Develop Reference

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.

Difference in behavior of android:gravity="center_vertical" for RelativeLayout and LinearLayout

Let's say I have two views that I want to center vertically. The first view is bigger than the second view.
I noticed that if I place theses two views inside a RelativeLayout with properties layout_height="wrap_content" and android:gravity="center_vertical" nothing happens. This is what I get :
In the opposite, if I place these two views inside a LinearLayour with properties layout_height="wrap_content" and android:gravity="center_vertical" the views are centered vertically :
Lastly, if I place these two views inside a RelativeLayout with properties layout_height with a fixed height and android:gravity="center_vertical" I get the same result as the LinearLayout. The views are centered vertically.
I would expect the views to be centered vertically in each case. Do you know the reason for this difference?

LinearLayout handles all its child object based on its orientation (Horizontal or vertical). So when you are saying gravity: "center_vertical". You are actually referencing based on your parent layout.
In case of RelativeLayout,it enables you to specify the location of child objects relative to each other (child A to the left of child B) or to the parent (aligned to the top of the parent).
Personally I would use gravity only in LinearLayouts and the centerInParent for RelativeLayouts.

In your first case it'll work with RelativeLayout as you expected if you use android:layout_centerVertical="true" to the child view which you want to be centred.
So in case of LinearLayout you need to specify the orientation first (i.e. horizontal/vertical) so that the child views are inflated based on the reference of your parent layout.
While in RelativeLayout, as the name says it all, you can specify the position with respect to the views which are the child of a parent RelativeLayout.
Now the views you want to achieve can be generated in many other ways too.
For example, set your parent layout as LinearLayout. Don't specify any gravity attribute in the parent layout. Hence, you set a layout_gravity attribute to the child to certer_vertical and this should work too.

Well, after some others tests, it seems that the behavior of android:gravity for a RelativeLayout is a bit random. I will just avoid to use is.

Is there any layout xml tag to specify the position of view in "dp"?

We can get the relative position of a View, in pixels, using getLeft(), getTop() etc.
Is there any tag to do the reverse in a layout xml?
Something like android:layout_left="30dp" or something?

You are certainly welcome to use padding (e.g., android:paddingLeft) and margins (e.g., android:layout_marginLeft) to influence the position. However, the position is controlled primarily by the layout rules for the particular container that you are using (LinearLayout, RelativeLayout, etc.).

Imageview floating left

How can I create ImageView floating to the left of the textView, something like html:
<div>
<img src="src" style="float:left"> Text here.....
</div>

You have two options:
With LinearLayout, setting the orientation to horizontal so the image is first and then come the rest.
With RelativeLayout, there you can indicate the position of a element relative to another with the attributes of the style android:layout_toLeftOft, android:layout_toRightOf, android:layout_below or android:layout_alignParentTop, ...
However, it is not so flexible as CSS for some actions and, for example, wrapping text around an image is not so easy to achieve.

There's really no concept of floating elements in Android, but you can easily put an image to the left of some text using the drawableLeft attribute of the TextView. Example:
Otherwise, for more complicated layouts, parent views determine how their children are laid out. For example, instead of the concept of a div which simply wraps it children and uses the float and display attributes of the children to determine how things look, Android has more complex parent views (ViewGroups as they are called, since that's the super class) to control things.
Check out the docs for LinearLayout and RelativeLayout for some examples.

The positioning of views depend on the kind of layout you use. In case if you are using a RelativeLayout and you want to float your view(in your case the ImageView) which is within this Relativelayout, you can use the attribute of the ImageView (android:layout_toLeftOf="") specifying the view id of your textview between the double quote.

Which Layout supports android:layout_gravity?

I came through many examples in internet.I found that neither Relative Layout nor Linear Layout supports android:layout_gravity.By it I mean the views inside these layouts does not support android:layout_gravity attribute. So any one having idea which layout supports android:layout_gravity and how to use it in better way?

Children (that is, direct descendants in the View hierarchy) of LinearLayout do use layout_gravity (see LinearLayout.LayoutParams), but only on the "secondary" axis. So, in a vertical LinearLayout, center_horiztonal will work, but center_vertical will do nothing.
Children of FrameLayout also support layout_gravity (see FrameLayout.LayoutParams).
Keep in mind that the layout_* parameters set values in a LayoutParams object provided by the view parent. So a layout_* parameter will only have an effect if the parent view supports the parameter.

Actually if you use RelativeLayout you don't need to use layout_gravity.Better way to position your layout's elements are android. Here you can get a good explanation how to use RelativeLayout.