Develop Reference

RecyclerView content goes behind and not over TextView above

Consider this video from material design on drag & dropping items:
Material design reordering list
In the video you see a RecyclerView and above it a TextView with the text "Playlist". When the row is dragged up you see it going over the Playlist.
In my code the row goes behind the Textview. I've put RecyclerView's layout_height to match_parent. And I used a FrameLayout and placed the TextView below the RecyclerView. Why isn't it going over it?
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="#+id/layoutRoot"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:id="#+id/txtTitle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="#dimen/general_margin"
android:text="#string/activity_bible_order_explanation"
android:layout_gravity="start|top" />
<androidx.recyclerview.widget.RecyclerView
android:id="#+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#android:color/transparent"
android:layout_marginTop="96dp"
android:orientation="vertical"
app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" />
</FrameLayout>

Use Constraint layout as a parent layout and add textview at the top by giving top constraints to the textview. then add
recyclerview's top constraint to the the bottom of textview and recyclerview bottom constraint to the bottom of parent then set height 0 it will work
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="#+id/layoutRoot"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:id="#+id/txtTitle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintTop_toTopOf="parent"
android:padding="#dimen/general_margin"
android:text="#string/activity_bible_order_explanation"
/>
<androidx.recyclerview.widget.RecyclerView
android:id="#+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="0dp"
app:layout_constraintTop_toBottomOf="#id/txtTitle"
android:background="#android:color/transparent"
android:layout_marginTop="96dp"
android:orientation="vertical"
app:layout_constraintBottom_toBottomOf="parent"
app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" />
</androidx.constraintlayout.widget.ConstraintLayout>

Have you tried elevation? the problem might be that in the z axis the TextView is higher than the RecyclerView. try adding android:elevation="10dp" or higher... like this:
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="#+id/layoutRoot"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:id="#+id/txtTitle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="#dimen/general_margin"
android:text="#string/activity_bible_order_explanation"
android:layout_gravity="start|top" />
<androidx.recyclerview.widget.RecyclerView
android:id="#+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:elevation="10dp" (if it doesn't work try maybe higher elevation, 20dp etc.)
android:background="#android:color/transparent"
android:layout_marginTop="96dp"
android:orientation="vertical"
app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" />
</FrameLayout>
Hope it works for you (: let me know!

What is the parent layout of this?
Because you set match_parent in recyclerview and list have many content that's why this issue facing.
Please give fix height or use relative layout and apply position above of text view in recyclerview

Problem with your layout code:
You're using a FrameLayout
Inside FrameLayout, you're also using top margin on your RecyclerView
android:layout_marginTop="96dp" <-- This causing the issue.
Solution:
You should LinearLayout instead of FrameLayout. And, also set LinearLayout's orientation as vertical.
android:orientation="vertical"
Also remove this line from your RecyclerView:-
android:layout_marginTop="96dp"
Solution Code:
<?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-auto"
android:id="#+id/layoutRoot"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:id="#+id/txtTitle"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="16dp"
android:text="Hello World"/>
<androidx.recyclerview.widget.RecyclerView
android:id="#+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#android:color/transparent"
app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" />
</LinearLayout>

The issue is not that the dragged view is going behind the TextView but, rather, the dragged view is not being drawn outside the bounds of the RecyclerView. The effect is the same.
To allow the RecyclerView to draw outside of itself, set the following for the parent of the RecyclerView:
android:clipChildren="false"
For the RecyclerView set
android:clipToPadding="false"
That should solve the problem.

The RecyclerView is above the text view in your FrameLayout. Currently, it appears that the underlying TextView is above the RecyclerView as you have given the background for the RecyclerView as:
android:background="#android:color/transparent"
To make this work as normal, either remove this line or change the background of RecyclerView to any value other than transparent.

There is 1 more issue. You need to place the recycler view first. Try this:
<?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-auto"
android:id="#+id/layoutRoot"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<androidx.recyclerview.widget.RecyclerView
android:id="#+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#android:color/transparent"
app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" />
<TextView
android:id="#+id/txtTitle"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="16dp"
android:text="Hello World"/>
</LinearLayout>

How do i use a scrollview?

