Develop Reference

How to create a system app for custom ROM

We have a custom ROM for a device we are making and what I want to do is make our own custom Settings app to replace the settings app that comes already built in android.
I cant really find any documentation on how to even create a system app. It does not appear you can create it in Android Studio, I tried looking on the AOSP site and didnt find anything there related to creating system apps. The few things I did find were on here where you would put android:sharedUserId="android.uid.system" in your manifest and then has to be signed with the same key as the ROM but after that I cant find anything.
The settings app I want to create would need access to the framework.jar to be able to use the hidden framework API's.
Does anyone have any information or know where I can find this information on how to go about actually creating a system app?

You may want to research how to make a Device Tree Overlay (dto). Basically it is your code, reformatting the base code, into what you want. https://source.android.com/devices/architecture/dto?hl=en

Since you said you use Gradle, you should put first build you apk and put them in you vender folder, and next important thing is to create a mk/bp file to tell the Android build system how to deal with this pre-built apk.
P.S. If you system app highly depends on Android hidden api, make sure you have the right version of framework.jar in you Android Studio project, or, as what I alway did, create apps directly in AOSP, in this case, you can just check the code structure of the original Settings app in AOSP
this tutorial may help you.

Xamarin.Forms (UWP and Android) directory selection outside of AppData

I've been pulling my hair out all day. I'm converting my WPF app to Xamarin.Forms (UWP, IOS and Android). I'm currently focused on UWP and Android (don't have an apple devices yet) and I got a lot of the program running but I ran into a huge snag with reading files that are not in my sandbox directory. Currently, my app allows you to choose a folder with images and you can move through them. To test out the reading functionality, I've placed the images into the "LocalState" directory and bingo, my code is working beautifully. Now the program doesn't have a default folder and when I originally created it in WPF, the user would have a pop up and they would choose their own folder but I cannot for the life of me get a similar function for Xamarin. Every time I try something, I get an "access denied". I've seen Android apps allow you to pick a folder for emulators and I'm 100% sure that you can setup UWP to do permissions with StorageApplicationPermissions but I just cannot get this to work for Xamarin.Forms. Please help me!

OK, Android first it is easier. You need to setup in the manifest permissions to read and write SD-card. On Marshmallow and later you need to ask for the permission at the runtime (you need to do that from the native project).
On UWP the easiest thing to do would be to set the broadfileaccess in the manifest. Other than that you pointed out to the proper APIs, just you need to run them natively.
In general native code is called by using dependency injection. If it gets too complicated you may consider to create Xamarin.Forms project as a shared project where you can use conditional compiling instead of dependency injection which is quite simpler.

Change Editor.java class in Android Studio, Save, Run, Debug -> Possible?

In Android Studio I can change the Android API code and save it. I assume this change is not pushed to the emulator?
Do I need to download the whole AOSP and compile a new android.jar or is there an easier way to change for example Editor.java and try out things in Android Studio?

You probably don't want to change an Android api... you wouldn't be able to install the app on anyone's phone.
But yah, Android is open source and you can download AOSP, change an api, and compile a device image.That'd be the only way to do it.
What is your end goal here? There probably is an easier way to accomplish it.

How to create batch command for : compile + sign + install + run

Background
As it turns out, it's quite an annoyance to test out in-app-billing (i've written about it here) . Each time you find a bug and need to test your app, you must have the following steps:
compile the app , preferably with the debuggable="true" flag on the manifest
sign the app with your real key you've created for the play store.
install the app on your device
run the app on your device, preferably debugging it via eclipse.
If you'd use a batch operation instead of doing all of those by yourself, it should make things easier to test such apps, and might also be useful for other apps as well.
The question
Is there any way to make the whole process fully automatic, so that with a single click of a button, it will do all of the above?
I know how to achieve 3+4 (using "adb.exe install -r *.apk" and "adb.exe shell am start -n FULL_ACTIVITY_PATH" ), but I don't know how to achieve the others.
If there is a solution that doesn't involve a third party app or plugin (like maven or Ant), it could be very nice. Of course, if that's what is available, I would like to know if it's possible to achieve via a batch file that will use them somehow.
Is there maybe a way to mimic how Eclipse does things in this regard, without being so specific on the project (maybe just the path of the project and that's it)?

Maybe you can package app by ant . You can do every thing you what int the ant code of packaging ,causing the core code is to make property configs in your app and referenced lib, such as build.xml,local.properties...,and replace channel in AndroidMainfest.xml ,so you can also replace any code in your android app,just as you want ,to change the flag of debug , because all the operation just handle local file before packaging your app,you can copy or modify any file in your app. After above process ,you get a app,then you can run other code to install the app. Recently ,I have been using the batch package method to get a lot of app for different channel .so hope it helps.

How to have different versions of an android project on the device?

I need to have different versions of my code on my device when I'm developing my android application. I want to have a copy of my latest code once in a week or so. The reason is that sometimes you make changes to the code and make it worse for a while, then after that you polish your code to include the features. So I need to have a copy of latest correctly running version of my code on the device.
I tried to change the app name, to see whether the ide deploys a new copy of my code, but it didn't work and the ide seems to be smarter than that and it replaces the app with the new name. I suspect that I need to change the package which is a little bit frustrating/risky. Anyone knows the solution?
Right now, I'm using Intellij to implement my android project but I think this doesn't matter.

You need to change the package name.
I use ant build script to do this easily. I created an ant task to rename the package name and make a build.

Typically I use svn to keep track of changes and label which ones are the ones that work then add features and use svn merge commands to combine new features with working code. Then if you want to undo something you can just roll back a revision.

The way I achieved that goal is:
- configure the original project as library
- create another project specifying a different package name in the manifest
- you also need to declare all the activities, receivers, etc with the full name in the new manifest
In that way switching the code of the original project through different branches you can create different apks and have different version of the app installed at the same on your device.

You need to change the package name, not the app name, it's pretty easy in Eclipse.

If your package name now is 'com.sina.perch', so just rename it to 'com.sina.perch1' or something else whatever in file 'AndroidManifest.xml' , which looks like:
It will be work.