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Android LinearLayout with 2 content, one should take 30dp and another the rest

I have vertical linearLayout. First element is a framelayout that has some content and second element is button that should always be at the very bottom.
While button should be at the bottom, the framelayout should take the rest space that is in the linearlayout

<LinearLayout
...>
<FrameLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1">
...
</FrameLayout>
<Button
.../>
</LinearLayout>
The key to this solution is the combination of 0dp height for your FrameLayout and the layout_weight attribute. This attribute allows a LinearLayout to divide the "extra" space up between its children. Your button takes up a fixed amount of space, and your FrameLayout takes up no space at all... so everything that isn't the button winds up being given to the FrameLayout and now it fills up the whole LinearLayout while leaving enough space for the Button below it.

Use android:layout_height="match_parent" on your FrameLayout

you should use RelativeLayout somthing Like that :
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<Button
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Test"
android:layout_alignParentBottom="true"
android:id="#+id/btnTest"
/>
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#color/colorAccent"
android:layout_above="#id/btnTest"
>
</FrameLayout>
</RelativeLayout>

Set width of parent linear layout as percentage of the screen

I am building an android application and I have a dialog fragment. The dialog fragment has a set width. However, the issue is that when the app is run on a device with a different screen size, the dialog fragment isn't centered properly.
Initially, what I had was:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="500dp"
android:layout_height="wrap_content">
<-- code goes here -->
</RelativeLayout>
As you can see, I have a relativeLayout with a defined width. Since I know that you can't use layout_weight in a relative layout, what I did was I wrapped that parent relative layout in a linear layout as such:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="0.8"
android:weightSum="1">
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content">
</RelativeLayout>
</LinearLayout>
However, that doesn't work since the dialog fragment is cut when I run the application on a device with smaller screen size.
How can I set the width a dialog fragment as a percentage of the screen size? Is this possible at all or would I have to resort to setting it programmatically?

This is a correct way, if you want RelativeLayout have 40% width of the screen, but this technique cant apply to the parent layout, because parent layout doesn't have parent layout and android:layout_weight doesn't affect
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:weightSum="100">
<RelativeLayout
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="40">
</RelativeLayout>
Since I know that you can't use layout_weight in a relative layout
We can use layout_weight in any view and layout, if it direct child of a LinearLayout

With the new percent support library, you can now use a PercentRelativeLayout.
Check out this link

The code below will make relative layout 1/2 of the parent and will position it horizontally centered:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center_horizontal">
<RelativeLayout
android:layout_width="0dp"
android:layout_weight="0.5"
android:layout_height="wrap_content">
</RelativeLayout>
</LinearLayout>
Hope it helps.

Android RelativeLayout above Fragment

I am trying to display a RelativeLayout above a Fragment without success.
And here my xml :
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:background="#5C5A59"
android:gravity="center_vertical"
android:orientation="horizontal"
android:padding="5dp" >
</RelativeLayout>
<fragment
android:id="#+id/map"
android:name="com.google.android.gms.maps.MapFragment"
android:layout_width="match_parent"
android:layout_height="match_parent" />
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center|bottom"
android:background="#5C5A59"
android:orientation="horizontal"
android:paddingLeft="10dp"
android:paddingRight="10dp" >
<TextView
android:id="#+id/lbl_altitude"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:text="#string/lbl_altitude"
android:textColor="#00FFFF"
android:layout_alignParentBottom="true" />
</RelativeLayout>
</LinearLayout>
Using this code the Fragment takes always the entire size of the screen and my RelativeLayout above isnt display.
Thank you in advance.

Try to use layout_weight property
You can read this guide: Linear Layout

if you are using the Linear layout then you have to set the weight this will set your screen according to the percentage for example if you give weightsum 1 to the parent and then you first give the layout weight 0.2 to your layout ang 0.8 to your fragement its means your layout acquires the 20% screen and other fragement acquire 80%
So i think you want to use the Relative layout as your parent and set first child aligment on the top of parent and set the fragement below to the relative layout.

I want to bring my relative layout in bottom

what changes shall I make to bring the relative layout in bottom?
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="86dp"
android:layout_alignParentBottom="true"
android:layout_weight="0.08"
android:gravity="bottom" >
</RelativeLayout>
</LinearLayout>

You need to change your parent Layout to be a RelativeLayout, since the attribute android:layout_alignParentBottom does nothing for LinearLayout children:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="86dp"
android:layout_alignParentBottom="true" >
</RelativeLayout>
</RelativeLayout>

You can achieve that by using a FrameLayout instead of a LinearLayout since it stacks childs from top to bottom by default.
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="86dp"
android:layout_gravity="bottom" >
</RelativeLayout>
</FrameLayout>

use
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<!-- replace this by any other layout or view. You might want something here, right? -->
<View android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="1"/>
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="86dp">
</RelativeLayout>
</LinearLayout>

