Develop Reference

Button not visible in AVD

Making an app. Last 2 button are visible in preview mode , but not in AVD(Google pixel 3a). Actually I'm new to Android. I just directly pasted the code if need more information let me know. Thank you
Snippet of code
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
tools:context=".MainActivity">
<Button
android:layout_width="match_parent"
android:layout_height="match_parent"
android:text="#string/courses_offered"
android:layout_marginTop="470dp"
android:layout_marginBottom="140dp"
android:layout_marginRight="30dp"
android:layout_marginLeft="230dp"
android:textColor="#color/white"
android:backgroundTint="#color/orange"
/>
<Button
android:id="#+id/Fee_structure"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:text="#string/fee_structure"
android:layout_marginTop="470dp"
android:layout_marginBottom="140dp"
android:layout_marginRight="230dp"
android:layout_marginLeft="30dp"
android:textColor="#color/white"
android:backgroundTint="#color/orange"
/>
</androidx.constraintlayout.widget.ConstraintLayout>

You're using ConstraintLayout but your buttons doesn't have any constraint to initialize where they should be
For example:
adding app:layout_constraintBottom_toBottomOf="parent" will position the bottom of the button to the bottom of parent, in this case, your layout
See this for further details and examples: https://developer.android.com/reference/androidx/constraintlayout/widget/ConstraintLayout?authuser=1

You have to use constraints in the Constraint layout just like
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintBottom_toBottomOf="parent"
enter link description here
Try this code it will show you in AVD.
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
>
<Button
android:id="#+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:backgroundTint="#color/teal_200"
android:text="asd"
android:textColor="#color/white"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.8"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.8" />
<Button
android:id="#+id/Fee_structure"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:backgroundTint="#color/teal_200"
android:text="asd"
android:textColor="#color/white"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintHorizontal_bias="0.2"
app:layout_constraintVertical_bias="0.8" />
</androidx.constraintlayout.widget.ConstraintLayout>

You have given height and width to match parent so your first button is taking all the space available in screen so you have to make them wrap content (height) and then constraint them accordingly

Android Activity_Main Float Button

This is Acitivity_main.xml of a PDF Reader on Android.
I am confused on how to add two floating Action Buttons(Page Up/Down Button) in the bottom right corner. The buttons need to display above PDF, and uses linear vertical layout.
Following XML separates PDF and Buttons into two parts which is wrong, any idea how to correct it?
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
tools:context=".MainActivity">
<LinearLayout
android:id="#+id/pdfstructure"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_alignParentBottom="true"
android:orientation="vertical"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent">
<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="#+id/DownButton"
android:layout_width="100dp"
android:layout_height="200dp"
android:layout_gravity="end|bottom"
android:layout_marginTop="600dp"
android:layout_marginRight="30dp"
android:clickable="true"
android:contentDescription="#string/upButton"
android:stateListAnimator="#null"
android:text="Down"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent" />
<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="#+id/upButton"
android:layout_width="100dp"
android:layout_height="200dp"
android:layout_gravity="end|bottom"
android:layout_marginTop="800dp"
android:layout_marginRight="30dp"
android:clickable="true"
android:text="Up"
android:stateListAnimator="#null"
android:contentDescription="#string/upButton"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent" />
</LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

Add this line in your linearLayout
android:gravity="bottom"
and delete those lines in your FloatingActionButton
android:layout_marginTop="600dp"
android:layout_marginTop="800dp"

Place ImageView on top of button in ConstraintLayout

I'm facing issues with constraintlayout when placing ImageView or FrameLayout on top of a Button.
When i try to inflate a fragment on FrameLayout, Buttons that are part of activity are visible on top of fragment UI.
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="button"/>
<FrameLayout
android:background="#color/blue"
android:layout_width="match_parent"
android:layout_height="match_parent">
</FrameLayout>
</android.support.constraint.ConstraintLayout>

To place any view above another view you have to give proper constraint like below.
Give top and bottom constraint proper and also as I give all constraint(start,end,top,bottom) to FrameLayout it's height and width give to 0dp(you can't use height to match_parent otherwise it's cover whole screen and overlap button).
>
<?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"
android:layout_width="match_parent" android:layout_height="match_parent">
<Button
android:id="#+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="button"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintBottom_toBottomOf="parent"/>
<FrameLayout
android:background="#color/colorPrimary"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintBottom_toTopOf="#+id/button"/>
</android.support.constraint.ConstraintLayout>

To set constraints of button and FrameLayout to,
Button:
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
FrameLayout :
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/button"

Your code looks ok but if you want to tell 1 view to be above another view in ConstraintLayout you can go to the design tab and move the higher above the lower view:
Example:
In the image, you can see that the button is placed below the frameLayout so he will be lower than the frameLayout (the frameLayout will be above the button)
Also, you were missing some constraints on your button :
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="button"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="#+id/frameLayout2" />
<FrameLayout
android:id="#+id/frameLayout2"
android:layout_width="match_parent"
android:layout_height="match_parent">
</FrameLayout>
And if all that didn't help you check this thread, as #Xavier Rubio Jansana said, it may be because of elevation.

