protected void dispatchDraw(Canvas canvas) {
super.dispatchDraw(canvas);
}

这个方法是ViewGroup在绘制子View的时候调用的，那么我们可以在这个时候进行阴影的绘制。

首先，这个方法已经为我们提供了这个View的画布：Canvas，我们可以直接在上面进行阴影的绘制，代码如下：

Paint shadowPaint = new Paint();
shadowPaint.setColor(Color.RED);
shadowPaint.setStyle(Paint.Style.FILL);
shadowPaint.setAntiAlias(true);
float left = 45;
float top = 45;
float right = getWidth() - 45;
float bottom = getHeight() - 45;

shadowPaint.setShadowLayer(45, 0, 0, getContext().getResources().getColor(R.color.color_000000));
RectF rectF = new RectF(left, top, right, bottom);
canvas.drawRoundRect(rectF, 0, 0, shadowPaint);
canvas.save();

• 创建画笔，设置画笔的颜色，风格
• 获取绘画的范围：ShadowCard的范围减去需要的阴影的范围，假如阴影的宽度为45px，则在ShadowCard内部的45px内进行绘制
• 给画笔设置阴影的颜色，阴影的模糊度，模糊度值越大越模糊，且不能为0
• 创建RectF，最后开始绘画。

这样阴影就可以成功绘制了，这个方法代码量很少，很简单，也很实用。

为了更好的封装，我们可以为上面需要的参数进行定制，比如阴影的颜色，阴影的宽度，阴影的上下偏移，阴影的模糊度。

代码如下：

public class ShadowViewCard extends FrameLayout {
private static final int DEFAULT_VALUE_SHADOW_COLOR = R.color.shadow_default_color;
private static final int DEFAULT_VALUE_SHADOW_CARD_COLOR = R.color.shadow_card_default_color;
private static final int DEFAULT_VALUE_SHADOW_ROUND = 0;

private static final int DEFAULT_VALUE_SHADOW_RADIUS = 10;
private static final int DEFAULT_VALUE_SHADOW_TOP_HEIGHT = 5;
private static final int DEFAULT_VALUE_SHADOW_LEFT_HEIGHT = 5;
private static final int DEFAULT_VALUE_SHADOW_RIGHT_HEIGHT = 5;
private static final int DEFAULT_VALUE_SHADOW_BOTTOM_HEIGHT = 5;
private static final int DEFAULT_VALUE_SHADOW_OFFSET_Y = 0;
private static final int DEFAULT_VALUE_SHADOW_OFFSET_X = DEFAULT_VALUE_SHADOW_TOP_HEIGHT / 3;

private int shadowRound;
private int shadowColor;
private int shadowCardColor;
private int shadowRadius;
private int shadowOffsetY;
private int shadowOffsetX;
private

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int shadowTopHeight;
private int shadowLeftHeight;
private int shadowRightHeight;
private int shadowBottomHeight;

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

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

public ShadowViewCard(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
initView(context, attrs);
}

private void initView(Context context, AttributeSet attrs) {
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ShadowViewCard);
shadowRound = a.getDimensionPixelSize(R.styleable.ShadowViewCard_shadowRound, DEFAULT_VALUE_SHADOW_ROUND);
shadowColor = a.getColor(R.styleable.ShadowViewCard_shadowColor, getResources().getColor(DEFAULT_VALUE_SHADOW_COLOR));
shadowCardColor = a.getColor(R.styleable.ShadowViewCard_shadowCardColor, getResources().getColor(DEFAULT_VALUE_SHADOW_CARD_COLOR));
shadowTopHeight = a.getDimensionPixelSize(R.styleable.ShadowViewCard_shadowTopHeight, dp2px(getContext(), DEFAULT_VALUE_SHADOW_TOP_HEIGHT));
shadowRightHeight = a.getDimensionPixelSize(R.styleable.ShadowViewCard_shadowRightHeight, dp2px(getContext(), DEFAULT_VALUE_SHADOW_RIGHT_HEIGHT));
shadowLeftHeight = a.getDimensionPixelSize(R.styleable.ShadowViewCard_shadowLeftHeight, dp2px(getContext(), DEFAULT_VALUE_SHADOW_LEFT_HEIGHT));
shadowBottomHeight = a.getDimensionPixelSize(R.styleable.ShadowViewCard_shadowBottomHeight, dp2px(getContext(), DEFAULT_VALUE_SHADOW_BOTTOM_HEIGHT));
shadowOffsetY = a.getDimensionPixelSize(R.styleable.ShadowViewCard_shadowOffsetY, dp2px(getContext(), DEFAULT_VALUE_SHADOW_OFFSET_Y));
shadowOffsetX = a.getDimensionPixelSize(R.styleable.ShadowViewCard_shadowOffsetX, dp2px(getContext(), DEFAULT_VALUE_SHADOW_OFFSET_X));
shadowRadius = a.getInteger(R.styleable.ShadowViewCard_shadowRadius, DEFAULT_VALUE_SHADOW_RADIUS);
a.recycle();
setPadding(shadowLeftHeight, shadowTopHeight, shadowRightHeight, shadowBottomHeight);
setLayerType(LAYER_TYPE_SOFTWARE, null);
}

public static int dp2px(Context context, float dipValue) {
final float scale = context.getResources().getDisplayMetrics().density;
return (int) (dipValue * scale + 0.5f);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
}

@Override
protected void dispatchDraw(Canvas canvas) {
Paint shadowPaint = new Paint();
shadowPaint.setColor(Color.WHITE);
shadowPaint.setStyle(Paint.Style.FILL);
shadowPaint.setAntiAlias(true);
float left = shadowLeftHeight;
float top = shadowTopHeight;
float right = getWidth() - shadowRightHeight;
float bottom = getHeight() - shadowBottomHeight;
shadowPaint.setShadowLayer(shadowRadius, shadowOffsetX, shadowOffsetX, shadowColor);
RectF rectF = new RectF(left, top, right, bottom);
canvas.drawRoundRect(rectF, shadowRound, shadowRound, shadowPaint);
canvas.save();
F = new RectF(left, top, right, bottom);
canvas.drawRoundRect(rectF, shadowRound, shadowRound, shadowPaint);
canvas.save();