I want to put a scroll view to scroll the content on my screen, but I can't understand what I'm doing wrong here. Any help would be highly appreciated. Do I need to put my constraint layout in a linear layout and then that linear view in the scroll view or I'm doing something else wrong here?
<ScrollView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:fillViewport="true">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<android.support.constraint.ConstraintLayout
android:id="#+id/layout_fragment_head"
android:layout_width="0dp"
android:layout_height="0dp"
android:background="#color/color_theme_main_dark"
app:layout_constraintBottom_toTopOf="#+id/guideline15"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent">
// I edited the code here for simplicity
</android.support.constraint.ConstraintLayout>
<fragment
android:id="#+id/fragment_deviceslist"
android:name="com.resatech.android.scoutandroid.master.fragments.DevicesListFragment"
android:layout_width="0dp"
android:layout_height="0dp"
app:defaultNavHost="true"
app:layout_behavior="#string/appbar_scrolling_view_behavior"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="#+id/guideline15"
tools:layout="#layout/fragment_devices_list" />
<android.support.constraint.Guideline
android:id="#+id/guideline15"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent=".3" />
</android.support.constraint.ConstraintLayout>
</ScrollView>

You should not put a ConstraintLayout inside a ScrollView. Otherwise, objects inside ConstraintLayout cannot scroll because they are constrained.
Put the ScrollView inside the ConstraintLayout, then, all objects inside ScrollView can be scrolled (because they are not constrained).
In this case, the only constrained view is the ScrollView and this is correct.
If you want a fixed header, put it outside the ScrollView constrained to top of the ConstraintLayout.

in ScrollView just add one layer!
To fix your problem edit your code like this :
<ScrollView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:fillViewport="true">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
/* Your Code of Constraint Layout Like :
<RelativeLayout />
<ImageView />
<LinerLayout/>
and .... */
</RelativeLayout>
</ScrollView>

Android - how to make a scrollable constraintlayout?

I want to make a layout that lets me scroll down using constraint layout, but I don't know how to go about it. Should the ScrollView be the parent of the ConstraintLayout like this?
<?xml version="1.0" encoding="utf-8"?>
<ScrollView 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"
android:fillViewport="true">
<android.support.constraint.ConstraintLayout
android:id="#+id/Constraint"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
Or the other way around? Maybe someone can point me to a good tutorial on this or give an example, I can't seem to find one.
Also, I don't know if this is a bug or some configuration that I don't have set up but I've seen images like this one :
where there are some components outside the blueprint "blue rectangle" yet they are visible, while on my side if I place a component on the "white space" I can't see it or move it anywhere, and it appears on the component tree.
UPDATE :
I found a way to make the constraint layout scrollable in the design tool, using a horizontal guideline to push down the constraint layout border and extend it beyond the device, after that, you can use the guideline as the new bottom of the constraint layout to anchor the components.

It seems that it is working, I don't know what dependency you were working with but in this one
compile 'com.android.support.constraint:constraint-layout:1.0.2'
Is working, this is what I did
<?xml version="1.0" encoding="utf-8"?>
<ScrollView 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"
tools:context=".MainActivity">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<android.support.design.widget.TextInputLayout
android:id="#+id/til_input"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:hint="Escriba el contenido del archivo"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toLeftOf="#+id/btn_save"
app:layout_constraintTop_toTopOf="#id/btn_save"
app:layout_constraintVertical_chainStyle="spread">
<EditText
android:layout_width="match_parent"
android:layout_height="match_parent" />
</android.support.design.widget.TextInputLayout>
<Button
android:id="#+id/btn_save"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="onClickButtonSave"
android:text="Guardar"
app:layout_constraintLeft_toRightOf="#+id/til_input"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="#+id/txt_content"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="0dp"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#id/til_input"
app:layout_constraintVertical_chainStyle="spread"
app:layout_constraintVertical_weight="1" />
<Button
android:id="#+id/btn_delete"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:onClick="onClickButtonDelete"
android:text="Eliminar"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#id/txt_content"
app:layout_constraintVertical_chainStyle="spread" />
</android.support.constraint.ConstraintLayout>
</ScrollView>
Scroll Top
Scroll Bottom

There is a type of constraint which breaks the scroll function:
Just make sure you are not using this constraint on any view when wanting your ConstraintLayout to be scrollable with ScrollView :
app:layout_constraintBottom_toBottomOf=“parent”
If you remove these your scroll should work.
Explanation:
Setting the height of the child to match that of a ScrollView parent is contradictory to what the component is meant to do. What we want most of the time is for some dynamic sized content to be scrollable when it is larger than a screen/frame; matching the height with the parent ScrollView would force all the content to be displayed into a fixed frame (the height of the parent) hence invalidating any scrolling functionality.
This also happens when regular direct child components are set to layout_height="match_parent".
If you want the child of the ScrollView to match the height of the parent when there is not enough content, simply set android:fillViewport to true for the ScrollView.

