Develop Reference

Android Studio how do I change layout_alignParentEnd for a view to be false programmatically?

So suppose I have a textview called tvPlaceholder with the following property android:layout_alignParentEnd="true". How would I go about setting this to be false in the code (Kotlin)? I know you can change things like visibility, text size, e.t.c. in the code but could not find a definitive answer for layout_alignParentEnd. I am also calling tvPlaceholder from a viewholder if that matters at all: viewholder.tvPlaceholder.

When a view is added to a parent, it adds a LayoutParams object that defines any rules for laying it out. You can get this object by calling getLayoutParams(). Each layout parent has their own layout params subclass, I'm assuming this is a relative layout. So for a RelativeLayout, the LayourParams has a function removeRule that can do this.
val params = view.getLayoutParams() as RelativeLayout.LayoutParams
params.removeRule(RelativeLayout.ALIGN_PARENT_END)

What is the parent of the layout element?

I'm new to Android development so this might be an obvious question, but I've looked at multiple sources and can't find the answer. If you put widget elements inside a layout element then the parent of those widget elements is the layout element, right? So what is the parent element of the layout? Because in the default generated file, activity_main.xml (for the layout), there is the attribute android:layout_width="match_parent" for the topmost constraint layout element. So, what is the parent of this constraint layout element?

Your ConstraintLayout is the root layout of your Activity but it is just part of a layout created by the system.
You can for example get the reference of the parent layout of your ConstraitLayout:
ViewGroup layoutRoot = findViewById(android.R.id.content);
When you set match_parent, your View (a Layout is a ViewGroup which is just a View) will try to fill the space available in its parent layout. So, in a certain way, it just tell to how much your view can grow in the X/Y-axis. In your case, your ConstraintLayout will grow in order to match that android.R.id.content
Of course, there's one View on the top of the stack (probably the decorView of your Window) which is created by the System and is assuming whole area available on the Window created by the system.

Basically we use parent layout as Linear Layout/Relative Layou/ConstraintLayout/Frame Layout etc. So all are classes which extends ViewGroup. Please refer to Google developer documentation for better understanding.
Read this -
https://developer.android.com/reference/android/view/ViewGroup.html

match_parent is a fixed value for views to stretch according to their parents.
For example let's say you have a Textview inside a RelativeLayout. If the Textview is set to match parent then it would stretch according to its parent/rootview which is the Relative layout.
So basically it would have as the same height and width as the parent layout.

How to add view in custom layout in runtime

I am creating custom layout (extends FrameLayout). I have a bunch of view defined in xml(it doesn't matter now).
What I need to do. My custom layout has custom defined attributes, let's assume that it named footer_banner_type.
I have different Banners classes some of them I quite different from one another, so I cannot place some base banner in xml. So I have to add some banner based on attribute value.
I am extending FrameLayout . I am newbie and this is my first custom layout.
I don't know how to improve performance.
As I understand Layout iterating and inflating all child views. But if I need to add view in runtime.I don't want to make layout reiterate view hierarchy, because it will be performance issue.
My question is how to implement my task in better way.

//First create your view:
View wonderfulView = new View(this.getApplicationContext());
//Then, create its LayoutParams
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, 0, 1);
//Set those layout params in your view
wonderfulView.setLayoutParams(params);
//finaly, add the view to your ViewGroup
yourLayout.addView(wonderfulView);
That's it.
If you want to change the view container, you'll have to remove it form the previous parent like this:
View movingThing = ((YourLayoutClass)findViewById(R.id.currentContainer)).getChildAt(WhereYourViewis);
((YourLayoutClass)findViewById(R.id.currentContainer)).removeView(movingThing);
((YourLayoutClass)findViewById(R.id.newContainer)).addView(movingThing);

android explaination addview & layoutparams during runtime

just want to know what happens when I set new layout params to a new instance of an ImageView and then add it on an empty layout. It seems like doing it reset the parent layout hosting the new ImageView and also reset position and size of every sibling ImageView.
So what happens? Parent layout takes the layoutparams of the child?
thanks

Layout params have direct affect to views which use that params. But that also affects other views under same parent. Think about width, height and weight attrs of linear layout params. For example, if you set a layout params which has a weight attr defined, this will all change the visual of your parent.
From the doc:
LayoutParams are used by views to tell their parents how they want to
be laid out.
Note:
A parent is a view group, a container for child views. There are some types of view groups, ie LinearLayout, RelativeLayout etc(subclasses of ViewGroup can be googled), and they have specific layout behaviours and LayoutParams. While adding views to view groups layout params are used (both on xml and programmatically) for setting position, width, height inside that view group(container).

dynamically adding a view to activity layout

I have a custom view (an extension of a TextView) that I want to dynamically add to my Layout (don't want to include it in the main.xml file).
The book says to fetch the RelativeLayout using findViewById() in my java code then create a new instance of my custom view, then use addView on the RelativeLayout to add the new view.
I'm not getting any errors, but when I click my button to add the new view, nothing is happening (view isn't being added). Do I need to set additional properties on my custom view (layout width, layout height for example) in order for it to be shown?
EDIT: adding code
// changed to an imageview as I thought it might be easier to see an image
RelativeLayout rel = (RelativeLayout) findViewById(R.id.rellay);
MyCustomImageView mciv = new MyCustomImageView(null);
mciv.setId(5);
LayoutParams p = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
mciv.setLayoutParams(p);
mciv.setImageResource(R.drawable.someImage);
rel.Addview(mciv);

Please post your code where you add the view.
But yes, you might be missing the params for width and height. Try something like
LayoutParams p = new LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.FILL_PARENT);
txtView.setLayoutParams(p);
or what you would like the width and height to be. Also in xml layout, layout_width and layout_height are required attributes.