Place ImageView over Button android

Issue

I am trying to place an ImageView over a Button using RelativeLayout. Here is my xml:

<RelativeLayout
                android:layout_width="0dip"
                android:layout_height="match_parent"
                android:layout_margin="10dp"
                android:layout_weight="0.50" >

                <Button
                    android:id="@+id/btnFindDaysInBetween"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:background="@color/blue_500"
                    android:text="@string/dt_text_days" />

                <ImageView
                    android:id="@+id/imageview_find_days_in_between"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_centerInParent="true"
                    android:contentDescription="@string/empty"
                    android:src="@drawable/ic_check_circle_white" />
            </RelativeLayout>

Here is the image screenshot:

As you can see, the ImageView's src image is not visible. However if i change the button at the back to an ImageView, the image of the top ImageView is visible. Please refer below..

Changed xml:

 <RelativeLayout
                android:layout_width="0dip"
                android:layout_height="match_parent"
                android:layout_margin="10dp"
                android:layout_weight="0.50" >

                <!-- 
                <Button
                    android:id="@+id/btnFindDaysInBetween"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:background="@color/blue_500"
                    android:text="@string/dt_text_days" />
                -->
                <ImageView
                    android:id="@+id/imageview_find_days"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_centerInParent="true"
                    android:contentDescription="@string/empty"
                    android:src="@drawable/ic_send_black" />

                <ImageView
                    android:id="@+id/imageview_find_days_in_between"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_centerInParent="true"
                    android:contentDescription="@string/empty"
                    android:src="@drawable/ic_check_circle_white" />
            </RelativeLayout>

Changed xml's screenshot:

What is it that i am doing wrong in the first layout?

Solution

The reason is actually very simple. :) We are so caught up thinking in 2D that we overlook the elevation - in Z.

There is nothing wrong with your first layout. The Button simply has a higher elevation than the ImageView - exactly 1dp higher. Therefore, no matter how you arrange the two views, the Button rises above.

A bit of proof:

A Button, by default gets the Widget.Material.Button style:

<!-- Bordered ink button -->
<style name="Widget.Material.Button">
    <item name="background">@drawable/btn_default_material</item>
    <item name="textAppearance">?attr/textAppearanceButton</item>
    <item name="minHeight">48dip</item>
    <item name="minWidth">88dip</item>
    <item name="stateListAnimator">@anim/button_state_list_anim_material</item>
    <item name="focusable">true</item>
    <item name="clickable">true</item>
    <item name="gravity">center_vertical|center_horizontal</item>
</style>

The attribute that introduces this elevation is android:stateListAnimator. StateListAnimator is similar to StateListDrawable, and provides state change animations. The complete xml is here: Link. But here's the base state of the button:

<!-- base state -->
<item android:state_enabled="true">
    <set>
        <objectAnimator android:propertyName="translationZ"
                        android:duration="@integer/button_pressed_animation_duration"
                        android:valueTo="0"
                        android:startDelay="@integer/button_pressed_animation_delay"
                        android:valueType="floatType"/>
        <objectAnimator android:propertyName="elevation"
                        android:duration="0"
                        android:valueTo="@dimen/button_elevation_material"
                        android:valueType="floatType" />
    </set>
</item>

As you can see, the elevation value for the button is set to @dimen/button_elevation_material:

<dimen name="button_elevation_material">1dp</dimen>

And that's how the ImageView ends up being behind/below the Button.

So, what can we do?

A straight-forward solution would be to set the ImageView's elevation to the same amount - 1dp.

Another solution, which will require a bit of work, is to remove the Button's elevation rather than change ImageView's. Based on the default StateListAnimator, we can create our own - and remove the elevation. Then, in your res/values-v21/styles.xml, define a style that inherits from Widget.Material.Button:

<style name="MyDepressedButtonStyle" parent="android:Widget.Material.Button">
    <item name="android:stateListAnimator">@anim/customized_state_animator</item>
</style>

Now, set this style on your Button:

<Button
    style="@style/MyDepressedButtonStyle"
    ....
    .... />

Edit:

Actually, we can apply the customized StateListAnimator directly:

<Button
    android:stateListAnimator="@anim/customized_state_animator"
    ....
    .... />

No need to take the scenic route!

Answered By - Vikram