效果

ShadowContainer 介绍：
Android 自定义Shadow 样式和size
ShadowContainer属性介绍
阴影的颜色：containerShadowColor
阴影的圆角半径，和View的background圆角保持一致即可：containerCornerRadius
阴影的模糊半径，值越大阴影越大：containerShadowRadius
阴影展示的区域位置：containerDeltaLength
是否展示阴影,通过修改enable可以实现阴影的显示与隐藏，达到类似于选择器的效果：enable
x方向的偏移量：deltaX
y方向的偏移量：deltaY

attrs.xml文件

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="ShadowContainer">
        <attr name="containerShadowColor" format="color"/>
        <attr name="containerShadowRadius" format="dimension"/>
        <attr name="containerDeltaLength" format="dimension"/>
        <attr name="containerCornerRadius" format="dimension"/>
        <attr name="deltaX" format="dimension"/>
        <attr name="deltaY" format="dimension"/>
        <attr name="enable" format="boolean"/>
    </declare-styleable>
</resources>

自定义view

public class ShadowContainer extends ViewGroup {
    private final float deltaLength;
    private final float cornerRadius;
    private final Paint mShadowPaint;
    private boolean drawShadow;

    public ShadowContainer(Context context) {
        this(context, null);
    }

    public ShadowContainer(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public ShadowContainer(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ShadowContainer);
        int shadowColor = a.getColor(R.styleable.ShadowContainer_containerShadowColor, Color.RED);
//        int shadowColor = Color.RED;
        float shadowRadius = a.getDimension(R.styleable.ShadowContainer_containerShadowRadius, 0);
        deltaLength = a.getDimension(R.styleable.ShadowContainer_containerDeltaLength, 0);
        cornerRadius = a.getDimension(R.styleable.ShadowContainer_containerCornerRadius, 0);
        float dx = a.getDimension(R.styleable.ShadowContainer_deltaX, 0);
        float dy = a.getDimension(R.styleable.ShadowContainer_deltaY, 0);
        drawShadow = a.getBoolean(R.styleable.ShadowContainer_enable, true);
        a.recycle();
        mShadowPaint = new Paint();
        mShadowPaint.setStyle(Paint.Style.FILL);
        mShadowPaint.setAntiAlias(true);
        mShadowPaint.setColor(shadowColor);
        mShadowPaint.setShadowLayer(shadowRadius, dx, dy, shadowColor);
    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        if (drawShadow) {
            if (getLayerType() != LAYER_TYPE_SOFTWARE) {
                setLayerType(LAYER_TYPE_SOFTWARE, null);
            }
            View child = getChildAt(0);
            int left = child.getLeft();
            int top = child.getTop();
            int right = child.getRight();
            int bottom = child.getBottom();
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                canvas.drawRoundRect(left, top, right, bottom, cornerRadius, cornerRadius, mShadowPaint);
            } else {
                Path drawablePath = new Path();
                drawablePath.moveTo(left + cornerRadius, top);
                drawablePath.arcTo(new RectF(left, top, left + 2 * cornerRadius, top + 2 * cornerRadius), -90, -90, false);
                drawablePath.lineTo(left, bottom - cornerRadius);
                drawablePath.arcTo(new RectF(left, bottom - 2 * cornerRadius, left + 2 * cornerRadius, bottom), 180, -90, false);
                drawablePath.lineTo(right - cornerRadius, bottom);
                drawablePath.arcTo(new RectF(right - 2 * cornerRadius, bottom - 2 * cornerRadius, right, bottom), 90, -90, false);
                drawablePath.lineTo(right, top + cornerRadius);
                drawablePath.arcTo(new RectF(right - 2 * cornerRadius, top, right, top + 2 * cornerRadius), 0, -90, false);
                drawablePath.close();
                canvas.drawPath(drawablePath, mShadowPaint);
            }
        }
        super.dispatchDraw(canvas);
    }

