Develop Reference

Unity - files .txt in Project are hidden into the .apk?

If I have a .txt file in a project folder of an Android project and I build this project as .apk. This file is accessible for the user from the Device Explorer or in hidden into the .apk?
(I apologize for my english)

You can also access the streamingAssetsPath (read only):
https://docs.unity3d.com/ScriptReference/Application-streamingAssetsPath.html
and the persistentDataPath (this is a folder where you can read/write data, accessible to your users... but you can access this folder only after the installation):
https://docs.unity3d.com/ScriptReference/Application-persistentDataPath.html
To read files in the streamingAssets on Android you need to use WWW class but everything is explained in the docs (with an example).
Read this to understand if you really need to use Resources folder or it's better to use streamingAssetsPath:
https://unity3d.com/learn/tutorials/topics/best-practices/resources-folder
if you want to undestand the difference I suggest to read this:
https://forum.unity.com/threads/resources-vs-streamingassets-for-mobile.494804/

Unity will produce a .apk-file.
Your .txt-file will be baked in if:
Its referenced by something in the scene
Its in the Resources folder

Using a linked file in Assets directory - FileNotFound Exception

I am trying to link to an external file from a shared repository between my iOS and Android apps. This does not present a problem for iOS, but it does for Android. My current solution is to create a copy of the file from the external repository and place it in my projects Assets folder. This solution works, but is not much of a good one in my opinion and involves too many extra steps.
Using Eclipse, I am able to link to a resource. It's as simple as copying a file into my Assets folder and being prompted to either copy the file or link to the resource. If I link to the resource and try to run my app, I get a FileNotFoundException. If I copy the file instead, the app file is found just fine.
Ideally, I'd like to link to the file so that when I pull a new update from git then I don't need to copy the file over every single time. I'd prefer to link to the file.

I don't know what Eclipse uses "under the covers" for "Link here" drag-and-drop stuff. However, it is an Eclipse-ism. Android's build tools are fairly isolated from Eclipse proper, and so they won't know about those links.
Using a hardlink, or perhaps a symlink, at the OS X filesystem level should work, as both Eclipse and Android's build tools should treat it like a local file.

Android - Secure place to store sensitive files inside the same project

Let's say I have an Android Project set up and a git repository for it. Where can I put my keystore (and other sensitive files) inside my project so I can be guaranteed that it will not end up inside the .apk when I export the app (but will be inside the git repository)?
For instance, I can't put sensitive files inside the assets folder because that will end up inside the .apk.
Is there a place to store these kind of files in the Android file hierarchy I am missing?

Don't put your keystore anywhere in your source control or project folder even if you have .gitignored it. It is a personal document, to be treated as you would your SSH keys.
It is better to keep it in an external location, such as your documents folder or personal keystore folder, which you manage yourself. As you have mentioned, these don't need to end up in the APK so they are purely supporting documents and files.

Is there a place to store these kind of files in the Android file hierarchy I am missing?
Put them in the project root. The only file from the project root that is incorporated directly into the APK is AndroidManifest.xml, and even that is moving with the new build system and project directory structure.
Or, create some other directory that does not collide with any existing directories and will not be used by Android, like _stuff/.
Or, have your project itself be in a subdirectory, putting other related files in a peer directory.
Also, make sure you set up your .gitignore properly such that files only go into the Git repo that you actually want in the Git repo.

Share assets among multiple projects

As we all know by now, Android's Eclipse+ADT IDE framework doesn't support (re)using assets from a Library Project.
Instead, any asset resources used by an application must be stored in the assets/ directory of the application project itself.
My problem is that I have 8 (eight!) applications using the same Library Project that uses assets and as a result of the aforementioned limitation I have to copy/duplicate and sync each of the asset files to all 8 (eight!) applications.
This is a maintenance nightmare and it is error prone, too.
Any idea how to work around this by sharing a single copy of each asset among multiple application projects?
Note: I'd like to avoid an OS or filesystem dependent solution like hardlinks or softlinks. I prefer a solution like context.getAssets(), if it indeed does the work.

