渐变透明和模糊css

需求是列表向上滚动时候， 顶部需要淡出效果，而且透明度渐变效果。这是需求背景。

第一种实现方式：系统Api提供的方法实现

 android:layout_width="match_parent" android:requiresFadingEdge="vertical" android:fadingEdgeLength="26dp" android:scrollbars="none" android:layout_height="match_parent"/> 

重点属性

android:requiresFadingEdge="vertical" android:fadingEdgeLength="26dp"
重点这两行代码，第一行代码意思是上下边缘渐变遮罩效果。第二行代码的意思是 遮罩高度26dp。 现在问题来了，UI不想要下边有遮罩效果，只想要顶部有这个效果，怎么办呢？

第二种实现方式：顶部RecyclerView顶部盖一个渐变View

这种实现方式不可控的地方太多，代码中逻辑比较恶心。直接Pass了

第三种实现方式： 利用View 提供的Api做自己的定制效果

其实RecyclerView继承ViewGroup,ViewGroup继承View。View给我们提供了方法。看源码：

 / * <p>Indicate whether the vertical edges are faded when the view is * scrolled horizontally.</p> * * @return true if the vertical edges should are faded on scroll, false * otherwise * * @see #setVerticalFadingEdgeEnabled(boolean) * * @attr ref android.R.styleable#View_requiresFadingEdge */ public boolean isVerticalFadingEdgeEnabled() { return (mViewFlags & FADING_EDGE_VERTICAL) == FADING_EDGE_VERTICAL; } / * <p>Define whether the vertical edges should be faded when this view * is scrolled vertically.</p> * * @param verticalFadingEdgeEnabled true if the vertical edges should * be faded when the view is scrolled * vertically * * @see #isVerticalFadingEdgeEnabled() * * @attr ref android.R.styleable#View_requiresFadingEdge */ public void setVerticalFadingEdgeEnabled(boolean verticalFadingEdgeEnabled) { if (isVerticalFadingEdgeEnabled() != verticalFadingEdgeEnabled) { if (verticalFadingEdgeEnabled) { initScrollCache(); } mViewFlags ^= FADING_EDGE_VERTICAL; } } / * Returns the strength, or intensity, of the top faded edge. The strength is * a value between 0.0 (no fade) and 1.0 (full fade). The default implementation * returns 0.0 or 1.0 but no value in between. * * Subclasses should override this method to provide a smoother fade transition * when scrolling occurs. * * @return the intensity of the top fade as a float between 0.0f and 1.0f */ protected float getTopFadingEdgeStrength() { return computeVerticalScrollOffset() > 0 ? 1.0f : 0.0f; } / * Returns the strength, or intensity, of the bottom faded edge. The strength is * a value between 0.0 (no fade) and 1.0 (full fade). The default implementation * returns 0.0 or 1.0 but no value in between. * * Subclasses should override this method to provide a smoother fade transition * when scrolling occurs. * * @return the intensity of the bottom fade as a float between 0.0f and 1.0f */ protected float getBottomFadingEdgeStrength() { return computeVerticalScrollOffset() + computeVerticalScrollExtent() < computeVerticalScrollRange() ? 1.0f : 0.0f; } / * Returns the strength, or intensity, of the left faded edge. The strength is * a value between 0.0 (no fade) and 1.0 (full fade). The default implementation * returns 0.0 or 1.0 but no value in between. * * Subclasses should override this method to provide a smoother fade transition * when scrolling occurs. * * @return the intensity of the left fade as a float between 0.0f and 1.0f */ protected float getLeftFadingEdgeStrength() { return computeHorizontalScrollOffset() > 0 ? 1.0f : 0.0f; } / * Returns the strength, or intensity, of the right faded edge. The strength is * a value between 0.0 (no fade) and 1.0 (full fade). The default implementation * returns 0.0 or 1.0 but no value in between. * * Subclasses should override this method to provide a smoother fade transition * when scrolling occurs. * * @return the intensity of the right fade as a float between 0.0f and 1.0f */ protected float getRightFadingEdgeStrength() { return computeHorizontalScrollOffset() + computeHorizontalScrollExtent() < computeHorizontalScrollRange() ? 1.0f : 0.0f; } 

这个时候我想你应该懂我意思了吧？ 我们自定义RecyclerView，然后去重写它的方法，强制返回0f,全透明 不就好了么？看下自定义RecyclerView 代码：

import android.content.Context import android.util.AttributeSet import androidx.recyclerview.widget.RecyclerView /* * create by guiping at 2021/6/11/2:24 下午 * description:顶部阴影半遮照Recyclerview */ class FadingEdgeRecyclerView(context: Context, attrs: AttributeSet?) : RecyclerView(context, attrs) { override fun getTopFadingEdgeStrength(): Float { //顶部走系统的，不做修改 return super.getTopFadingEdgeStrength() } override fun getBottomFadingEdgeStrength(): Float { //底部全透明 return 0f } } 

这样就好了。

最终效果