如何使用 Snaphelper 更改中心元素的背景颜色

Question

我想更改居中位置元素的背景颜色。 我使用了垂直 snapHelper 并且我的中心元素也被检测到，所以我只想为中心元素添加背景颜色。

LinearSnapHelper snapHelper = new LinearSnapHelper() {

        @Override
        public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {
            View centerView = findSnapView(layoutManager);
            if (centerView == null)
                return RecyclerView.NO_POSITION;

            int position = layoutManager.getPosition(centerView);
            int targetPosition = -1;
            if (layoutManager.canScrollHorizontally()) {
                if (velocityX < 0) {
                    targetPosition = position - 1;

                } else {
                    targetPosition = position + 1;
                }
            }

            if (layoutManager.canScrollVertically()) {
                if (velocityY < 0) {
                    targetPosition = position - 1;
                } else {
                    targetPosition = position + 1;
                    centerView.getBackground().setColorFilter(Color.parseColor("#000000"), PorterDuff.Mode.DARKEN);
                }
            }

            final int firstItem = 0;
            final int lastItem = layoutManager.getItemCount() - 1;
            targetPosition = Math.min(lastItem, Math.max(targetPosition, firstItem));
            return targetPosition;
        }
    };

}

Answer 1

您可以将滚动侦听器注册到RecyclerView ，并在滚动结束时（即处于SCROLL_STATE_IDLE状态）； snapHelper.findSnapView()不应返回null值； 所以你可以设置它的颜色。

final LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
        super.onScrollStateChanged(recyclerView, newState);

        // Detect scroll end
        if (newState == RecyclerView.SCROLL_STATE_IDLE) {

            // Resetting all the loaded items to the default color 
            int firstVisibleItemPosition =
                    layoutManager.findFirstVisibleItemPosition();
            int lastVisibleItemPosition = layoutManager.findLastVisibleItemPosition();
            for (int i = firstVisibleItemPosition; i <= lastVisibleItemPosition; i++) {
                RecyclerView.ViewHolder holder = recyclerView.findViewHolderForAdapterPosition(i);
                // Rest to the default color of other loaded items
                holder.itemView.setBackgroundColor(ResourcesCompat.getColor(getResources(), R.color.defaultColor, null));
            }

            // Changing the background of the snaphelper centered item
            View centerView = snapHelper.findSnapView(layoutManager);
            if (centerView != null)
                centerView.setBackgroundColor(ResourcesCompat.getColor(getResources(), R.color.centerColor, null));
        }

    }
});

科特林：

val layoutManager = recyclerView.layoutManager as LinearLayoutManager
recyclerView.addOnScrollListener(object : RecyclerView.OnScrollListener() {
    override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {
        super.onScrollStateChanged(recyclerView, newState)
        
        // Detect scroll end
        if (newState == RecyclerView.SCROLL_STATE_IDLE) {
            val firstVisibleItemPosition =
                layoutManager.findFirstVisibleItemPosition()
            val lastVisibleItemPosition = layoutManager.findLastVisibleItemPosition()
            for (i in firstVisibleItemPosition..lastVisibleItemPosition) {
                val holder =
                    recyclerView.findViewHolderForAdapterPosition(i) 

                // Resetting all the loaded items to the default color 
                 holder!!.itemView!!.setBackgroundColor(ResourcesCompat.getColor(resources, R.color.defaultColor, null))
            }

            // Changing the background of the snaphelper centered item
            val centerView: View? = snapHelper.findSnapView(layoutManager)
            centerView.setBackgroundColor(ResourcesCompat.getColor(resources, R.color.centerColor, null))

        }

    }
})

---

更新：

有一种比遍历所有 RV 加载的项目更好的方法来重置为默认颜色； 只需将之前的 centralView 保存到本地字段中，并更新其颜色而不是更新整个加载的项目：

View previousCenteredView;
final LinearLayoutManager layoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
        super.onScrollStateChanged(recyclerView, newState);

        // Detect scroll end
        if (newState == RecyclerView.SCROLL_STATE_IDLE) {

            // Changing the background of the snaphelper centered item
            View centerView = snapHelper.findSnapView(layoutManager);
            previousCenteredView = centerView;
            
            if (centerView != null)
                centerView.setBackgroundColor(ResourcesCompat.getColor(getResources(), R.color.centerColor, null));

        } else if (previousCenteredView != null){
            // Resetting previous centered view to the default color 
            previousCenteredView.setBackgroundColor(ResourcesCompat.getColor(getResources(), R.color.defaultColor, null));
            previousCenteredView = null;
        }

    }
});

科特林：

var previousCenteredView: View? = null
val layoutManager = recyclerView.layoutManager as LinearLayoutManager
recyclerView.addOnScrollListener(object : RecyclerView.OnScrollListener() {
    override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {
        super.onScrollStateChanged(recyclerView, newState)
        
        // Detect scroll end
        if (newState == RecyclerView.SCROLL_STATE_IDLE) {

            // Changing the background of the snaphelper centered item
            val centerView: View? = snapHelper.findSnapView(layoutManager)
            previousCenteredView = centerView
            centerView.setBackgroundColor(ResourcesCompat.getColor(resources, R.color.centerColor, null))
            
        } else if (previousCenteredView != null){
            // Resetting previous centered view to the default color 
            previousCenteredView.setBackgroundColor(ResourcesCompat.getColor(resources, R.color.defaultColor, null))
            previousCenteredView = null
        }


    }
})

Answer 2

要设置视图的背景色，可以使用多种方法进行检查

根据您的代码，您正在设置视图的背景颜色，但仅在if语句内部，因此请尝试提取位于if之后的行。

...
if (layoutManager.canScrollVertically()) {
      if (velocityY < 0) {
            targetPosition = position - 1;
      } else {
            targetPosition = position + 1;
      }
}

centerView.getBackground().setColorFilter(Color.parseColor("#000000"), PorterDuff.Mode.DARKEN);

final int firstItem = 0;
final int lastItem = layoutManager.getItemCount() - 1;
targetPosition = Math.min(lastItem, Math.max(targetPosition, firstItem));
return targetPosition;
...