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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.

Android layout webview between another two views

What's the best approach to have a layout with the following:
Spinner (default height)
WebView (all the space between the two views (Spinner and Button))
Button (default height)
How is it possible to specify the height of the WebView to take all the space between the two (i.e. if they are resized later, the WebView will automatically be adjusted).
Thanks!

Use a RelativeLayout as the parent layout. Align the Spinner to the top of the parent. Align the Button to the bottom of the parent. For the WebView, set the height as match_parent and then use the android:layout_below and android:layout_above attributes to make sure that it always lies between the two views.
(I would've given the code but I don't want to. You might just copy paste with zero learning.)

When a layout is nested inside another layout, does it inherit the params of the parent?

Ok so imagine a situation where you have the main layout as LinearLayout (which supports layout_weight) and inside it you have nested RelativeLayout (which does not support layout_weight).
Now since the RelativeLayout is nested inside LinearLayout, will it be able to use layout_weight ? And if the case was reversed (Linear inside Relative), will LinearLayout be able to use layout_below, layout_toParentLeft, etc ?

Now since the RelativeLayout is nested inside LinearLayout, will it be able to use layout_weight ?
No. Since RelativeLayout is nested inside LinearLayout it can be given a weight to be weighted inside the LinearLayout but it does not inherit the property of weightSum to hand out to children.
And if the case was reversed (Linear inside Relative), will LinearLayout be able to use layout_below, layout_toParentLeft, etc ?
Yes, you are inside of a RelativeLayout so any child can be given those properties. However, any child inside of that nested LinearLayout can not use the properties of RelativeLayout as in your example.
WeighSum Docs
a number greater than 0.0f, or a number lower than or equals to 0.0f
if the weight sum should be computed from the children's
layout_weight"
android:weightSUme="aNumber" can be used to "sum" the weight of the children. If all the childrens sums will add up to 1 then you don't need this property. As I recently learned from #RomainGuy through a discussion with #Squonk, it is really only needed if they won't equal 1 and you want some empty space in your layout. Otherwise, the cpu will determine what the weightSum will be.

The answer to your question is not very complicated. I can synthesize it in two letters: NO

Android: My text is being displayed vertically in a programmatically set TextView

I'm adding a TextView to a parent LinearLayout RelativeLayout programmtically. There is enough space for the text to be displayed horizontally but for some reason the text is displayed vertically. Does anyone know what's going on here?

It's really hard to say what is the problem without looking at your XML and layout code. In my experience, sometimes it happens when the parent layout has height set as WRAP_CONTENT and the views inside it have wrong weight config, for example one has weight but WRAP_CONTENT and the other MATCH_PARENT (Sorry, I don't remember the case exactly). I suggest you check your LayoutParams carefully or set a fixed width and height for the TextView to see what is the problem.
You can also post your code here so we can have a look at it

Is there a way in Android XML to make a row of buttons with widths set to widest button?

Is there a way to declare a row of buttons in XML so that all the buttons have the same width, which is equal to the wrap_content width of the widest button? I'm familiar with the trick of setting all the widths to 0 and assign them all a weight of 1, but that won't work if the parent layout width is set to wrap_content. I don't want to set the parent width to fill_parent because I don't want the buttons stretched more than necessary.
The only way I can think of doing this is in code (either with onMeasure logic in each button that communicates with the other buttons or with a custom layout class).

I think you'd have to do this in code.
Creating a custom layout class would be the way to go. Override onMeasure() and make it look something like this:
Call setLayoutParams on all children to set their layout_widths to WRAP_CONTENT.
Call super.onMeasure()
Iterate child views to find the one with the biggest getMeasuredWidth().
Iterate all other child views calling setLayoutParams() with the widest pixel width.
Call super.onMeasure() again. :)
That should work but I won't stake my reputation on it... happy to help you further if it doesn't.

To get the buttons in a row in the XML you need to add the buttons with in a LinearLayout and change the orietnation to horizontal i.e.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
</LinearLayout>
As for getting the widest button and changing all the other buttons to match I am not sure as a guess you would have to have some sort of method within your activity to get the widest button and then programaticaly set all the other buttons to be the same.

Just find out what your widest button is, but it in a view with a horizontal width to match, and then use layout_width="match_parent" or "fill_parent" in < 2.3.
It'll make them all use the width assigned.
If you want to do it programatically, you need to iterate over all the sections, find the max, than iterate again and set it.