Android-View阴影Drawable

背景：

CardView是Google的Material Design风格，并且开放出来的自定义api极少，无法满足非Material Design风格的UI阴影设计

public class ShadowDrawable extends Drawable {

  private Paint mShadowPaint;
  private Paint mBgPaint;
  private int mShadowRadius;
  private int mShape;
  private int mShapeRadius;
  private int mOffsetX;
  private int mOffsetY;
  private int mBgColor[];
  private RectF mRect;

  public final static int SHAPE_ROUND = 1;
  public final static int SHAPE_CIRCLE = 2;

  private ShadowDrawable(int shape, int[] bgColor, int shapeRadius, int shadowColor, int shadowRadius, int offsetX, int offsetY) {
      this.mShape = shape;
      this.mBgColor = bgColor;
      this.mShapeRadius = shapeRadius;
      this.mShadowRadius = shadowRadius;
      this.mOffsetX = offsetX;
      this.mOffsetY = offsetY;

      mShadowPaint = new Paint();
      mShadowPaint.setColor(Color.TRANSPARENT);
      mShadowPaint.setAntiAlias(true);
      mShadowPaint.setShadowLayer(shadowRadius, offsetX, offsetY, shadowColor);
      mShadowPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_ATOP));

      mBgPaint = new Paint();
      mBgPaint.setAntiAlias(true);
  }

  @Override
  public void setBounds(int left, int top, int right, int bottom) {
      super.setBounds(left, top, right, bottom);
      mRect = new RectF(left + mShadowRadius - mOffsetX, top + mShadowRadius - mOffsetY, right - mShadowRadius - mOffsetX,
              bottom - mShadowRadius - mOffsetY);
  }

  @Override
  public void draw(@NonNull Canvas canvas) {
      if (mBgColor != null) {
          if (mBgColor.length == 1) {
              mBgPaint.setColor(mBgColor[0]);
          } else {
              mBgPaint.setShader(new LinearGradient(mRect.left, mRect.height() / 2, mRect.right,
                      mRect.height() / 2, mBgColor, null, Shader.TileMode.CLAMP));
          }
      }

      if (mShape == SHAPE_ROUND) {
          canvas.drawRoundRect(mRect, mShapeRadius, mShapeRadius, mShadowPaint);
          canvas.drawRoundRect(mRect, mShapeRadius, mShapeRadius, mBgPaint);
      } else {
          canvas.drawCircle(mRect.centerX(), mRect.centerY(), Math.min(mRect.width(), mRect.height())/ 2, mShadowPaint);
          canvas.drawCircle(mRect.centerX(), mRect.centerY(), Math.min(mRect.width(), mRect.height())/ 2, mBgPaint);
      }
  }

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

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

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

  public static void setShadowDrawable(View view, Drawable drawable) {
      view.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
      ViewCompat.setBackground(view, drawable);
  }

  /**
     * 为指定View添加阴影
     * @param view 目标View
     * @param shapeRadius View的圆角
     * @param shadowColor 阴影的颜色
     * @param shadowRadius 阴影的宽度
     * @param offsetX 阴影水平方向的偏移量
     * @param offsetY 阴影垂直方向的偏移量
     */
  public static void setShadowDrawable(View view, int shapeRadius, int shadowColor, int shadowRadius, int offsetX, int offsetY) {
      ShadowDrawable drawable = new ShadowDrawable.Builder()
              .setShapeRadius(shapeRadius)
              .setShadowColor(shadowColor)
              .setShadowRadius(shadowRadius)
              .setOffsetX(offsetX)
              .setOffsetY(offsetY)
              .builder();
      view.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
      ViewCompat.setBackground(view, drawable);
  }

