Develop Reference

I don't understand how this is working (MATCH_PARENT at View with layout_weight)

When I set MATCH_PARENT for View + layout_weight the elements of the view behave strangely. Please can you explain why this is so. For example, here is the code on which to experiment. I can't understand the pattern. The less put the weight of the item, so it is more, however, it is unclear how many times.
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
android:weightSum="5"
tools:context=".MainActivity">
<LinearLayout
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#FF0"
android:orientation="horizontal">
</LinearLayout>
<LinearLayout
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#F00"
android:orientation="horizontal">
</LinearLayout>
<LinearLayout
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#F0F"
android:orientation="horizontal">
</LinearLayout>
<LinearLayout
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#0FF"
android:orientation="horizontal">
</LinearLayout>
<LinearLayout
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#00F"
android:orientation="horizontal">
</LinearLayout>
</LinearLayout>
Testing for Yellow layout: set layout_weight= 0.5 > large?? how much bigger? Did the elements shrink in size or were they thrown out of the parent?
<LinearLayout
android:layout_weight="0.5"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#F00"
android:orientation="horizontal">
</LinearLayout>
Update, PS: I know how the layout will work with WRAP_CONTENT (or 0dp). If you set MATCH_PARENT + layout_weight, then we have a fixed element size regardless of the content, when WRAP_CONTENT does not guarantee a fixed size from the content (and if the content size is increased, the element will be stretched). I am only interested in the pattern MATCH_PARENT + layout_weight, because it guarantees the block size when the content size is exceeded.

The less put the weight of the item, so it is more...
The reason is because you are assigning android:layout_width as match_parent to all LinearLayout. To get android:layout_weight worked, you should set android:layout_width as 0.

First of all, give android:weightSum to the parent of the layouts.
Secondly, if you have layouts placed horizontally next to each other then set android:layout_width="0dp" for each element.
Similarly, for vertically aligned elements, give android:layout_height="0dp".

Right way to have a scrollable bottom view

What is the right way to have a scrollable bottom view?
<ScrollView 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:fillViewport="true">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#color/white"
android:orientation="vertical">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
style="#style/wrnTextP1"
android:text="Ok , batatas "/>
<android.support.v4.widget.Space
android:layout_height="0dp"
android:layout_width="match_parent"
android:layout_weight="1" />
<Button
style="#style/FullWidthButton"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:text="Chega por hoje!" />
</LinearLayout>
at the preview its looke fine, but when I run it on the emulator the view(in this case: the button) its not at bottom at all.

I think you are misunderstanding how ScrollView is meant to be used. The LinearLayout child of your ScrollView has a match_parent height... if your ScrollView is the same height as the screen, and the LinearLayout is the same height as the ScrollView, then there's nothing to scroll (all the content is the size of the screen).
The direct child of a ScrollView should either have a fixed height (like 2000dp) or use wrap_content.
If you use a fixed height, then the rest of your code "works"; you can have a Space element that uses layout_weight to push the last view down to the bottom of your LinearLayout. However, if you're using wrap_content, this is impossible (since there's no "extra" space for the layout_weight to consume).
Instead, you could consider putting the "bottom" view outside of the ScrollView. Something like this:
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<ScrollView
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1">
...
</ScrollView>
<Button
style="#style/FullWidthButton"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Chega por hoje!" />
</LinearLayout>

recycleview takes all the space over relative layout

I have a layout like this
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:padding="16dp"
>
<android.support.v7.widget.RecyclerView
android:layout_height="match_parent"
android:layout_width="match_parent"
android:id="#+id/rv"
>
</android.support.v7.widget.RecyclerView>
<RelativeLayout
android:id="#+id/relative_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
>
<com.google.android.gms.ads.AdView
android:id="#+id/adView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
ads:adSize="BANNER"
ads:adUnitId="#string/banner_ad_unit_id" />
</RelativeLayout>
</LinearLayout>
I want the admob view to take the space and then all the remaining view should be taken by the recycleview.
What's happening is recycleview takes all the space and admobview is not shown at all, unless I put hard coded height for recyleview,,
What's the best way to fit this relativelayout or just the admob view below the recycleview without hardcoding..?

You can use a RelativeLayout instead of a LinearLayout. Within the relative layout, align the AdView along the bottom of the screen, and put the RecyclerView on top of the AdView.

<android.support.v7.widget.RecyclerView
android:layout_weight="1"
android:layout_height="0dp"
android:layout_width="match_parent"
android:id="#+id/rv"
>
when you put match parent it will take all the room, but specifying weight will allow it to expand. You also don't need RelativeLayout here as well.

HorizontallScrollView with auto-fitting pages

I am trying to create a HorizontalScrollView which will be exactly 3 times the screen width. I have tried the following code, but the scrollview will not scroll. How can I set it so that the 3 LinearLayouts inside the main LinearLayout are each the width of the screen?
<HorizontalScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fillViewport="true">
<!-- the container for all the pages -->
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="100dp"
android:background="#000000"/>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="100dp"
android:background="#FF0000"/>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="100dp"
android:background="#FF0000"/>
</LinearLayout>
Note that if I give the LinearLayouts a specific width, the scrollview does scroll, but I need each LinearLayout to have the width of the screen.

You will need to use wrap_content on your top LinearLayout, and set the width of the child LinearLayouts programmatically.

Undesired FrameLayout Stretch to Wrap All Children's Heights (Instead of Respecting Just One Child's Height)

Basically, I am using a popular tactic to center a background image for a LinearLayout by wrapping it in a FrameLayout and adding an ImageView that uses fill_parent before the LinearLayout. However, when I add a ScrollView in the game, everything changes.
The issues is that the image is quite large vertically and the ScrollView respects the height of both the ImageView and the LinearLayout due to the wrap_content height attribute. This causes an undesired blank space below the LinearLayout when the image is vertically larger than it.
How do I make the FrameLayout's height stretch only to the child LinearLayout's height? If possible, without mangling with its sizes programmatically on run-time/draw-time.
<ScrollView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:fillViewport="true" >
<FrameLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content" >
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="#drawable/cash_bg" />
<LinearLayout
android:id="#+id/main_ll"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:orientation="vertical" >
<!-- My main views are here -->
</LinearLayout>
</FrameLayout>
</ScrollView>

I had the same problem and gone nuts searching for an answer. I think this is an Android bug. What I have done is to include the FrameLayout into a LinearLayout to be able to force "fill_parent" on FrameLayout's height, which increases the complexity, but solves the problem. Did not find any other (better) solution.
Try this:
<ScrollView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:fillViewport="true" >
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<FrameLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent" >
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="#drawable/cash_bg" />
<LinearLayout
android:id="#+id/main_ll"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:orientation="vertical" >
<!-- My main views are here -->
</LinearLayout>
</FrameLayout>
</LinearLayout>
</ScrollView>