Android how to create gradient shadow on left and right of view?

问题

I cannot use Lollipop elevation property because I need to target devices from API 18.

I need to place a shadow on the right and left side of a view.

This is what I tried so far :

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <item>
        <shape >
            <!-- set the shadow color here -->
            <stroke
                android:width="2dp"
                android:color="#7000" />

            <!-- setting the thickness of shadow (positive value will give shadow on that side) -->

            <padding
                android:bottom="0dp"
                android:left="2dp"
                android:right="2dp"
                android:top="0dp" />

            <corners android:radius="3dp" />
        </shape>
    </item>

    <!-- Background -->

    <item>
        <shape>
            <solid android:color="@color/salmon" />
            <corners android:radius="3dp" />
        </shape>
    </item>
</layer-list>

I tried adding gradients in the first item, but it spreads from left to right which is not what I am looking fo. Is there a way to isolate two unique gradients for each side ?

This is what I am trying to do :

回答1:

You can try using a gradient contained within a rectangle, which you can size: example below is a shadow I used for Navigation Drawer. You can modify this to fit your needs.

<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <gradient android:startColor="#111"
        android:endColor="#00000000">
    </gradient>
    <size
        android:height="@dimen/activity_vertical_margin"
        android:width="5dp">
    </size>
</shape>

Alternitively, you can use a CardView from v7 CardView support library, which supports Android api 7+.

回答2:

You need to create your own drawable for it.

<?xml version="1.0" encoding="utf-8"?>
    <shape xmlns:android="http://schemas.android.com/apk/res/android"
           android:shape="rectangle">
        <gradient
            android:angle="270"
            android:endColor="@android:color/white"
            android:startColor="@color/black_38">
        </gradient>
        <size

            android:height="4dp">
        </size>
    </shape>

And then for example with help of relative layout to align to any side of your layout. For me it's bottom shadow of custom app bar.

...
    <View
            android:layout_alignParentBottom="true"
            android:layout_width="match_parent"
            android:background="@drawable/app_bar_shadow"
            android:layout_height="4dp"/> // height of your shadow

来源：https://stackoverflow.com/questions/34401334/android-how-to-create-gradient-shadow-on-left-and-right-of-view