other option.

There's no way to do that. Your app have to be installed to be able to check user's environment. You could try to to enforce SD card installation of your app, so if there's none Google Play might (not tested) simply not allow app installation at all, but it will not solve your problem as user will still do not know why. Solution is to clearly state in product description that SD card is mandatory. But note, that requiring SD card is risky as many devices does not have any while still offer external storage. My suggestion - just add note about storage requirements and let system deal with it.

I think it is NOT POSSIBLE . You are checking the sdcard on splash screen and prevent user for next process is the right solution or Use android:installLocation for install android application on sdcard.
Beginning with API Level 8, you can allow your application to be
installed on the external storage (for example, the device's SD card).
This is an optional feature you can declare for your application with
the android:installLocation manifest attribute.
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
android:installLocation="preferExternal"
... >
If you declare "preferExternal", you request that your application be
installed on the external storage, but the system does not guarantee
that your application will be installed on the external storage. If
the external storage is full, the system will install it on the
internal storage. The user can also move your application between the
two locations.
When your application is installed on the external storage:
There is no effect on the application performance so long as the
external storage is mounted on the device.
The .apk file is saved on the external storage, but all private user
data, databases, optimized .dex files, and extracted native code are
saved on the internal device memory.
The unique container in which your application is stored is encrypted
with a randomly generated key that can be decrypted only by the
device that originally installed it. Thus, an application installed
on an SD card works for only one device.
The user can move your application to the internal storage through
the system settings.
Look Here for more details .

Android: Download media or store in app?

I'm currently working on an app which downloads a zip file and extracts a bunch of tiny mp3 files into external storage. The zip file is about 2.5 meg.
I'm wondering if it would be a better idea to include the zip file in app resources, and then copy and extract it to the file system. This would prevent the headaches involved in something like a lost connection during the zip file download. I noticed some missing mp3s, and when I checked the zip file, the last 20 or so weren't it it. So, it looks like the system can successfully (an unfortunately) extract from an incomplete zip file.
The downside including the zip file in app resources is obviously that it increases the size of the app by 2.5 meg. I've specified "prefer external storage" in the manifest. This isn't supported before release 2.2, i.e. level 8, so I'm already excluding those phones, which probably is 20 or 30 percent of the total android phones. But I'm not entirely sure if that guarantees the app will be installed on the SD card.
It could be worth including the zip file in the app just to avoid the potential hassles in handling the download of the zip file; but if the size of the app makes it unappealing, then that's kind of a drawback - especially if the "prefer external" doesn't work on some devices.
Which is the better option? Ideas are welcome.

One of the highest selling Android Games, Gun Bros, does what you are thinking about: small app, that downloads the content on demand. This allows them to also serve ads while the content is downloading. In addition, by downloading the content you can make use of compression over the network thus reducing the total amount of data a user will actually have to use to get the whole application. I think in the end it is a personal preference, but I would lean towards downloading over the network.

Given the fact that there are some apps/games out in Android market which are in the 40+ MB range a 2 MB additional download doesn't seem that much to me. Anyway your users have to download it one way or another. Either in the apk or as a separate zip file.
Now Android market allows you to post updates to your application so your users can get new content through there.
Regarding prefer external storage, it is just that - a preference. If external storage is not available or a user is installing it on a 2.1 phone, then it will install the application to internal storage, as long as your applications minimum SDK level is 7 or lower.
But there are other applications I can think off that download content from sites, things like scripting for android (SL4A). If you want to update your game content that way, I would recommend creating a mechanism whereby your application can first discover the size of the download, then do the download. And once the download completes, verify the size against the size it should be. If the two don't match, then run the download again or something like that.