Use NestedScrollView with viewport true is working good for me
<android.support.v4.widget.NestedScrollView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fillViewport="true">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="700dp">
</android.support.constraint.ConstraintLayout>
</android.support.v4.widget.NestedScrollView>
for android x use this
<androidx.core.widget.NestedScrollView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fillViewport="true">
<androidx.constraintlayout.widget.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
.....other views....
</androidx.constraintlayout.widget.ConstraintLayout>
</androidx.core.widget.NestedScrollView>

To summarize, you basically wrap your android.support.constraint.ConstraintLayout view in a ScrollView within the text of the *.xml file associated with your layout.
Example activity_sign_in.xml
<?xml version="1.0" encoding="utf-8"?>
<ScrollView 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"
tools:context=".SignInActivity"> <!-- usually the name of the Java file associated with this activity -->
<android.support.constraint.ConstraintLayout
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"
android:background="#drawable/gradient"
tools:context="app.android.SignInActivity">
<!-- all the layout details of your page -->
</android.support.constraint.ConstraintLayout>
</ScrollView>
Note 1: The scroll bars only appear if a wrap is needed in any way, including the keyboard popping up.
Note 2: It also wouldn't be a bad idea to make sure your ConstraintLayout is big enough to the reach the bottom and sides of any given screen, especially if you have a background, as this will ensure that there isn't odd whitespace. You can do this with spaces if nothing else.

Just use constraint layout inside NestedScrollView or ScrollView.
<android.support.v4.widget.NestedScrollView
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">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#color/white">
</android.support.constraint.ConstraintLayout>
</android.support.v4.widget.NestedScrollView>
thats it. enjoy your coding.

A lot of answers here, nothing really simple. It's important that the ScrollView's layout_height is set to match_parent while the layout_height of the ContraintLayout is wrap_content
<ScrollView
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<androidx.constraintlayout.widget.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
...

Please use below solution it has taken my lots of time to fix.
Enjoy your time :)
<?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"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#color/white"
>
<ScrollView
android:id="#+id/mainScroll"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#color/white"
android:fillViewport="true"
android:layout_alignParentTop="true"
android:layout_alignParentEnd="true"
android:layout_alignParentStart="true">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom"
android:layout_alignParentStart="true"
android:layout_alignParentTop="true"
android:layout_alignParentBottom="true"
android:layout_alignParentEnd="true"
>
</android.support.constraint.ConstraintLayout>
</RelativeLayout>
</ScrollView>
</RelativeLayout>
Use Exactly like this u will definitely find your solution...

TO make a scrollable layout, the layout is correct. It will not be scrollable until there is reason to scroll(just like in any other layout). So add enough content and it will be scrollable, just like with any layout(Linear, Relative, etc). However, you cannot scroll properly in Blueprint or design-mode when designing with ConstraintLayout and ScrollView.
Meaning:
You can make a scrollable ConstraintLayout, but it will not scroll properly in the editor due to a bug/scenario that wasn't considered. But even though scrolling doesn't work in the editor, it works on devices. (I have made several scrolling COnstraintLayouts, so I have tested it)
Note
Regarding your code. The ScrollView is missing a closing tag, I don't know if it is the case in the file or if it is a copy-paste miss, but you may want to look at it.

