Using <include> with <merge> in ConstraintLayout

问题

I am having trouble using tags <include> and <merge> inside a ConstraintLayout.

I want to create a flat view hierarchy (hence Constraints) but still have elements that are reusable. So I use <include> in my layout and <merge> in the included layouts to avoid having nested layouts (especially avoiding nested ConstraintLayouts)

So I wrote this: Parent layout

<android.support.constraint.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <include
        android:id="@+id/review_1"
        layout="@layout/view_movie_note"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/review_2"/>

    <include
        layout="@layout/view_movie_note"
        android:id="@+id/review_2"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginLeft="7dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toRightOf="@+id/review_1"
        app:layout_constraintRight_toRightOf="parent"
        />

</android.support.constraint.ConstraintLayout>

and this view_movie_note :

<merge>

    <TextView
        android:id="@+id/note_origin"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="15dp"
        android:layout_marginStart="5dp"
        app:layout_constraintStart_toStartOf="@+id/cardView2"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginLeft="5dp" />


    <android.support.v7.widget.CardView
        android:id="@+id/five_star_view_container"
        android:layout_width="0dp"
        android:layout_height="52dp"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="10dp"
        android:elevation="3dp"
        app:cardUseCompatPadding="true"
        app:contentPaddingTop="22dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHeight_min="52dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/note_origin">

        <FiveStarsView
            android:id="@+id/five_star_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal" />

    </android.support.v7.widget.CardView>

    <android.support.v7.widget.CardView
        android:id="@+id/cardView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        app:cardBackgroundColor="@color/colorPrimary"
        app:contentPaddingLeft="15dp"
        app:contentPaddingRight="15dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/note_origin">

        <TextView
            android:id="@+id/grade"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="12sp" />

    </android.support.v7.widget.CardView>


</merge>

I am expecting this

Instead I got this

Clearly the constraints that I put in the <include> tag are overriden by the constraints in the included layout.

Is this the expected behaviour ? If yes, how are we supposed to keep a flat layout using <include> and ConstraintLayout ?

回答1:

Short answer

The best move will be replacing <merge> block with a (nested) ConstraintLayout rather than using redundant layout structure.

---

ConstraintLayout is great but it doesn't work well with composition and separation of responsibilities of each piece

That is wrong. ConstraintLayout does work well with reusing layouts. Any layout in which all child views are laid out according to relationships between sibling views and the parent layout, behaves exactly like this. This is true even for RelativeLayout.

Then, where is the problem?

Let's take a closer look at what <merge> is.

The doc says

The <merge/> tag helps eliminate redundant view groups in your view hierarchy when including one layout within another.

It will have the same effect as replacing the <include> element with the contents of <merge> block. In other words, the views in the <merge/> block is directly placed to the parent layout without an intermediate view group. Therefore, the constraints of the <include> element is completely ignored.

In this particular example, the views in the including layout is added two times to the parent as the second one on top of another.

Conclusion

Layout resource files are intended to be used independently. To qualify the term reusable, it should not depend on it's parent (The view group in which it will be added in future). It would be looking okay if you had to include the layout only one time. But </merge> won't be a good idea in that case too because you can't place it in any different layout in a different position.

Obviously, flat layout hierarchies have better performance. However, sometimes we may have to sacrifice it.

回答2:

Android documentation says

The <merge /> tag helps eliminate redundant view groups in your view hierarchy when including one layout within another

and has an example too

If your main layout is a vertical LinearLayout in which two consecutive views can be re-used in multiple layouts, then the re-usable layout in which you place the two views requires its own root view. However, using another LinearLayout as the root for the re-usable layout would result in a vertical LinearLayout inside a vertical LinearLayout. The nested LinearLayout serves no real purpose other than to slow down your UI performance.

Also see this answer, which will make you understand merge tag more.

Problem in your layout

For the child layout

You put constraints on child elements inside <merge tag. That's not okay. Because that constraints are destroyed at run time when both child layout are merged inside your parent layout. (You tell me if you can do this without include tag, will your constraints work?)

For parent layout

Same for <include tag, you are giving constraints/custom attributes to <include tag, that will be lost, because <merge tag is joined to the root view, so you can not apply custom attributes to the <include with <merge tag. That's why Bahman answer will work.

Attributes on <include tag works when you have root element inside child layout and no <merge tag.

Conclusion

As this is clear, you are not using <merge and <include, as it should be. You have understand what <include and <merge tag do. So use them appropriately.

If you ask solution

ConstraintLayout was introduced to solve complex layout. Not to increase complexity. So when you can do this easily with LinearLayout why to choose Constraints.

Parent Layout

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <include
        android:id="@+id/review_1"
        layout="@layout/view_movie_note"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        />

    <include
        android:id="@+id/review_2"
        layout="@layout/view_movie_note"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginLeft="7dp"
        android:layout_weight="1"
        />

</LinearLayout>

view_movie_note.xml

<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="wrap_content"
    android:layout_height="wrap_content">

    <TextView
     .../>

    <android.support.v7.widget.CardView
    ...
    </android.support.v7.widget.CardView>

    <android.support.v7.widget.CardView
    ...
    </android.support.v7.widget.CardView>
</android.support.constraint.ConstraintLayout>

I hope I could make you understand well.

