Android: The mystery of ConstraintLayout

Recently I was working on a simple login layout. Nothing really special a ConstraintLayout which has LinearLayout, a few Edit Text, Text View and Button. A very common use case for most applications nowadays. So I took the first EditText from the palette and dropped it into the ConstraintLayout connecting it’s constrained and setting the width to 0dp

<TextView
    android:id="@+id/text_view_application_name"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginStart="@dimen/margin_normal"
    android:layout_marginEnd="@dimen/margin_normal"
    android:layout_marginBottom="@dimen/margin_normal"
    android:text="@string/text_application_name"
    android:textAlignment="center"
    android:textSize="@dimen/size_text_title"
    app:layout_constraintBottom_toTopOf="@+id/linearLayout"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

Looks pretty standard right? But let’s pay attention to the lines below

<TextView
    ...
    android:layout_width="0dp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

From the official Google documentation

Build a Responsive UI with ConstraintLayout | Android Developers

we can read that ConstrainLayout size can be adjusted as follows

• Fixed: You specify a specific dimension in the text box below or by resizing the view in the editor.
• Wrap Content: The view expands only as much as needed to fit its contents.
• Match Constraints: The view expands as much as possible to meet the constraints on each side (after accounting for the view’s margins). However, you can modify that behavior with the following attributes and values (these attributes take effect only when you set the view width to match constraints)

And just after this, we are reading following warring

Note: You cannot use match_parent for any view in a ConstraintLayout. Instead use "match constraints" (0dp).

Interesting, so Google is not allowing us to use match_parent , but what if I set match_parent instead of 0dp and remove constraints to end and start , will I get Lint errors, will my UI look broken on the devices, what will happen 🤔?

<TextView
    ...
    android:layout_width="match_parent"
    a̶p̶p̶:̶l̶a̶y̶o̶u̶t̶_̶c̶o̶n̶s̶t̶r̶a̶i̶n̶t̶E̶n̶d̶_̶t̶o̶E̶n̶d̶O̶f̶=̶"̶p̶a̶r̶e̶n̶t̶"̶
    a̶p̶p̶:̶l̶a̶y̶o̶u̶t̶_̶c̶o̶n̶s̶t̶r̶a̶i̶n̶t̶S̶t̶a̶r̶t̶_̶t̶o̶S̶t̶a̶r̶t̶O̶f̶=̶"̶p̶a̶r̶e̶n̶t̶"̶ />

For now, I can only see an error in the layout editor

but when I run the application on the phone UI is not broken and Lint is also not complaining about the match_parent or missing constrains.

Okay if everything works fine, then maybe there is some performance impact when using match_parent , I thought. I found a very good article about ConstrainLayout performance

Constraint Layout performance

In the article 👆 you can find source code, to check your layout performance, I checked out the source code, modified it a bit and run the performance tests, for ConstraintLayout with match_parent and with 0dp and constraints. The results were pretty interesting

I used the latest version of ConstraintLayout 2.0.0. Each test was run 1 000 times and shown results are the average of them, as you can see the view without constraints is rendered faster than the one with constraints, which is a bit surprising as it’s not a recommended way of using it.

Maybe Nicolas Roard can help, to understand how this works and why I should not use the match_parent even if the performance is a bit better?

If someone knows the answer please leave it in the comments 🥰