In XML layout, the button is always displayed on the top layer

I encountered a very strange problem when I was writing a project recently , stay Relativelayout in Button Always at the top , Cannot be overridden by other controls , Then I tried Android 5.0 The following versions can be overwritten normally , after many setbacks , At last I found the reason .

The reasons causing ：stateListAnimator attribute
Google in Material Design Launched in , Is a very simple way to achieve a smooth transition between visual states . This property can be through android:stateListAnimator Set it up , It can make the control produce different interaction when clicking . about Button, When clicked, there is a shadow effect by default to indicate the pressed state （5.0 It used to be simple discoloration ）.
Reference link :android5.0 StateListAnimator

resolvent ：

If Button There are already customized selector Style without the need for native post press shadow effects , have access to android:stateListAnimator=”@null” Remove the shadow effect and make Button Can be covered normally
If you want to keep Button And want it to be covered , You should use a separate Linearlayout Yes Button Package , Remember to give Button And left and right sides of the . Although this method does not conform to the specification , But for the sake of demand, we have to do so .
Code example ：

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="52dp">

                <Button         
                android:id="@+id/btn"
                android:layout_width="match_parent"
                android:layout_height="42dp"
                android:layout_marginLeft="30dp"
                android:layout_marginRight="30dp"
          android:background="@drawable/login_btn_selector"
                android:text="@string/app_login"
                tools:ignore="UselessParent"
                android:textColor="@color/white"
                android:textSize="20sp" />
        </LinearLayout>

source ： Wuziye's blog