首先，这是基于api 21的elevation来实现的阴影，21以下的机型暂不讨论

阴影是绘制在控件外部，所以，首先要保证该控件的直属parent有足够的空间让其绘制阴影。（如果是非直属的，则parent的clipChildren和clipPadding需要设置为false）

看完本篇文章，你将会：

1.知道如何控制elevation来实现自己的阴影

2.知道如何控制elevation的阴影的alpha（感觉这个是重点，当初使用cardView就是UI认为太重，无法调节alpha而最终自己用shadow绘制

3.控制阴影的同时，也具有圆角效果

首先，我们需要两个属性，一个是圆角大小，一个是alpha的值，直接定义style为

<declare-styleable name="ShadowLayout">
        <attr name="shadow_radius" format="dimension" />
        <attr name="shadow_alpha" format="float" />
</declare-styleable>

然后，简单实现下圆角代码。（为何这样实现，不再复述）

public class ShadowLayout extends FrameLayout {
    private RectF mRectF;
    private Path mPath;
    private Paint mPaint;
    private float mRadius;
    private float mAlpha;

    public ShadowLayout(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        final TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.ShadowLayout, 0, 0);
        mRadius = a.getDimensionPixelSize(R.styleable.ShadowLayout_shadow_radius, 0);
        mAlpha = a.getFloat(R.styleable.ShadowLayout_shadow_alpha, 1f);
        if (mAlpha < 0) {
            mAlpha = 0;
        } else if (mAlpha > 1f) {
            mAlpha = 1f;
        }
        a.recycle();
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
        mRectF = new RectF();
        mPath = new Path();
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        mRectF.set(0, 0, getWidth(), getHeight());
        mPath.addRect(mRectF, Path.Direction.CCW);
        mPath.addRoundRect(mRectF, mRadius, mRadius, Path.Direction.CW);
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        final int layerId = canvas.saveLayer(mRectF, null, Canvas.ALL_SAVE_FLAG);
        super.dispatchDraw(canvas);
        canvas.drawPath(mPath, mPaint);
        canvas.restoreToCount(layerId);
    }
}

然后，布局文件如下

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

    <dora.felix.shadowlayoutdemo.ShadowLayout
        android:layout_width="150dp"
        android:layout_height="150dp"
        android:elevation="10dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:shadow_alpha="0.5"
        app:shadow_radius="16dp">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#00ffff"
            android:gravity="center"
            android:text="Hello World!" />
    </dora.felix.shadowlayoutdemo.ShadowLayout>

</android.support.constraint.ConstraintLayout>

看下效果如下：

圆角.png

嗯嗯，圆角很ok，但是，阴影哪去了？
别急，网上说了，要设置背景，才能显示，于是，在初始化完毕，直接，设置个背景就行了
setBackground(new ColorDrawable(Color.WHITE));
再看下效果

设置颜色背景的阴影效果.png

public class RoundDrawable extends Drawable {
    private Paint mPaint;
    private float mRadius;
    private float mAlpha;
    private RectF mRectF;

    public RoundDrawable(float radius, float alpha) {
        mRadius = radius;
        mAlpha = alpha;
        mRectF = new RectF();
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
        mPaint.setColor(Color.WHITE);
    }

    @Override
    public void setColorFilter(@Nullable ColorFilter colorFilter) {
        mPaint.setColorFilter(colorFilter);
    }

    @Override
    public int getOpacity() {
        return PixelFormat.TRANSLUCENT;
    }

    @Override
    public void setAlpha(int alpha) {
        mPaint.setAlpha(alpha);
    }

    @Override
    public void setBounds(@NonNull Rect bounds) {
        super.setBounds(bounds);
        mRectF.set(bounds);
    }

    @Override
    public void draw(@NonNull Canvas canvas) {
        canvas.drawRoundRect(mRectF, mRadius, mRadius, mPaint);
    }
}

当然，为了保证自己的drawable不会被动到，直接重写android.view.View#setBackground，代码如下：

    @Override
    public void setBackground(Drawable background) {
        if (background instanceof RoundDrawable) {
            super.setBackground(background);
        } else {
            super.setBackground(new RoundDrawable(mRadius, mAlpha));
        }
    }

然后，继续运行下效果

自定义Drawable.png

    @Override
    public void getOutline(@NonNull Outline outline) {
        super.getOutline(outline);
        outline.setRoundRect(getBounds(), mRadius);
        outline.setAlpha(mAlpha);
    }

第一个设置，可以得到阴影效果，alpha设置就可以直接设置阴影的alpha了，直接看效果图

alhpa=0.5.png

alpha=0.15.png