I'd recommend just including it in the apk file. Many apps on the market are like 5-10 MB, and games can be even bigger (I've seen games range anywhere from 1 MB to 40 MB), so adding 2,5 MB to your app shouldn't really be a problem, even if they don't support installing to sd-cards.
My phone for example have 2.1 and therefor can't install to the sd-card and I've so far never run out of space even after installing a bunch of 2-5 MB apps.
And no "prefer external storage" does not guarantee that it's installed on the sd-card, if the user doesn't have an sd-card for example, or if they choose themselves to move it to the internal storage.
Point is, adding 2,5 MB to your apk isn't anything anyone will really notice or care about, and you shouldn't need to either.

App to SD functionality on Android

I'm getting more and more requests about being able to use an App to SD feature that must be part of Android 2.2, but I really have no experience with it. I also don't know where to find any documentation regarding how to make my apps compatible with this feature. People have told me my apps cannot be moved to the SD card.
My mind is also telling me that this is a really bad idea for paid apps that don't have license protection of any kind.
Has anyone had any experience with this, know of any documentation, or have any tips regarding what would stop an app from being compatible with this feature?
Note: I do not use copy protection on my apps.

You can allow your app to be installed to the SD card using the android:installLocation manifest attribute. This can be set to preferExternal or auto, depending on whether you would like to recommend that it be installed on the SD card, or simply allow it. By default, applications cannot be installed to the SD card for backwards-compatibility reasons, so you must opt-in to this feature if you want your users to be able to use it.
From the documentation:
When your application is installed on the external storage:
There is no effect on the application performance so long as the external storage is mounted on the device.
The .apk file is saved on the external storage, but all private user data, databases, optimized .dex files, and extracted native code are saved on the internal device memory.
The unique container in which your application is stored is encrypted with a randomly generated key that can be decrypted only by the device that originally installed it. Thus, an application installed on an SD card works for only one device.
The user can move your application to the internal storage through the system settings.
Thus, you shouldn't worry about license protection too much; there is encryption built into the feature. You also generally shouldn't worry about license protection because any form of copy protection or DRM tends to be more harmful to honest users than to pirates. As long as someone is able to use your app, someone will be able to pirate it; it is well nigh impossible to create an unbreakable DRM scheme. If your app is already in internal storage, it's likely already pirated. Most users are honest, however, and will buy the app from the Market, so you won't really gain much from stopping piracy (most people who download pirated apps are those who don't have access to paid apps in the Market; it's still not available in many countries).
Anyhow, the upshot is that this should be about as secure as your app already is, and just allows users more flexibility in where to store their app. It's not enabled by default in case of bugs that cause applications that aren't expecting it to break, but it should be perfectly safe to enable.

Here you go.
And documented here.
The Android platform now allows
applications to request installation
onto the device's external storage
media (such as the SD card), as an
alternative to installation onto the
device's internal memory.
Application developers can express the
preferred installation location for
their applications by means of a new
attribute of in the
manifest file,
android:installLocation. The attribute
supports three values: "internalOnly",
"preferExternal", and"auto". At
install time, the system checks the
value of android:installLocation and
installs the application .apk
according to the preferred location,
if possible. If the application has
requested external installation, the
system installs it into a private,
encrypted partition in the external
media. Once an application .apk is
installed externally, the system lets
the user change the storage location
of the .apk and move it onto the
device's internal memory if needed
(and vice versa), through Manage
Applications in the user settings.
By default, the system installs all
applications onto the device's
internal memory, except for those that
explicitly request external
installation. This means that the
system will always install legacy
applications onto internal memory,
since they do not have access to
theandroid:installLocation attribute.
However, it is possible to configure
and compile a legacy application such
that it is installed internally on
older versions of the platform and
externally on Android 2.2 and later
platforms, if necessary.
Note that requesting installation onto
the device's external media is not
suitable for all applications,
particularly because the external
media may be removable and
unmounting/remounting may disrupt the
user experience and system settings.
For more information about setting a
preferred install location for your
application, including a discussion of
what types of applications should and
should not request external
installation, please read the App
Install Location document.Install Location document.