There are two issues here. The first being that
android:gravity
is wrong. This sets the gravity for the contents of this Views children. So for example used in a TextView it would place the text at the bottom if the TextView. What you want instead is
android:layout_gravity
this sets the Views gravity within its parent.
The second problem is that android:gravity="bottom" doesn't work as you would expect in a LinearLayout who's orientation is vertical. I'm not sure I can explain why well so I will try to find a link in a bit that explains it. But if you change the orientation of the LinearLayout to horizontal you will see this.
So your best options are to change the orientation if that's a possibility and if not then change the root ViewGroup to a RelativeLayout and use the property android:alignParentBottom="true" as someone already suggested.
Edit for explanation
This blog post and this SO answer, among others explain it a little. But basically, if the orientation is horizontal then the Views are layed out from left to right as we expect so you can't control the left and right gravity. With a vertical orientation, the same is true for top and bottom gravity because where they are placed vertically is already determined by Android when you set that orientation. I hope this makes sense.

How to make several RelativeLayouts each occupying a potion of the screen?

I want to have a relative layout inside another full-screen relative layout, occupying full width but 50% of its parent's height, preferably done with XML and not java code.
I have figured out how how to align parent's center, and how to fill up the width, but is there a way to get 50% of parent's height? What about 30%? 6.2834%?
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent" android:layout_height="fill_parent">
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="??????????"
android:layout_centerInParent="true" >
The reason I'm trying to do percentage is that, if I specify it with "dip", while the object will remain the same size, the layout will look a lot different on different screen sizes (e.g. a phone and a tablet).
EDIT:
Thank you for all the answers about using LinearLayout and weighting. I have looked at that before, too. I feel I might have over-simplified the problem. Say I need something like this:
I suppose I could use complicated LinearLayout and weighting to outline the center square, then having the center square to fill_parent, like so:
But then what should I do with the other 3 squares (layouts)? Can I have another "layer" of LinearLayout for another square? Or should I divide up the whole screen into many, many small cells and having these sublayouts span over multiple cells (not sure if this is even possible)?

Try to use LinearLayout with weightSum
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:weightSum="2"
android:gravity="center_vertical"
android:orientation="vertical" >
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:background="#FF0000">
</RelativeLayout>
</LinearLayout>

If you don't absolutely need it nested in one RelativeLayout you can use weight in a LinearLayout as others have pointed out. I just added in an additional RelativeLayout above and below so you can use the rest of the screen if you are trying to. If not, just remove the other RelativeLayouts.
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/ParentLinearLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:weightSum="10" >
<RelativeLayout
android:id="#+id/RelativeLayoutTop"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="2.5"
android:background="#color/torange" >
</RelativeLayout>
<RelativeLayout
android:id="#+id/RelativeLayoutMid"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="5"
android:background="#color/tpurple"
android:padding="8dp" >
<TextView
android:id="#+id/description"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
android:text="#string/describe"
android:textAppearance="?android:attr/textAppearanceLarge" />
</RelativeLayout>
<RelativeLayout
android:id="#+id/RelativeLayoutBottom"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="2.5"
android:background="#color/torange" >
</RelativeLayout>

I usually go with a LinearLayout for this and set the weight to a certain percentage :
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical">
<View
android:layout_width="fill_parent"
android:layout_height="0dp"
android:layout_weight="25"/>
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="0dp"
android:layout_weight="50">
</RelativeLayout>
<View
android:layout_width="fill_parent"
android:layout_height="0dp"
android:layout_weight="25"/>
</LinearLayout>
To your edit:
At some point you need to determine the layout. Start by taking the layout in groups. Look for patterns. In your simple explanation we have devised a way using a linearlayout to group 3 objects with one in the middle. With your new layout, could you group those items in any way?
Once you have simple layout patterns set, maybe add specific spacing that you are looking for by defining weights. Then you might want to add a relative layout and start anchoring views to specific views. Ask yourself do they overlap? Does one view always position on top of other views or on the sides. What defines the bounds of your views and then take it from there using linear layouts, weights, relative layouts, toLeftOf, toRightOf, bellow, above, margins, and padding.
Here is an example of what I mean by grouping like objects. It's by no means the best solutions but that all depends on how you define the positioning parameters.
Yellow = vertical linear layout
Green = horizontal linear layouts
You have 1 large vertical layout and inside two horizontal layouts with multiple objects inside of that. From there you can break it down into easier to manage portions on how to arrange and item within that layout. Now with relative layouts you could position items relative to another object, you could remove some of the work handled by the linear layouts but you will then be defining their distance relative to the other objects and might have to fiddle to get the layout to adjust properly on different screen sizes (reason to not use static positioning).

Maybe try using a LinearLayout with 3 layouts inside with android:layout_weight set to 1, 2, 1.
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<RelativeLayout
android:background="#FF0000"
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="0dip" >
</RelativeLayout>
<RelativeLayout
android:background="#00FF00"
android:layout_weight="2"
android:layout_width="match_parent"
android:layout_height="0dip" >
</RelativeLayout>
<RelativeLayout
android:background="#FF0000"
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="0dip" >
</RelativeLayout>
</LinearLayout>

RelativeLayout does not support percentage of width and height for children. Use LinearLayout with android:layout_weight attribute.