Maybe things have changed since you posted the question, but I was able to reuse asset files from another project in my Android project. I created my project's assets folder as a linked folder. I posted a sample project to show how it works.

how to reference an asset in a library project

In a class belonging to a Library project I call:
webview.loadUrl("file:///android_asset/info.html", null);
Unfortunately, this only works if I duplicate the file info.html into the Application's project asset folder as well.
Is there a way to tell an Android library code: "look for this file in the library's assets folder, not in the application's assets folder" ?

This answer is out of date, the gradle build system and AAR files support assets.
From the Android Docs:
Library projects cannot include raw assets
The tools do not support the use of raw asset files (saved in the assets/ directory) in a library project. Any asset resources used by an application must be stored in the assets/ directory of the application project itself. However, resource files saved in the res/ directory are supported.
If you want to include files from a Library project, you'll need to put it in the resources instead of the assets. If you're trying to load HTML files from your library project into a WebView, this means that you'll have to go a more roundabout method than the usual asset URL. Instead you'll have to read the resource data and use something like loadData.

This is now possible using the Gradle build system.
Testing with Android Studio 0.5.0 and v0.9 of the Android Gradle plugin, I've found that files such as
MyLibProject/src/main/assets/test.html
are correctly packaged in the final application and can be accessed at runtime via the expected URL:
file:///android_asset/test.html

You can achieve this by creating a symbolic link in the project's asset folder that points to the directory in the library project.
Then you can access as below:
webview.loadUrl("file:///android_asset/folder_in_a_libary_project/info.html", null);

Okay. Ive been stressing out and losing sleep about this for a while. Im the type of person that loves API creation, and HATES complicated integration.
There arent many solutions around on the internet, so im quite proud of what Ive discovered with a bit of Eclipse Hackery.
It turns out that when you put a file in the Android Lib's /assets folder. The target apk will capture this and place it on the root of the APK archive. Thus, making general access fail.
This can be resolved by simply creating a Raw Java Library, and placing all assets in there, ie (JAVALIB)/assets/fileX.txt.
You can in turn then include this as a Java Build Path Folder Source in
Project > Properties > Java Build Path > Source > Link Source.
Link Source
Click on Variables. and Add New Variable, ie VAR_NAME_X. location : ../../(relative_path_to_assets_project)
Click Ok
Now, when you build and run your app, the assets folder in the APK will contain your (GLOBAL Library) files as you intended.
No need to reconfigure android internals or nothing. Its all capable within a few clicks of Eclipse.

I confirm that Daniel Grant's approach works for at least the following situation: target project does NOT have an asset folder (or the folder is empty, so you can safely delete it).
I did not setup any variable.
Simply setup a LinkSource as follows (just an example)
Linked folder location: /home/matthew/workspace_moonblink/assetsForAdvocacy/assets
Folder name : assets
The "assetsForAdvocacy" is a Java project, (created with New- Project - Java Project) with empty src folder, and a new folder named "assets", which now provides the entire assets folder for the target project.
This is a fairly straightforward way within Eclipse to provide assets re-use across many different projects IF they do not already have assets, good enough to get going with. I would probably want to enhance it to become a content provider in the long run, but that is a lot more development.
My project accesses the assets with the following code:
String advocacyFolderInAssets = "no_smoking/"; //a folder underneath assets/
String fn =advocacyFolderInAssets+imageFilename;
Bitmap pristineBitmapForAdvocacy = getBitmapFromAsset(context, fn);
I use Motodev Studio 3.1.0 on Ubuntu. It would not let me 'merge' a new assets folder in the new assets-only project onto an existing assets folder in the target project.

If you want to use a setup where multiple derivate products are created from one library you might consider using svn:externals or similar solution in your SCM system. This will also do the trick that static assets like online help may be versioned seperately from the android source code.

I found this older question, it might help you, too.
This is the official way Google uses to archive this (from the above post): Link