Try the code below
<?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:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:id="#+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginLeft="8dp"
android:layout_marginTop="8dp"
android:layout_marginEnd="8dp"
android:layout_marginRight="8dp"
android:text="button"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<FrameLayout
android:layout_width="match_parent"
android:layout_height="638dp"
android:layout_marginStart="8dp"
android:layout_marginLeft="8dp"
android:layout_marginEnd="8dp"
android:layout_marginRight="8dp"
android:layout_marginBottom="8dp"
android:background="#color/appGrey"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/button">
</FrameLayout>
</android.support.constraint.ConstraintLayout>

Using ListView inside ConstraintLayout

I have read on Android Developers that ConstraintLayout can be used to design a responsive layout for an application. There is a parent ConstraintLayout which houses a toolbar and two other ConstraintLayouts. The first child ConstraintLayout is going to act as empty view for my ListView. The second ConstraintLayout holds my listview and a floating action button. Currently, the the listview appears under the Toolbar, rather than below it. Also as seen in the screenshot, the floating action button appears outside visible area.
See the screenshot below:
And this is the app layout when the list is empty:
This is the code for my layout:
<?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"
android:id="#+id/content"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
>
<android.support.v7.widget.Toolbar xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="#+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
android:theme="#style/ThemeOverlay.AppCompat.Dark.ActionBar"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintRight_toRightOf="parent"/>
<android.support.constraint.ConstraintLayout
app:layout_constraintTop_toBottomOf="#id/toolbar"
android:id="#+id/empty_view"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="?attr/actionBarSize">
<TextView
app:layout_constraintTop_toTopOf="parent"
android:id="#+id/tv_empty_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/list_is_empty"
android:textSize="#dimen/large_text"
android:textStyle="bold"
/>
<ImageView
android:id="#+id/iv_empty_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintTop_toBottomOf="#id/tv_empty_view"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
android:contentDescription="#string/list_is_empty"
android:src="#drawable/emptybox" />
</android.support.constraint.ConstraintLayout>
<android.support.constraint.ConstraintLayout
android:id="#+id/main_area"
android:layout_marginTop="50dp"
android:layout_marginBottom="?actionBarSize"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintTop_toBottomOf="#id/toolbar"
app:layout_constraintBottom_toBottomOf="parent">
<ListView
android:id="#+id/listview"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginBottom="?attr/actionBarSize"
android:layout_marginLeft="#dimen/margin_side"
android:layout_marginStart="#dimen/margin_side"
android:layout_marginRight="#dimen/margin_side"
android:layout_marginEnd="#dimen/margin_side"
android:layout_marginTop="?attr/actionBarSize"
/>
<android.support.design.widget.FloatingActionButton
android:layout_below="#+id/listview"
android:id="#+id/fab"
android:layout_width="#android:dimen/notification_large_icon_width"
android:layout_height="#android:dimen/notification_large_icon_height"
android:layout_gravity="end|bottom"
android:src="#drawable/plus"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintRight_toRightOf="parent"
/>
</android.support.constraint.ConstraintLayout>
</android.support.constraint.ConstraintLayout>

While using ConstraintLayout you have to add constraints for 'constraintTop',constraintRight,constraintBottom,constraintLeft or constraintStart and constraintEnd. only if you constraint all your four sides the constraint layout (or Constraint Start or End with other References) works well. Otherwise the layout will not work correctly
For Further Reference https://codelabs.developers.google.com/codelabs/constraint-layout/index.html?index=..%2F..%2Findex#0
https://developer.android.com/training/constraint-layout/index.html

There are a couple of things that you can improve with the XML and make the design easier.
First, the main layout will match the screen, to have the preview correctly simulate that, you can set its width/height to match_parent
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="#+id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
>
Then, Android Studio should be giving you warnings/errors and saying The view is not constrained horizontally/vertically. In ConstraintLayout, you have to use constraints to specify how your views are placed. If you don't, by default they will position at 0/0 and most probably will look different when it runs on the device:
<ListView
android:id="#+id/listview"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintBottom_toBottomOf="parent"
android:layout_marginBottom="0dp" />
Now, you should be able to that the main_area overlaps with the toolbar. To fix it, you can change main_area height to match the constrains:
android:id="#+id/main_area"
android:layout_height="0dp"
You should be able to obtain a design similar to what you intended.

