工具栏不会在设备上的应用中投射阴影（Android Studio）

Question

I have a toolbar layout and mainlayout . In my mainlayout I include my toolbar layout :

Toolbar layout :

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/tool_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@color/PrimaryButtonsAndHeadscolor"
android:titleTextAppearance="@color/ColorPrimaryDark"
app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
android:elevation="8dp">
</android.support.v7.widget.Toolbar>

My main layout (the part with the toolbar):

  <LinearLayout


    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainScreen"
    android:orientation="vertical">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:minHeight="?attr/actionBarSize"
        >

      >
    <include
    android:id="@+id/tool_bar"
    layout="@layout/tool_bar"
    ></include>
    </LinearLayout>

On andorid studio at the "preview" screen my tool bar cast a shadow , but when I run it on my device it doesn't. I know that elevation attribute is suitable for device running on api 21+ , I am using lgg-4 api22 so it should cast shadow beneath the toolbar (I also see shadow beneath the toolbar in other apps ). What I am doing worng ?

thanks !

Answer 1

Okay , so finally I found the problem : my mistake was the fact I put my toolbar in an internal LinearLayout (and thats somehow Cancelling the elevation effect when running on device).

Previous Layout :

    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainScreen"
    android:orientation="vertical">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:minHeight="?attr/actionBarSize"
        >
            >
            <include
                android:id="@+id/tool_bar"
                layout="@layout/tool_bar"
                ></include>
    </LinearLayout>

New layout :

    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainScreen"
    android:orientation="vertical">

            >
            <include
                android:id="@+id/tool_bar"
                layout="@layout/tool_bar"
                ></include>

It works,there is a shadow on my toolbar !