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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.

How to put an image on two different layouts

How can I achieve this layout?

Use a FrameLayout that act as a parent of a LinearLayout( say layout ll_par) & an ImageView..In your ll_par add your two remaining layout .& remember put the Imageview as the last child of FrameLayout & set its layout_gravity to right|center_vertical & give some margin to your Imageview according to the need ..

Centering a Linear Layout Horizontal and Vertical

Im running into a speed bump in my android App. I want to center my Linear Layout in the center of the screen (Horisontally only) and I want to center another element only vertically. I haven't seen an easy apparent way to do this in the program.
http://i.stack.imgur.com/gfMBD.png
I want the grey Box to be centered horisontally in the app

I'm not sure, how your layouts are now, but I'd do something like this:
Vertical Linearlayout
RelativeLayout, centered horizontally
RelativeLayout, set to match parent
RelativeLayout, centered vertically

Use RelativeLayout as a root ViewGroup, and then just add android:layout_centerHorizontal="true" to your first LinearLayout, and android:layout_centerVertical="true" to your second one.

Align a layout top of a image that is centered

I have a relative layout, inside of it is a ImageView that is centered in its parent.
I want to add a share up to the image. If I put this attribute android:layout_alignTop="#+id/myImage" the button will appear over the image. I want to be upper than this.

Use
android:layout_below="#+id/shareId"
in your <ImageView> tag. Or
android:layout_above="#+id/myImage"
in your share's View tag depending on what works better for you. If this doesn't fix it then please post the xml and a picture of what you want/have if possible.
RelativeLayout Docs shows the different properties that can be used.

Android XML: Good examples to use gravity rather than layout_gravity

I have done some basic xml layout from the Internet and I am confused as to what android:gravity would be used for vs android:layout-gravity. The UI designer seems to respond well when I change the layout_gravity, but I get no response to the regular gravity? What is up with this?
Is this similar to layout parameters with fill-parent and match-parent? one is gone now.

android:gravity is a statement from a parent to its content, indicating where the content should slide within the available parent space (if there is room for such sliding).
android:layout_gravity is a request from a child to its parent, asking that it be slid in a certain direction (if there is room for such sliding).

layout_gravity - It contains and it belongs to component's parent layout. Like if you set one TextView in LinearLayout, LinearLayout's width is fill_parent, but TextView basically will set at left-top. So now if you would give layout_gravity=right then your TextView will be shifted to top-right corner. But the text will not change its place. i.e. you had been wrote "Hello world" then "hello world" will not be changed to right, but its parent textView will be changed to right. So here in this case it will belongs to LinearLayout, means parent Layout.
gravity- It contains and it belongs to TextView as per above explanation. if you will give gravity="right" then "Hello-world" text will go to right-top corner of TextView not LinearLayout.