I have a layout activity in my project in which I am using a constraint layout and basically only 3 view, first is custom toolbar second a Recycler View and last one is a fragment view. I constrained recycler view to top toolbar and top of fragment view but I goes below fragment and even under screen. This is my layout 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"
    android:theme="@style/Theme.Music.Font">


    <androidx.appcompat.widget.Toolbar
        android:id="@ id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <TextView
            android:id="@ id/header_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:fontFamily="@font/bold"
            android:text="@string/app_name"
            android:textSize="26sp" />

        <androidx.appcompat.widget.SearchView
            android:id="@ id/searchView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="end"
            app:searchIcon="@drawable/search_icon" />
    </androidx.appcompat.widget.Toolbar>


    <androidx.recyclerview.widget.RecyclerView
        android:id="@ id/listView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:scrollbarThumbVertical="@drawable/scroll_bar"
        android:scrollbars="vertical"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/toolbar" />
    
    <androidx.fragment.app.FragmentContainerView
        android:id="@ id/nowPlaying"
        android:name="com.example.music.NowPlaying"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        tools:layout="@layout/fragment_now_playing" />

</androidx.constraintlayout.widget.ConstraintLayout>

And when I constrained with fragment also it goes on top overriding toolbar. As shown in picture.

CodePudding user response：

When you use wrap_content for the height or width, you're saying the view should expand to be as large as necessary to "display" the content. "Display" in air quotes because if there's a lot of content, parts of the view might end up off the screen.

Your constraints don't limit the size here, just the positioning. If you constrain one edge to something, you'll pin it, and it will expand off the screen at the other end. If you constrain opposite edges, then you're basically centering it around those constraints.

If you want to match those constraints, you need to set the height to 0dp. That way your constrained edges will actually be pinned where you want them, and that makes the view a certain height. It fits the space between its constrained edges.

Looking at your layout, I'm guessing the "now playing" part at the bottom is your FragmentContainerView. That's set to wrap_content but its contents seem to be a fixed size, so that's fine - exactly what you want! Same goes for the toolbar - it's wrap_content, but its contents are a fixed size.

So your RecyclerView also needs its bottom to be constrained to the top of nowPlaying, and it needs a layout_height of 0dp. That way, it's constrained to the space between the toolbar and the now playing bit, and its size fills that area. The other two take up as much space as they need, and the RecyclerView fills what's left - which is what you want, right?

Generally this is how it works for anything that scrolls, like a RecyclerView or ScrollView - the view in the layout is a window into the scrolling content, so the actual size of the view (layout_height and layout_width) controls the size of that window. The content just scrolls up and down behind it. If that window is the same size as the content, wrap_content, then the whole thing is visible (but possibly off the screen) and there's nothing to scroll! So for scrolling stuff, you always want to limit the size of the view.