回答3:

Wrap include tags with ConstraintLayout tags then move attributes of include tags to these new ConstraintLayout tags:

    <android.support.constraint.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    <android.support.constraint.ConstraintLayout
            android:id="@+id/review_1"
            android:layout_width="0dp"
            android:layout_height="0dp"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toLeftOf="@+id/review_2">

                    <include  layout="@layout/view_movie_note"  />

   </android.support.constraint.ConstraintLayout>

    <android.support.constraint.ConstraintLayout
            android:id="@+id/review_2"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_marginLeft="7dp"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toRightOf="@+id/review_1"
            app:layout_constraintRight_toRightOf="parent">

                       <include layout="@layout/view_movie_note" />

    </android.support.constraint.ConstraintLayout>

  </android.support.constraint.ConstraintLayout>

回答4:

As a solution

Parent 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:layout_width="match_parent"
 android:layout_height="match_parent">


<android.support.v7.widget.LinearLayoutCompat
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:weightSum="2">

    <include
        android:id="@+id/review_1"
        layout="@layout/view_movie_note"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/review_2"
        app:layout_constraintTop_toTopOf="parent" />

    <include
        android:id="@+id/review_2"
        layout="@layout/view_movie_note"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="7dp"
        android:layout_weight="1"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toRightOf="@+id/review_1"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

 </android.support.v7.widget.LinearLayoutCompat>


</android.support.constraint.ConstraintLayout>

view_movie_note

<?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="wrap_content">

<TextView
    android:id="@+id/note_origin"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="15dp"
    android:layout_marginLeft="5dp"
    android:layout_marginStart="5dp"
    app:layout_constraintStart_toStartOf="@+id/cardView2"
    app:layout_constraintTop_toTopOf="parent" />


<android.support.v7.widget.CardView
    android:id="@+id/five_star_view_container"
    android:layout_width="wrap_content"
    android:layout_height="52dp"
    android:layout_marginBottom="8dp"
    android:layout_marginTop="10dp"
    android:elevation="3dp"
    app:cardUseCompatPadding="true"
    app:contentPaddingTop="22dp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHeight_min="52dp"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/note_origin">

    <!--<FiveStarsView-->
    <!--android:id="@+id/five_star_view"-->
    <!--android:layout_width="wrap_content"-->
    <!--android:layout_height="wrap_content"-->
    <!--android:layout_gravity="center_horizontal" />-->

    <RatingBar
        android:id="@+id/ratingBar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</android.support.v7.widget.CardView>

<android.support.v7.widget.CardView
    android:id="@+id/cardView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="20dp"
    app:cardBackgroundColor="@color/colorPrimary"
    app:contentPaddingLeft="15dp"
    app:contentPaddingRight="15dp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="@+id/note_origin">

    <TextView
        android:id="@+id/grade"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="12sp" />

  </android.support.v7.widget.CardView>
</android.support.constraint.ConstraintLayout>

回答5:

merge is a tag and not a ViewGroup, so all the parameter passed to the include will be ignored... You can flat this ViewGroup only with a duplicated layout, if you need to manage it, you can create a Group... XML attributes from merge layout to RelativeLayout via inflate

回答6:

file view_movie_note.xml should have a single xml node at it's root - else there won't be two layout nodes which could be aligned with constraints (the <merge> tag might be useless). having two identical layouts with identical resId adds to the problem - because constraints are always being defined relative to a resId, which is not unique here.

alternative approaches would be, a) to use a LinearLayout in order to distribute two items horizontally - or b) in case there should even be some more child nodes (which I'd assume), better use CardViews in a GridLayout with two columns, within a GridView / RecyclerView.

a single CardView as the root node of such a child-layout would be the least to align properly.

according to the expected result, a ConstraintLayout is not required to align nodes just alike that - with the GridLayoutManager one can even adjust the column count according to the display size. A StaggeredGridLayoutManager would also be an option, when the height of the elements varies.

in principle, it is always easier to work with the framework, than to work against the framework.

回答7:

Some issues with your question:

1. As per android documentation link

You can also override all the layout parameters (any android:layout_* attributes) of the included layout's root view by specifying them in the <include/> tag So any constraint you put into include tag will be removed.

2. Any android:id in include will NOT be overridden if merge tag is used in your included layout.

3. Chaining and adding constraint works on views with different ids. So for including same view multiple times with equal weight will not work via include tag.

That being said, you can either copy paste the entire

Therefore, you can not use include in this fashion.

You are left with 3 options:

1. Use some other ViewGroup (LinearLayout and then constraint layout for example)
2. Copy paste the content of include layout with different ids of views
3. Modify ConstraintLayout code to support spread-chains so that entire included layout is copied horizontally.

IMO, 1st option is best if you have a small number of these layouts, 2nd option is best if you have only one layout (asked in question) and 3rd option is best if you have large number of layouts.

回答8:

You can use FrameLayout to include common layouts. Try out this:

<?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">

<FrameLayout
    android:id="@+id/review_1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toLeftOf="@+id/review_2"
    app:layout_constraintTop_toTopOf="parent">

    <include
        layout="@layout/view_movie_note"
        android:layout_width="0dp"
        android:layout_height="0dp" />
</FrameLayout>

<FrameLayout
    android:id="@+id/review_2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toRightOf="@+id/review_1"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent">

    <include
        layout="@layout/view_movie_note"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginLeft="7dp" />
</FrameLayout>
</android.support.constraint.ConstraintLayout>

Hope, it will help you.

来源：https://stackoverflow.com/questions/46447621/using-include-with-merge-in-constraintlayout