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How to add functionality to existing classes?

Currently, I am developing an android library. Basically the idea is to add some functionality to any android widget that there is. The library is used to create compound views, and the user should be able to convert every existing android widget/view into a compound view with some additional functionality I want to add dynamically.
Every one of these views should still be useable inside XML files, which means I cannot change the constructor too much.
Another requirement is that I need an option for the user to provide some classes for the views. By that, I mean that the created compound view is going to have a public variable named viewStore. Thy type of viewStore would either be the user's implementation of the ViewStore interface (preferred way but I am pretty sure this would require code generation as discussed later) or would be provided via generics.
In the end, a compound view would have this folder structure like this:
MyView Folder
MyView extends CoolViewWithViewStore extends AndroidWidgetChoosenByUser
MyViewStore implements ViewStore <- used in CoolViewWithViewStore
One option is to extend every single widget. I don't think I need to explain why this is a bad idea. Furthermore the user couldn't provide the additional classes that are needed.
Another one I thought of was Annotation with code generation. The problem I came across here was that the user needs access to variables of CoolViewWithViewStore inside MyView which wouldn't be possible because CoolViewWithViewStore would be generated at compile-time and furthermore the user could accidentally use his class inside XML instead of our generated one.
I would like to hear if anybody has a better idea of how to handle this or if there even is a clean solution to this to achieve this kind of architecture. If anybody has a better idea of how to structure my library I would like to hear this as well.

Using Kotlin extension function you could extend a class with new functionality without having to inherit from the class.

Best approach for handling same layouts for different activities

I have been working on application which has 3 types of accounts related to it. We create a single layout and view/hide items on it depending on, from which account you are currently logged in.
With this approach, we have activities/fragments doing a lot of different things, they handle all cases wrapped in if/else checks etc. With growing project, it is becoming hard to maintain these classes.
Say, if I have to hide a view in certain scenario, I have to look around many if/else checks just to hide a single button because if I hide it on one place other check will make it visible again, really hard to maintain.
I am looking forward for best advises on this issue from the experts.

If you are struggling with a lot of if/else scattered in the code, maybe you should use polymorphism in your code.
Create an abstract class for the Activity, then specialize it for each particular type.
Use the Factory method pattern for creating objects of this hierarchy. This method will use the parameters for deciding which concrete class to instantiate, and then it will initialize the instance being returned.
Use the Template Method pattern if there is an algorithm common to all sub-classes but that contains some open steps that should be implemented by each class.
Use the State/Strategy pattern if you need polymorphic code that may be modified at runtime.

If your separate apps require minor customization and theme changes, but are really the same base app, multiple flavors is definitely the way to go. However, if both apps require a lot of custom code differences, you might want to rethink using multiple flavors strategy.
Also, take notice of the difference between flavors and build types. Use flavors for situations where you might need different versions of the same app in the Play Store, for example, free and pro, or for situations where you are customizing the same app for multiple clients.
for details http://www.androidauthority.com/building-multiple-flavors-android-app-706436

you have create new xml files in which has common view's for your activity and fragment then need to use include tag in xml for adding those common view's into your activities & fragments xml.

Create different xml for same layout and use <include layout="#"/>
Tag to create the layout, it will reduce if/else and also provide you the code re-usability

I think you should create separate layout for all 3 types of account and you can create PickLayout static class/method to pick the layout by type
int getLayout(int type){
return layoutMap.get(type);
}
if you have re-usable layout then you should use include, merge or you can use ViewStub also.
if you have chain of if/else then you should use Map link that will be scale-able, error-prone free.
And try to follow android suggested design-pattern that will be helpful for writing test case also.

Dynamically change values of colors.xml to change look and feel of android app

Problem :
I am trying to change the look and feel of my Android app, on the fly. Something like, the app starts up and gets a set of values from the server. These values are the elements that typically go inside colors.xml. What I am looking for, is a way to dynamically change the elements inside the colors.xml and update it with these new values received from the server. My understanding is that normally, this cannot be done directly. But has anyone found a workaround?
What I want to avoid if possible :
I would like to avoid setting color values inside each activity's onCreate() method for each element in that view. If at all possible, I would like to avoid this.
Any thoughts?

You can achive this change by newly introduce firebase remote config which provide remote config to change theme color or any other values necessary for app like promotion,updates etc
You can refer this Example

Unfortunately all color values (and other resources) inside the resources directory are hardcoded as static final ints. This means there is no way to change the values at runtime. You can however use one of the previously suggested solutions or have a look at this excellent explanation: https://stackoverflow.com/a/33992017/3662251
For a nice workaround that overrides the activity's getResources method and implements a custom Resources class which is in my opinion the most seamless solution: https://stackoverflow.com/a/34178187/3662251

I have did that in my app getting Hex color code like #06FF67 from my server and stored in sharedpreferences - https://stackoverflow.com/a/23024962/4741746
And when need to set new value that coming from server just override same shared preferences value with new data and set to app
Or you can use Random Color genrater also -https://stackoverflow.com/a/5280929/4741746

How to present an Android project when only the layout has to be done

I have to make a new design for an Android App, but I only have to create the visual part (native Android). The app logic would be created by another guy based on what I present to him.
My question is? How would this be managed correctly? Do I have to make the XML's of each layout? How could I show the other person my progress?
What things should I consider when doing this?
Thanks

You need to mock the app first (create a prototype) as suggested by NoChinDeluxe. Then if you guys go ahead and decide to code it, the answer to your problem is separation of responsibilities. As Jeffrey said UI work is not only about layouts, but code as well. What I would suggest is that you and the other guy get together first and define some contracts (interfaces) that will allow you guys to split the work and work in parallel. Therefore, he can create the business logic of the app without worrying about the UI implementation. You, on the other hand, will have to mock that business logic he's implementing at the beginning so it doesn't block your UI work.

you could create layout XML files for all the Activities/screens, using resources (icons, etc as suggested by #NoChinDeluxe). However since you'd want to run the mock app, you might want to also create a "throw-away" Activity that allows you navigate to different screens of the app. Here you can add a number of buttons and when you click on each button, your app shows a specific activity. This way, you will be able to show your colleague all the screens you have created. I hope this helps.

This may not be what you want to hear, but creating Android layouts isn't a design task. They are closely tied to the code, and the design of them is going to depend on how the engineer chooses to implement the app.
Here's an example. You might have a grid with 4 cells. You could use a RelativeLayout, a LinearLayout, or GridLayout, or a GridViewLayout. Which will use choose?
I'd suggest providing your engineer with mockups and graphical assets where required. Let him / her take those and create the layouts. If you want to create layouts as a (visual-only) reference for engineering, great, but it's certainly a non-optimal tool for that task.

Things You will consider when doing visual part:-
You have to work on the resource folder of your application
Layout : All Layout you have to prepare.
Drawable : Images and drawable .xml.
Inside Values folder you will find
dimen.xml : For different devices dimen you can set.
string.xml : You can store string for hint or other purpose.
style.xml : For designing or theme or custom design.
color.xml : Color which are going to used in the application.

Android Settings app preference files

I've read Android's settings tutorial, and want to use the new "headers" feature in order to accomplish something like the left image. To be exact, I want to have a global "enabled" checkbox, and below it some preference headers surrounded with a category header.
Can I make it be just by using XMLs ?

Yes it is possible by writing your own adapter class that handles headers. I found this nice pretty straightforward example for native settings looking like preference.
It's working on handset but it's not fixed for tablet (app crashes as it tries to use two-pane layout).