Here is your desired result, I've made some changes like margins, src just to make it work in my studio, so you'll have to choose whatever you were using, just replace mine with your src's and margins etc...
<?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"
android:id="#+id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
>
<android.support.v7.widget.Toolbar
android:id="#+id/toolbar"
android:layout_width="0dp"
android:layout_height="56dp"
android:background="?attr/colorPrimary"
android:theme="#style/ThemeOverlay.AppCompat.Dark.ActionBar"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintLeft_toLeftOf="parent"/>
<android.support.constraint.ConstraintLayout
app:layout_constraintTop_toBottomOf="#+id/toolbar"
android:id="#+id/empty_view"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent">
<TextView
app:layout_constraintTop_toTopOf="parent"
android:id="#+id/tv_empty_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
android:text="list_is_empty"
android:textSize="30sp"
android:textStyle="bold"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent" />
<ImageView
android:id="#+id/iv_empty_view"
android:layout_width="20dp"
android:layout_height="20dp"
app:layout_constraintTop_toBottomOf="#+id/tv_empty_view"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
android:background="#fff222" />
</android.support.constraint.ConstraintLayout>
<android.support.constraint.ConstraintLayout
android:id="#+id/main_area"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintTop_toBottomOf="#+id/toolbar"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent">
<ListView
android:id="#+id/listview"
android:layout_width="0dp"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent" />
<android.support.design.widget.FloatingActionButton
android:layout_marginEnd="30dp"
android:layout_marginBottom="30dp"
android:id="#+id/fab"
android:layout_width="64dp"
android:layout_height="64dp"
android:src="#drawable/bg"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintRight_toRightOf="parent"
/>
</android.support.constraint.ConstraintLayout>
</android.support.constraint.ConstraintLayout>
Here's the output:

I was also facing the same issue.
I made a mistake of keeping layout_width of ListView as fixed width. I changed it to match_constraint and it's coming correctly without any cut.
For height I was facing similar issue so I changed it to match_constraint and it worked.

Layout_anchor equivalent for ConstraintLayout

How would I go about getting my fab to straddle two layouts in a ConstraintLayout like I could in a CoordinatorLayout?
XML:
<?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:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#eeeeee"
android:orientation="vertical">
<android.support.v7.widget.Toolbar
android:id="#+id/myToolbar"
android:layout_width="384dp"
android:layout_height="56dp"
android:background="#color/colorPrimary"
android:elevation="0dp"
tools:layout_editor_absoluteX="0dp"
tools:layout_editor_absoluteY="0dp"/>
<android.support.design.widget.FloatingActionButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="#dimen/activity_horizontal_margin"
app:layout_anchor="#+id/tvTest"
app:layout_anchorGravity="end|bottom|right"
tools:layout_editor_absoluteX="0dp"
tools:layout_editor_absoluteY="0dp"/>
<CustomViews.FontText
android:id="#+id/tvTest"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginEnd="16dp"
android:layout_marginStart="16dp"
android:layout_marginTop="50dp"
android:text="TestText"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#+id/myToolbar"/>

For Example, You have two Layout ,
Layout 1 -> id="#+id/above_layout" ,
Layout 2 -> id="#+id/below_layout" ,
Then , And you want to set Fab Action Button in between Layout 1 and Layout 2,
app:layout_constraintRight_toRightOf="parent"
-> This will take view to right side
app:layout_constraintBottom_toBottomOf="#+id/above_layout",
app:layout_constraintTop_toBottomOf="#+id/above_layout"
--> This two prop will achieve effect of app:layout_anchor="#+id/tvTest" ,app:layout_anchorGravity="end|bottom|right" , and id's of layout should be same.
<android.support.design.widget.FloatingActionButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="#dimen/activity_horizontal_margin"
android:scaleType="center"
android:src="#drawable/ic_star"
app:fabSize="mini"
app:layout_constraintBottom_toBottomOf="#+id/above_layout"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#+id/above_layout"/>
I hope this helps.