Android 图片实现阴影效果的若干种方法

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

<!--底层的左边距离上层左边3dp, 底层的顶部，距离上层的顶部6dp,如果不做这个控制，底层和上层的左侧和上侧会重合在一起-->
<item android:left="3dp"
      android:top="6dp">
    <shape>
        <solid android:color="#b4b5b6"/>
    </shape>
</item>

<!--上层的右边距离底层的右边3dp, 上层的底部距离底层的底部6dp-->
<item android:bottom="6dp"
      android:right="3dp">
    <shape>
        <solid android:color="#fff"/>
    </shape>
</item>

</layer-list>

<TextView 
	    android:id="@+id/test_shadow"
	    android:layout_width="wrap_content"
	    android:layout_height="wrap_content"
	    android:textSize="60sp"
	    android:textColor="#cc000000"
	    android:text="Test Shadow"
	    android:layout_gravity="center"
	    android:shadowColor="#aa22ff22"
	    android:shadowRadius="10"
	    android:shadowDx="0"
	    android:shadowDy="0"
	    />

<TextView
android:id="@+id/btn_test_performance"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:elevation="5dp"
android:text="@string/hello"
android:background="@drawable/shape_round_white"
android:padding="20dp"
android:layout_marginTop="10dp"
android:layout_gravity="center"/>

/**
 * ShadowLayout.java
 * <p>
 * Created by lijiankun on 17/8/11.
 */

public class ShadowLayout extends RelativeLayout {

    public static final int ALL = 0x1111;

    public static final int LEFT = 0x0001;

    public static final int TOP = 0x0010;

    public static final int RIGHT = 0x0100;

    public static final int BOTTOM = 0x1000;

    private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);

    private RectF mRectF = new RectF();

    /**
     * 阴影的颜色
     */
    private int mShadowColor = Color.TRANSPARENT;

    /**
     * 阴影的大小范围
     */
    private float mShadowRadius = 0;

    /**
     * 阴影 x 轴的偏移量
     */
    private float mShadowDx = 0;

    /**
     * 阴影 y 轴的偏移量
     */
    private float mShadowDy = 0;

    /**
     * 阴影显示的边界
     */
    private int mShadowSide = ALL;

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

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

    public ShadowLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(attrs);
    }

    /**
     * 获取绘制阴影的位置，并为 ShadowLayout 设置 Padding 以为显示阴影留出空间
     */
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);

        float effect = mShadowRadius + dip2px(5);
        float rectLeft = 0;
        float rectTop = 0;
        float rectRight = this.getWidth();
        float rectBottom = this.getHeight();
        int paddingLeft = 0;
        int paddingTop = 0;
        int paddingRight = 0;
        int paddingBottom = 0;

        if (((mShadowSide & LEFT) == LEFT)) {
            rectLeft = effect;
            paddingLeft = (int) effect;
        }
        if (((mShadowSide & TOP) == TOP)) {
            rectTop = effect;
            paddingTop = (int) effect;
        }
        if (((mShadowSide & RIGHT) == RIGHT)) {
            rectRight = this.getWidth() - effect;
            paddingRight = (int) effect;
        }
        if (((mShadowSide & BOTTOM) == BOTTOM)) {
            rectBottom = this.getHeight() - effect;
            paddingBottom = (int) effect;
        }
        if (mShadowDy != 0.0f) {
            rectBottom = rectBottom - mShadowDy;
            paddingBottom = paddingBottom + (int) mShadowDy;
        }
        if (mShadowDx != 0.0f) {
            rectRight = rectRight - mShadowDx;
            paddingRight = paddingRight + (int) mShadowDx;
        }
        mRectF.left = rectLeft;
        mRectF.top = rectTop;
        mRectF.right = rectRight;
        mRectF.bottom = rectBottom;
        this.setPadding(paddingLeft, paddingTop, paddingRight, paddingBottom);
    }

    /**
     * 真正绘制阴影的方法
     */
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawRect(mRectF, mPaint);
    }

    /**
     * 读取设置的阴影的属性
     *
     * @param attrs 从其中获取设置的值
     */
    private void init(AttributeSet attrs) {
        setLayerType(View.LAYER_TYPE_SOFTWARE, null);  // 关闭硬件加速
        this.setWillNotDraw(false);                    // 调用此方法后，才会执行 onDraw(Canvas) 方法

        TypedArray typedArray = getContext().obtainStyledAttributes(attrs, R.styleable.ShadowLayout);
        if (typedArray != null) {
            mShadowColor = typedArray.getColor(R.styleable.ShadowLayout_shadowColor,
                    ContextCompat.getColor(getContext(), android.R.color.black));
            mShadowRadius = typedArray.getDimension(R.styleable.ShadowLayout_shadowRadius, dip2px(0));
            mShadowDx = typedArray.getDimension(R.styleable.ShadowLayout_shadowDx, dip2px(0));
            mShadowDy = typedArray.getDimension(R.styleable.ShadowLayout_shadowDy, dip2px(0));
            mShadowSide = typedArray.getInt(R.styleable.ShadowLayout_shadowSide, ALL);
            typedArray.recycle();
        }
        mPaint.setAntiAlias(true);
        mPaint.setColor(Color.TRANSPARENT);
        mPaint.setShadowLayer(mShadowRadius, mShadowDx, mShadowDy, mShadowColor);
    }

    /**
     * dip2px dp 值转 px 值
     *
     * @param dpValue dp 值
     * @return px 值
     */
    private float dip2px(float dpValue) {
        DisplayMetrics dm = getContext().getResources().getDisplayMetrics();
        float scale = dm.density;
        return (dpValue * scale + 0.5F);
    }
}

<declare-styleable name="ShadowLayout">
    <attr name="shadowColor" format="color"/>
    <attr name="shadowRadius" format="dimension"/>
    <attr name="shadowDx" format="dimension"/>
    <attr name="shadowDy" format="dimension"/>
    <attr name="shadowSide" format="integer"/>
</declare-styleable>