Android-自定义-RecyclerView.LayoutManager

不得不说RecyclerView真的很强大，例如无限轮播Banner，滑动卡片等都有RecyclerView的版本，他们是怎么做的呢？答案是基于RecyclerView.LayoutManager，我们可以自定义RecyclerView.LayoutManager，然后控制RecyclerView内部Item的位置以及大小达到我们想要的效果，为了简单，我们先自定义一个RecyclerView.LayoutManager，模仿LinearLayoutManager的效果。

开整：

• 1.自定义类TestLayoutManager继承RecyclerView.LayoutManager，实现抽象方法：

public class TestLayoutManager extends RecyclerView.LayoutManager {

    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return null;
    }
}

• 2.generateDefaultLayoutParams()就是RecyclerView 子 item 的 LayoutParameters，一般包裹内容即可：

    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return  new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
    }

• 3.关键：重写onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state)方法，对Item按照指定位置添加到视图中，有些类似于自定义ViewGroup的onLayout方法：

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        //如果没有item，直接返回
        if (getItemCount() <= 0) return;
        // 跳过preLayout，preLayout主要用于支持动画
        if (state.isPreLayout()) {
            return;
        }
        //在布局之前，将所有的子View先Detach掉，放入到Scrap缓存中
        detachAndScrapAttachedViews(recycler);
        //定义竖直方向的偏移量
        int offsetY = 0;
        for (int i = 0; i < getItemCount(); i++) {
            //这里就是从缓存里面取出
            View view = recycler.getViewForPosition(i);
            //将View加入到RecyclerView中
            addView(view);
            measureChildWithMargins(view, 0, 0);
            int width = getDecoratedMeasuredWidth(view);
            int height = getDecoratedMeasuredHeight(view);
            //最后，将View布局
            int left = (getWidth() - width) / 2;
            layoutDecorated(view, left, offsetY, getWidth() - left, offsetY + height);
            //将竖直方向偏移量增大height
            offsetY += height;
        }
    }

运行效果：

• 4.此时，Item能正常显示，但是无法响应滚动事件，所以还需要处理滚动事件，重写canScrollVertically()以及scrollVerticallyBy()方法：

    @Override
    public boolean canScrollVertically() {
        return true;
    }
    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
        offsetChildrenVertical(dy);
        return dy;
    }

• 5.此时Item可以滚动了，但是有两个问题，a.滑动方向与手势相反 b.滑动没有边界限制 编辑下面代码解决问题 :

    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
        //实际要滑动的距离
        int travel = dy;

        //如果滑动到最顶部
        if (verticalScrollOffset + dy < 0) {
            travel = -verticalScrollOffset;
        } else if (verticalScrollOffset + dy > totalHeight - getVerticalSpace()) {//如果滑动到最底部
            travel = totalHeight - getVerticalSpace() - verticalScrollOffset;
        }

        //将竖直方向的偏移量+travel
        verticalScrollOffset += travel;

        // 平移容器内的item
        offsetChildrenVertical(-travel);
        return travel;
    }

    /**
     * 获取RecyclerView在垂直方向上的可用空间，即去除了padding后的高度
     *
     * @return
     */
    private int getVerticalSpace() {
        return getHeight() - getPaddingBottom() - getPaddingTop();
    }

现在除了还没做Item缓存，基本和LinearLayoutManager效果差不多了。

完整源码：

/**
 * Created by Chao  2018/8/15 on 16:15
 * description
 */
public class TestLayoutManager extends RecyclerView.LayoutManager {
    private int totalHeight = 0;
    private int verticalScrollOffset = 0;

    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return  new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
    }

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        //如果没有item，直接返回
        if (getItemCount() <= 0) return;
        // 跳过preLayout，preLayout主要用于支持动画
        if (state.isPreLayout()) {
            return;
        }
        //在布局之前，将所有的子View先Detach掉，放入到Scrap缓存中
        detachAndScrapAttachedViews(recycler);
        //定义竖直方向的偏移量
        int offsetY = 0;
        totalHeight = 0;
        for (int i = 0; i < getItemCount(); i++) {
            //这里就是从缓存里面取出
            View view = recycler.getViewForPosition(i);
            //将View加入到RecyclerView中
            addView(view);
            measureChildWithMargins(view, 0, 0);
            int width = getDecoratedMeasuredWidth(view);
            int height = getDecoratedMeasuredHeight(view);
            //最后，将View布局
            int left = (getWidth() - width) / 2;
            layoutDecorated(view, left, offsetY, getWidth() - left, offsetY + height);
            //将竖直方向偏移量增大height
            offsetY += height;

            totalHeight += height;
        }
    }

    @Override
    public boolean canScrollVertically() {
        return true;
    }

    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
        //实际要滑动的距离
        int travel = dy;

        //如果滑动到最顶部
        if (verticalScrollOffset + dy < 0) {
            travel = -verticalScrollOffset;
        } else if (verticalScrollOffset + dy > totalHeight - getVerticalSpace()) {//如果滑动到最底部
            travel = totalHeight - getVerticalSpace() - verticalScrollOffset;
        }

        //将竖直方向的偏移量+travel
        verticalScrollOffset += travel;

        // 平移容器内的item
        offsetChildrenVertical(-travel);
        return travel;
    }

    /**
     * 获取RecyclerView在垂直方向上的可用空间，即去除了padding后的高度
     *
     * @return
     */
    private int getVerticalSpace() {
        return getHeight() - getPaddingBottom() - getPaddingTop();
    }
}