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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.

Android ImageView in HorizontalScrollView get's twice the width of screen

I'm trying to make a ImageView inside a HorizontalScrollView, but the imageView gets twice the width of the screen and end halfway out of bounds.
My XML:
<?xml version="1.0" encoding="utf-8"?>
<HorizontalScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/horizontalScrollView1"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="top"
android:baselineAligned="true"
android:orientation="vertical" >
<ImageView
android:id="#+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="#drawable/penguins" />
</LinearLayout>
</HorizontalScrollView>
And a image of what I get (click me!)
Does anyone know a solution to this problem?
Greetings

I copied your xml and using my resource it looks perfect, i believe your resource is bigger than you think it is, you probably should use layout_width and layout_height as fix size and also set scaleType to centerInside.

How can I get my Android Gallery to fit within my Relative Layout?

I'm currently stuck on my landscape layout. I am implementing a Gallery, and would like it to fit the relative layout's height, and wrap the content via the width. I tried scaleType centerInside, messing with different types of layout_height/width.
Here's where the relevant part of my XML looks like (I apologize for the lack of formatting):
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_height="fill_parent"
android:layout_width="fill_parent"
android:padding="10dip"
android:orientation="vertical">
<RelativeLayout
android:orientation="horizontal"
android:layout_height="wrap_content"
android:layout_width="fill_parent">
<Gallery xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/gallery"
android:layout_width="wrap_content"
android:layout_height="fill_parent"
android:layout_alignParentLeft="true"
android:scaleType="centerInside"
/>
Here is the image
of the cutoff.

I don't believe you can apply scaleType to a Gallery: looking at http://developer.android.com/reference/android/widget/Gallery.html I don't see scaleType as one of the supported attributes.
EDIT: You should apply the scaleType attribute on the ImageView you are putting in the Gallery.

I don't think wrap_content works well with the width of a Gallery view.
Instead use android:layout_toRightOf, it will ensure that the Gallery doesn't extend into the area of your textviews. The following code is an example, but I have not tested it.
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_height="fill_parent"
android:layout_width="fill_parent">
<LinearLayout
android:id="#+id/right_side"
android:layout_height="fill_parent"
android:layout_width="wrap_content"
android:layout_alignParentLeft="true">
*put your textviews here*
</LinearLayout>
<Gallery
android:layout_width="wrap_content"
android:layout_height="fill_parent"
android:layout_toLeftOf="#id/right_side"
/>
</RelativeLayout>

screen with top banner, webview and bottom banner

I'm trying to create a very simple android screen with the following content: top banner, webview and bottom banner. for some reason, i can't do it. my xml look like:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout android:id="#+id/FrameLayout02"
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content" android:layout_height="wrap_content"
android:orientation="vertical"
android:fitsSystemWindows="true" android:isScrollContainer="false">
<ImageView android:layout_width="wrap_content" android:layout_above="#+id/Web"
android:layout_height="fill_parent" android:layout_gravity="top"
android:src="#drawable/logo"></ImageView>
<WebView android:layout_gravity="center" android:id="#+id/Web"
android:layout_height="fill_parent" android:layout_width="fill_parent"></WebView>
<ImageView android:layout_width="wrap_content" android:layout_below="#+id/Web"
android:layout_height="fill_parent" android:layout_gravity="bottom"
android:src="#drawable/logo"></ImageView>
</LinearLayout>
what is wrong?
Thanks,
Daniel

Short answer all is wrong. Your code is a mess.
Long answer. You are trying to have an ImageView above a WebView and at the bottom another ImageView. I'm not exactly sure what of the mistakes broke it but that's also not important. Take a look at my example XML code.
<?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">
<ImageView android:id="#+id/topimage"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:src="#drawable/icon"/>
<ImageView android:id="#+id/bottomimage"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:src="#drawable/icon"/>
<WebView android:id="#+id/web"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_below="#id/topimage"
android:layout_above="#id/bottomimage"/>
</RelativeLayout>
Now what my XML does is simple. It uses a RelativeLayout as a container to allow a more flexible child alignment. The ImageView called topimage will adjust its height to its content and fill the parent in width. It is aligned to its parents top. The second ImageView is similar with the only exception that it is pinned to its parents bottom. The WebView is aligned in between both ImageViews. It adjust it's height to the 'ImageView`s heights.
Try to always keep some kind of structure in your XML and also Java code so you and also others can throw a look at it without starting to cry.

Android: can't remove vertical gap between images in LinearLayout

I have a simple LinearLayout in Android with two images vertically:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res/com.eataly.android"
android:orientation="vertical"
android:background="#android:color/white"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<ImageView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:src="#drawable/img_header1"
/>
<ImageView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:src="#drawable/img_header2"
/>
</LinearLayout>
As you can see at the following link, I can't get rid of a a gap at the top and at the bottom of the images:
http://img185.imageshack.us/img185/8484/senzanomev.png
I've tried everything imaginable on the LinearLayout and the ImageViews, i.e.:
android:padding="0px"
android:top="0px"
android:bottom="0px"
android:top="0px"
android:layout_margin="0px"
with no success.
What can I do to remove these empty borders?

I found the solution. I needed to add this attribute to the ImageView:
android:adjustViewBounds="true"

add android:scaleType="fitXY" in xml layout.

To do this in a linearlayout
for the top ImageView: android:gravity="bottom"
for the bottom ImageView: android:gravity="top"
Are you sure you don't want to be using a RelativeLayout? If you're trying to align two images to each other, that seems the way to go for down the road imho.