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This is useful when you want to constrain a view to a set of views rather than to one specific view. The only difference between Barrier and Guideline is that Barrier 's position is flexible and always based on the size of multiple UI elements contained within it, whereas Guideline 's position is always fixed.
Guidelines in Constraint Layout are invisible lines that are not visible to users but help developers to design the layout easily and constrain views to these guidelines, so that design can be more clear and interactive.
To define a Barrier , you can select one or more View components from the “Design” view, open the “Guidelines” menu and select the Barrier . If you want to add it directly in the XML, you can use the following code snippet: The resulting layout looks like the screenshot of the “Design” layout editor view from below.
A Barrier references multiple widgets as input, and creates a virtual guideline based on the most extreme widget on the specified side. For example, a left barrier will align to the left of all the referenced views.
Assume you have two TextView widgets with dynamic heights and you want to place a Button just below the tallest TextView:
The ONLY way to implement that directly in the layout is to use a horizontal Barrier. That Barrier allows you to specify a constraint based on the height of those two TextViews. Then you constrain the top of your Button to the bottom of the horizontal Barrier.
<?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">
<TextView
android:id="@+id/left_text_view"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:text="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
android:textSize="16sp"
android:background="#AAA"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toStartOf="@+id/right_text_view"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/right_text_view"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:layout_marginStart="8dp"
android:layout_marginEnd="8dp"
android:text="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
android:textSize="16sp"
android:background="#DDD"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toEndOf="@+id/left_text_view"
app:layout_constraintTop_toTopOf="parent" />
<androidx.constraintlayout.widget.Barrier
android:id="@+id/barrier"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:barrierDirection="bottom"
app:constraint_referenced_ids="left_text_view,right_text_view" />
<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:text="Button"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="@+id/barrier" />
</androidx.constraintlayout.widget.ConstraintLayout>
Assume you want to restrict the above-mentioned TextView heights to 30% of screen height, no matter the content they have.
To implement that you should add horizontal Guideline with percentage position and constrain the TextView bottom to that Guideline.
<?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">
<TextView
android:id="@+id/left_text_view"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="#AAA"
android:text="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
android:textSize="16sp"
app:layout_constraintBottom_toTopOf="@+id/guideline"
app:layout_constraintEnd_toStartOf="@+id/right_text_view"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/right_text_view"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:layout_marginBottom="8dp"
android:background="#DDD"
android:text="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
android:textSize="16sp"
app:layout_constraintBottom_toTopOf="@+id/guideline"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toEndOf="@+id/left_text_view"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:text="Button"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="@+id/guideline" />
<androidx.constraintlayout.widget.Guideline
android:id="@+id/guideline"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent="0.3" />
</androidx.constraintlayout.widget.ConstraintLayout>
The only difference between Barrier and Guideline is that Barrier's position is flexible and always based on the size of multiple UI elements contained within it and Guideline's position is always fixed.
Official documentation on Barrier:
A Barrier references multiple widgets as input, and creates a virtual guideline based on the most extreme widget on the specified side. For example, a left barrier will align to the left of all the referenced views.
Training docs on Barrier:
Similar to a guideline, a barrier is an invisible line that you can constrain views to. Except a barrier does not define its own position; instead, the barrier position moves based on the position of views contained within it. This is useful when you want to constrain a view to the a set of views rather than to one specific view.
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