For completing the previous answers I am adding the following example, which also takes into account the use of the AppBar. With this code, the Android Studio design editor seems to work fine with the ConstraintLayout.
<?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-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
android:background="#drawable/bg"
android:orientation="vertical">
<android.support.design.widget.AppBarLayout
android:id="#+id/app_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="#style/AppTheme.ActionBar.AppOverlayTheme">
<android.support.v7.widget.Toolbar
android:id="#+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
app:popupTheme="#style/AppTheme.PopupOverlay"/>
</android.support.design.widget.AppBarLayout>
<ScrollView
android:layout_width="match_parent"
android:layout_height="wrap_content">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<ImageView
android:id="#+id/image_id"
android:layout_width="match_parent"
android:layout_height="#dimen/app_bar_height"
android:fitsSystemWindows="true"
android:scaleType="centerCrop"
android:src="#drawable/intro"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="parent" />
<TextView
android:id="#+id/desc_id"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="#dimen/text_margin"
android:text="#string/intro_desc"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/image_id" />
<Button
android:id="#+id/button_scan"
style="?android:textAppearanceSmall"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:backgroundTint="#color/colorAccent"
android:padding="8dp"
android:text="#string/intro_button_scan"
android:textStyle="bold"
app:layout_constraintTop_toBottomOf="#+id/desc_id" />
<Button
android:id="#+id/button_return"
style="?android:textAppearanceSmall"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginTop="8dp"
android:backgroundTint="#color/colorAccent"
android:padding="8dp"
android:text="#string/intro_button_return"
android:textStyle="bold"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintTop_toBottomOf="#+id/button_recycle" />
<Button
android:id="#+id/button_recycle"
style="?android:textAppearanceSmall"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:backgroundTint="#color/colorAccent"
android:padding="8dp"
android:text="#string/intro_button_recycle"
android:textStyle="bold"
app:layout_constraintTop_toBottomOf="#+id/button_scan" />
</android.support.constraint.ConstraintLayout>
</ScrollView>
</LinearLayout>

you need surrounded my constraint-layout with a ScrollView tag and gave it the property android:isScrollContainer="true".

Take out bottom button from the nestedscrollview and take linearlayout as parent. Add bottom and nestedscrollview as thier children. It will work absolutely fine. In manifest for the activity use this - this will raise the button when the keyboard is opened
android:windowSoftInputMode="adjustResize|stateVisible"
<?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-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<androidx.core.widget.NestedScrollView xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:fillViewport="true">
<androidx.constraintlayout.widget.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<com.google.android.material.textfield.TextInputLayout
android:id="#+id/input_city_name"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginStart="20dp"
android:layout_marginTop="32dp"
android:layout_marginEnd="20dp"
android:hint="#string/city_name"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintTop_toTopOf="parent">
<com.google.android.material.textfield.TextInputEditText
android:id="#+id/city_name"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:digits="abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ"
android:lines="1"
android:maxLength="100"
android:textSize="16sp" />
</com.google.android.material.textfield.TextInputLayout>
</androidx.constraintlayout.widget.ConstraintLayout>
</androidx.core.widget.NestedScrollView>
<Button
android:id="#+id/submit"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#color/colorPrimary"
android:onClick="onSubmit"
android:padding="12dp"
android:text="#string/string_continue"
android:textColor="#FFFFFF"
app:layout_constraintBottom_toBottomOf="parent" />
</LinearLayout>

There is a bug in version 2.2 that makes it impossible to scroll the ConstraintLayout. I guess it still exists. You can use LinearLayout or RelativeLayout alternatively.
Also, check out: Is it possible to put a constraint layout inside a ScrollView.

Constraintlayout is the Default for a new app. I am "learning to Android" now and had a very hard time figuring out how to handle the default "sample" code to scroll when a keyboard is up. I have seen many apps where I have to close the keyboard to click "submit" button and sometimes it does not goes away. Using this [ScrollView / ContraintLayout / Fields] hierarchy it is working just fine now. This way we can have the benefits and ease of use from ConstraintLayout in a scrollable view.

You can use HorizontalScrollView and it'll work as well!

This is how I resolved it:
If you are using Nested ScrollView i.e. ScrollView within a ConstraintLayout then use the following configuration for the ScrollView instead of "WRAP_CONTENT" or "MATCH_PARENT":
<ScrollView
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintBottom_toTopOf="#+id/someOtherWidget"
app:layout_constraintTop_toTopOf="parent">

in scrollview make height and width 0
add Top_toBottomOfand Bottom_toTopOf constraints
that's it.

For me, none of the suggestions about removing bottom constraints nor setting scroll container to true seemed to work. What worked: expand the height of individual/nested views in my layout so they "spanned" beyond the parent by using the "Expand Vertically" option of the Constraint Layout Editor as shown below.
For any approach, it is important that the dotted preview lines extend vertically beyond the parent's top or bottom dimensions

How to place a view exactly above the center of its parent view/layout in android

What I want to achieve is that I want to place a View "above" the center of its parent layout. consider the image below:
The blue rectangle it where I want my View to be. as you can see it is exactly above the (vertical)center of the screen.
The challenge is that we don't know the height of the view (so I can't use a bottom margin for it) and I want to do it in layout xml file (so I can't use code to determine the height of the view and set it a margin programmatically)
with these restrictions, is there any way to do this?