    public void setDrawShadow(boolean drawShadow){
        if (this.drawShadow == drawShadow){
            return;
        }
        this.drawShadow = drawShadow;
        postInvalidate();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        if (getChildCount() != 1) {
            throw new IllegalStateException("子View只能有一个");
        }
        int measuredWidth = getMeasuredWidth();
        int measuredHeight = getMeasuredHeight();
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        View child = getChildAt(0);
        LayoutParams layoutParams = (LayoutParams) child.getLayoutParams();
        int childBottomMargin = (int) (Math.max(deltaLength, layoutParams.bottomMargin) + 1);
        int childLeftMargin = (int) (Math.max(deltaLength, layoutParams.leftMargin) + 1);
        int childRightMargin = (int) (Math.max(deltaLength, layoutParams.rightMargin) + 1);
        int childTopMargin = (int) (Math.max(deltaLength, layoutParams.topMargin) + 1);
        int widthMeasureSpecMode;
        int widthMeasureSpecSize;
        int heightMeasureSpecMode;
        int heightMeasureSpecSize;
        if (widthMode == MeasureSpec.UNSPECIFIED){
            widthMeasureSpecMode = MeasureSpec.UNSPECIFIED;
            widthMeasureSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        }else {
            if (layoutParams.width == LayoutParams.MATCH_PARENT) {
                widthMeasureSpecMode = MeasureSpec.EXACTLY;
                widthMeasureSpecSize = measuredWidth - childLeftMargin - childRightMargin;
            } else if (LayoutParams.WRAP_CONTENT == layoutParams.width) {
                widthMeasureSpecMode = MeasureSpec.AT_MOST;
                widthMeasureSpecSize = measuredWidth - childLeftMargin - childRightMargin;
            } else {
                widthMeasureSpecMode = MeasureSpec.EXACTLY;
                widthMeasureSpecSize = layoutParams.width;
            }
        }
        if (heightMode == MeasureSpec.UNSPECIFIED){
            heightMeasureSpecMode = MeasureSpec.UNSPECIFIED;
            heightMeasureSpecSize = MeasureSpec.getSize(heightMeasureSpec);
        }else {
            if (layoutParams.height == LayoutParams.MATCH_PARENT) {
                heightMeasureSpecMode = MeasureSpec.EXACTLY;
                heightMeasureSpecSize = measuredHeight - childBottomMargin - childTopMargin;
            } else if (LayoutParams.WRAP_CONTENT == layoutParams.height) {
                heightMeasureSpecMode = MeasureSpec.AT_MOST;
                heightMeasureSpecSize = measuredHeight - childBottomMargin - childTopMargin;
            } else {
                heightMeasureSpecMode = MeasureSpec.EXACTLY;
                heightMeasureSpecSize = layoutParams.height;
            }
        }
        measureChild(child, MeasureSpec.makeMeasureSpec(widthMeasureSpecSize, widthMeasureSpecMode), MeasureSpec.makeMeasureSpec(heightMeasureSpecSize, heightMeasureSpecMode));
        int parentWidthMeasureSpec = MeasureSpec.getMode(widthMeasureSpec);
        int parentHeightMeasureSpec = MeasureSpec.getMode(heightMeasureSpec);
        int height = measuredHeight;
        int width = measuredWidth;
        int childHeight = child.getMeasuredHeight();
        int childWidth = child.getMeasuredWidth();
        if (parentHeightMeasureSpec == MeasureSpec.AT_MOST){
            height = childHeight + childTopMargin + childBottomMargin;
        }
        if (parentWidthMeasureSpec == MeasureSpec.AT_MOST){
            width = childWidth + childRightMargin + childLeftMargin;
        }
        if (width < childWidth + 2 * deltaLength){
            width = (int) (childWidth + 2 * deltaLength);
        }
        if (height < childHeight + 2 * deltaLength){
            height = (int) (childHeight + 2 * deltaLength);
        }
        if (height != measuredHeight || width != measuredWidth){
            setMeasuredDimension(width, height);
        }
    }

    static class LayoutParams extends MarginLayoutParams{

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }

        public LayoutParams(int width, int height) {
            super(width, height);
        }

        public LayoutParams(MarginLayoutParams source) {
            super(source);
        }

        public LayoutParams(ViewGroup.LayoutParams source) {
            super(source);
        }
    }

    @Override
    protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }

    @Override
    protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
        return new LayoutParams(p);
    }

    @Override
    public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        View child = getChildAt(0);
        int measuredWidth = getMeasuredWidth();
        int measuredHeight = getMeasuredHeight();
        int childMeasureWidth = child.getMeasuredWidth();
        int childMeasureHeight = child.getMeasuredHeight();
        child.layout((measuredWidth - childMeasureWidth) / 2, (measuredHeight - childMeasureHeight) / 2, (measuredWidth + childMeasureWidth) / 2, (measuredHeight + childMeasureHeight) / 2);
    }
}

用法

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <example.chenj.com.shadow.ShadowContainer
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:containerDeltaLength="10dp"
        app:containerShadowRadius="10dp"
        app:containerShadowColor="#f00"
        app:enable="true"
        >

        <TextView
            android:layout_width="300dp"
            android:layout_height="200dp"
            android:background="#fff"
            />
    </example.chenj.com.shadow.ShadowContainer>
</LinearLayout>