API 27及以下版本如何修改视图（含MaterialCardView）阴影颜色？

一、原生实现方式 #

由于API 28以下系统不支持View#setOutlineAmbientShadowColor()和View#setOutlineSpotShadowColor()方法，可通过自定义绘制模拟带颜色的阴影，步骤如下：

1. 关闭MaterialCardView原生阴影

materialCardView.setCardElevation(0f);
materialCardView.setUseCompatPadding(true); // 可选，避免布局偏移

2. 自定义阴影Drawable
   通过Paint.setShadowLayer()实现带颜色的模糊阴影，同时匹配CardView圆角：

public class ColoredShadowDrawable extends Drawable {
    private final Paint mShadowPaint;
    private RectF mShadowRect;
    private final float mCornerRadius;
    private final int mShadowColor;
    private final float mShadowRadius;
    private final float mShadowOffsetX;
    private final float mShadowOffsetY;

    public ColoredShadowDrawable(int shadowColor, float shadowRadius, float offsetX, float offsetY, float cornerRadius) {
        mShadowColor = shadowColor;
        mShadowRadius = shadowRadius;
        mShadowOffsetX = offsetX;
        mShadowOffsetY = offsetY;
        mCornerRadius = cornerRadius;

        mShadowPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mShadowPaint.setColor(Color.TRANSPARENT);
        mShadowPaint.setShadowLayer(mShadowRadius, mShadowOffsetX, mShadowOffsetY, mShadowColor);
    }

    @Override
    public void draw(@NonNull Canvas canvas) {
        canvas.drawRoundRect(mShadowRect, mCornerRadius, mCornerRadius, mShadowPaint);
    }

    @Override
    protected void onBoundsChange(Rect bounds) {
        super.onBoundsChange(bounds);
        // 留出阴影绘制空间
        mShadowRect = new RectF(
            bounds.left + mShadowRadius,
            bounds.top + mShadowRadius,
            bounds.right - mShadowRadius,
            bounds.bottom - mShadowRadius
        );
    }

    @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;
    }
}

3. 将自定义Drawable设置给CardView
   可通过setForeground()或ViewOverlay添加阴影：

ColoredShadowDrawable shadowDrawable = new ColoredShadowDrawable(
    Color.parseColor("#FF6200EE"), // 自定义阴影颜色
    12f, // 阴影模糊半径
    0f, // X轴偏移
    4f, // Y轴偏移
    materialCardView.getRadius() // 和CardView圆角保持一致
);
materialCardView.setForeground(shadowDrawable);

二、性能优异的第三方库 #

如果不想手动实现，以下第三方库经过性能优化，兼容低版本API：

• CardViewEx
  扩展原生CardView，直接支持通过XML属性设置阴影颜色、半径和偏移，内部采用硬件加速绘制，性能接近原生：

  <com.example.cardviewex.CardViewEx
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      app:cardCornerRadius="8dp"
      app:shadowColor="@color/custom_shadow"
      app:shadowRadius="12dp"
      app:shadowOffsetY="4dp">
      <!-- 子布局 -->
  </com.example.cardviewex.CardViewEx>
  

• DynamicShadowLayout
  轻量级阴影布局容器，支持任意子View（包括MaterialCardView），支持自定义阴影颜色、方向、模糊度，兼容API 16+，全程硬件加速，性能优异：

  <com.example.dynamicshadowlayout.DynamicShadowLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      app:shadow_color="@color/blue_shadow"
      app:shadow_blur="10dp"
      app:shadow_offset="5dp">
  
      <com.google.android.material.card.MaterialCardView
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:layout_margin="10dp"
          app:cardCornerRadius="8dp">
          <!-- 子布局 -->
      </com.google.android.material.card.MaterialCardView>
  
  </com.example.dynamicshadowlayout.DynamicShadowLayout>
  

内容的提问来源于stack exchange，提问作者Mateus Bauer