You can follow the following layout to place your view at the desired location-
<?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">
<View android:id="#+id/dummy_view"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_centerInParent="true"
/>
<View
android:id="#+id/your_view"
android:layout_width="20dp"
android:layout_height="20dp"
android:layout_above="#+id/dummy_view"
android:layout_centerHorizontal="true"
/>
</RelativeLayout>

I recommend you to use ConstraintLayout, it can be done using guidelines easily (instead of using a View as the anchor point):
XML example:
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<android.support.constraint.Guideline
android:id="#+id/mid_guideline"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent="0.5"
/>
<View
android:layout_width="50dp"
android:layout_height="50dp"
android:background="#F00"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintBottom_toTopOf="#+id/mid_guideline"
/>
</android.support.constraint.ConstraintLayout>

Is it possible to place one view over another in android?

Can we place a small view over another large view? For example, I have a VideoView which is playing a file in the background. Over this, somewhere in the middle/corner, I want to place another ImageView.
But in Linear/Relative Layout, views can be placed only one after another or relative to each other, and AbsoluteLayout is advised against. So what do I do?

The FrameLayout is the simplest ViewGroup and stacks the Views in the order they're defined in layout XML (or added programmatically); the first will be lower, and the last will be on top.
Here is an example where two Views are stacked and offset to better illustrate the point:
Here is the actual layout XML with the two overlapping TextView boxes. The offset of the two boxes is done using android:layout_gravity while android:gravity is used for centering the text itself within each box.
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="100dp"
android:layout_height="100dp">
<TextView
android:layout_width="60dp"
android:layout_height="60dp"
android:layout_gravity="top|left"
android:background="#android:color/holo_blue_light"
android:gravity="center"
android:text="First is below"/>
<TextView
android:layout_width="60dp"
android:layout_height="60dp"
android:layout_gravity="bottom|right"
android:background="#android:color/holo_green_light"
android:gravity="center"
android:text=" Last is on top"/>
</FrameLayout>

Just in case if you want to place a view on top of a ButtonView then use this;
android:elevation="7dp" for the view which needs to be placed on top of the button.

FrameLayouts let you pile each view on top of the one below. This can also be achieved with a RelativeLayout.

You can also do it by using ConstraintLayout a new layout introduced by google.
ConstraintLayout allows you to create large and complex layouts with a flat view hierarchy (no nested view groups).
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
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:id="#+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:layout_weight="4"
tools:context="com.edalat.example.MainActivity">
<VideoView
android:id="#+id/videoView"
android:layout_width="283dp"
android:layout_height="349dp"
app:layout_constraintBottom_toBottomOf="parent"
android:layout_marginBottom="24dp"
app:layout_constraintTop_toTopOf="parent"
android:layout_marginTop="24dp"
android:layout_marginRight="24dp"
app:layout_constraintRight_toRightOf="parent"
android:layout_marginLeft="24dp"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintHorizontal_bias="0.509"/>
<ImageView
android:id="#+id/imageView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:srcCompat="#mipmap/ic_launcher"
app:layout_constraintTop_toTopOf="parent"
android:layout_marginTop="24dp"
app:layout_constraintBottom_toBottomOf="parent"
android:layout_marginBottom="24dp"
android:layout_marginLeft="24dp"
app:layout_constraintLeft_toLeftOf="parent"
android:layout_marginRight="24dp"
app:layout_constraintRight_toRightOf="parent"/>
</android.support.constraint.ConstraintLayout>

<?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"
android:padding="#dimen/dp_20"
android:background="#color/yellow"
>
<VideoView
android:id="#+id/videoview"
android:layout_width="wrap_content"
android:layout_centerInParent="true"
android:layout_height="300dp"
android:contentDescription="#string/app_name"
/>
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="#drawable/camera"
android:layout_margin="30dp"
android:layout_alignParentEnd="true"
android:layout_centerVertical="true"
android:id="#+id/imageView" />
</RelativeLayout>

Create Relative Layout and place views inside it and add
android:elevation="2dp"
for the view whcich is to be over another one

It can be done easily by putting the back layouts before forwarded layouts in XML file.
for example if I want to cardView1 comes over cardView2 I write this code:
<androidx.cardview.widget.CardView
android:id="#+id/cardView2"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<androidx.cardview.widget.CardView
android:id="#+id/cardView1"
android:layout_width="match_parent"
android:layout_height="wrap_content">
Since cardView2 comes before cardView1 , cardView1 is the one that comes over the other one.