  /**
     * 为指定View设置带阴影的背景
     * @param view 目标View
     * @param bgColor View背景色
     * @param shapeRadius View的圆角
     * @param shadowColor 阴影的颜色
     * @param shadowRadius 阴影的宽度
     * @param offsetX 阴影水平方向的偏移量
     * @param offsetY 阴影垂直方向的偏移量
     */
  public static void setShadowDrawable(View view, int bgColor, int shapeRadius, int shadowColor, int shadowRadius, int offsetX, int offsetY) {
      ShadowDrawable drawable = new ShadowDrawable.Builder()
              .setBgColor(bgColor)
              .setShapeRadius(shapeRadius)
              .setShadowColor(shadowColor)
              .setShadowRadius(shadowRadius)
              .setOffsetX(offsetX)
              .setOffsetY(offsetY)
              .builder();
      view.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
      ViewCompat.setBackground(view, drawable);
  }

  /**
     * 为指定View设置指定形状并带阴影的背景
     * @param view 目标View
     * @param shape View的形状 取值可为：GradientDrawable.RECTANGLE， GradientDrawable.OVAL， GradientDrawable.RING
     * @param bgColor View背景色
     * @param shapeRadius View的圆角
     * @param shadowColor 阴影的颜色
     * @param shadowRadius 阴影的宽度
     * @param offsetX 阴影水平方向的偏移量
     * @param offsetY 阴影垂直方向的偏移量
     */
  public static void setShadowDrawable(View view, int shape, int bgColor, int shapeRadius, int shadowColor, int shadowRadius, int offsetX, int offsetY) {
      ShadowDrawable drawable = new ShadowDrawable.Builder()
              .setShape(shape)
              .setBgColor(bgColor)
              .setShapeRadius(shapeRadius)
              .setShadowColor(shadowColor)
              .setShadowRadius(shadowRadius)
              .setOffsetX(offsetX)
              .setOffsetY(offsetY)
              .builder();
      view.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
      ViewCompat.setBackground(view, drawable);
  }

  /**
     * 为指定View设置带阴影的渐变背景
     * @param view
     * @param bgColor
     * @param shapeRadius
     * @param shadowColor
     * @param shadowRadius
     * @param offsetX
     * @param offsetY
     */
  public static void setShadowDrawable(View view, int[] bgColor, int shapeRadius, int shadowColor, int shadowRadius, int offsetX, int offsetY) {
      ShadowDrawable drawable = new ShadowDrawable.Builder()
              .setBgColor(bgColor)
              .setShapeRadius(shapeRadius)
              .setShadowColor(shadowColor)
              .setShadowRadius(shadowRadius)
              .setOffsetX(offsetX)
              .setOffsetY(offsetY)
              .builder();
      view.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
      ViewCompat.setBackground(view, drawable);
  }

  public static class Builder {
      private int mShape;
      private int mShapeRadius;
      private int mShadowColor;
      private int mShadowRadius;
      private int mOffsetX;
      private int mOffsetY;
      private int[] mBgColor;

      public Builder() {
          mShape = ShadowDrawable.SHAPE_ROUND;
          mShapeRadius = 12;
          mShadowColor = Color.parseColor("#4d000000");
          mShadowRadius = 18;
          mOffsetX = 0;
          mOffsetY = 0;
          mBgColor = new int[1];
          mBgColor[0] = Color.TRANSPARENT;
      }

      public Builder setShape(int mShape) {
          this.mShape = mShape;
          return this;
      }

      public Builder setShapeRadius(int ShapeRadius) {
          this.mShapeRadius = ShapeRadius;
          return this;
      }

      public Builder setShadowColor(int shadowColor) {
          this.mShadowColor = shadowColor;
          return this;
      }

      public Builder setShadowRadius(int shadowRadius) {
          this.mShadowRadius = shadowRadius;
          return this;
      }

      public Builder setOffsetX(int OffsetX) {
          this.mOffsetX = OffsetX;
          return this;
      }

      public Builder setOffsetY(int OffsetY) {
          this.mOffsetY = OffsetY;
          return this;
      }

      public Builder setBgColor(int BgColor) {
          this.mBgColor[0] = BgColor;
          return this;
      }

      public Builder setBgColor(int[] BgColor) {
          this.mBgColor = BgColor;
          return this;
      }

      public ShadowDrawable builder() {
          return new ShadowDrawable(mShape, mBgColor, mShapeRadius, mShadowColor, mShadowRadius, mOffsetX, mOffsetY);
      }
  }
}