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系列文章

android tv常见问题（一）焦点查找规律
android tv常见问题（二）如何监听ViewGroup子View的焦点状态
android tv常见问题（三）RecyclerView的焦点记忆
android tv常见问题（四）焦点变化时，Recyclerview是如何进行滚动的

github地址

https://github.com/Geekholt/TvFocus

期望结果

Recyclerview滚动时，聚焦的item位置保持在中间。

4.1.gif

实际结果

4.2.gif

问题分析

需要在计算RecyclerView滑动距离的方法中进行重写，控制每次滑动的距离。先来看看RecyclerView原生的滑动距离计算方法。

RecyclerView#requestChildRectangleOnScreen

当RecyclerView的某个子View需要被定位在屏幕的某个矩形范围时，调用此方法。

    /**
     * 通过该方法设置选中的item居中
     * 
     * 最终计算出的dy，dx的实际意义就是在滚动中上下和左右滑动的距离
     *
     * @param child     发出请求的子View
     * @param rect      子View坐标系内的矩形，即此子View希望在屏幕上的定位
     * @param immediate 设为true，则禁止动画和平滑移动滚动条
     * @return 进行了滚动操作的这个ViewGroup，是否处理此操作
     */
public boolean requestChildRectangleOnScreen(RecyclerView parent, View child, Rect rect,
        boolean immediate) {
    final int parentLeft = getPaddingLeft();
    final int parentTop = getPaddingTop();
    final int parentRight = getWidth() - getPaddingRight();
    final int parentBottom = getHeight() - getPaddingBottom();
    final int childLeft = child.getLeft() + rect.left - child.getScrollX();
    final int childTop = child.getTop() + rect.top - child.getScrollY();
    final int childRight = childLeft + rect.width();
    final int childBottom = childTop + rect.height();

    final int offScreenLeft = Math.min(0, childLeft - parentLeft);
    final int offScreenTop = Math.min(0, childTop - parentTop);
    final int offScreenRight = Math.max(0, childRight - parentRight);
    final int offScreenBottom = Math.max(0, childBottom - parentBottom);

    // Favor the "start" layout direction over the end when bringing one side or the other
    // of a large rect into view. If we decide to bring in end because start is already
    // visible, limit the scroll such that start won't go out of bounds.
    final int dx;
    if (getLayoutDirection() == View.LAYOUT_DIRECTION_RTL) {
        dx = offScreenRight != 0 ? offScreenRight
                : Math.max(offScreenLeft, childRight - parentRight);
    } else {
        dx = offScreenLeft != 0 ? offScreenLeft
                : Math.min(childLeft - parentLeft, offScreenRight);
    }

    // Favor bringing the top into view over the bottom. If top is already visible and
    // we should scroll to make bottom visible, make sure top does not go out of bounds.
    final int dy = offScreenTop != 0 ? offScreenTop
            : Math.min(childTop - parentTop, offScreenBottom);

    if (dx != 0 || dy != 0) {
        if (immediate) {
            parent.scrollBy(dx, dy);
        } else {
            parent.smoothScrollBy(dx, dy);
        }
        return true;
    }
    return false;
}

解决方案

    public boolean requestChildRectangleOnScreen(View child, Rect rect, boolean immediate) {
        //计算偏移量
        int selectedItemOffsetStart = 0;
        int selectedItemOffsetEnd = 0;
        selectedItemOffsetStart = !isVertical() ? (getFreeWidth() - child.getWidth()) : (getFreeHeight() - child.getHeight());
        selectedItemOffsetStart /= 2;
        selectedItemOffsetEnd = selectedItemOffsetStart;
        
        final int parentLeft = getPaddingLeft();
        final int parentTop = getPaddingTop();
        final int parentRight = getWidth() - getPaddingRight();
        final int parentBottom = getHeight() - getPaddingBottom();
        final int childLeft = child.getLeft() + rect.left - child.getScrollX();
        final int childTop = child.getTop() + rect.top - child.getScrollY();
        final int childRight = childLeft + rect.width();
        final int childBottom = childTop + rect.height();


        final int offScreenLeft = Math.min(0, childLeft - parentLeft - mSelectedItemOffsetStart);
        final int offScreenRight = Math.max(0, childRight - parentRight + mSelectedItemOffsetEnd);

        final int offScreenTop = Math.min(0, childTop - parentTop - mSelectedItemOffsetStart);
        final int offScreenBottom = Math.max(0, childBottom - parentBottom + mSelectedItemOffsetEnd);

        // Favor the "start" layout direction over the end when bringing one side or the other
        // of a large rect into view. If we decide to bring in end because start is already
        // visible, limit the scroll such that start won't go out of bounds.
        final int dx;
        if (getLayoutDirection() == View.LAYOUT_DIRECTION_RTL) {
            dx = offScreenRight != 0 ? offScreenRight
                    : Math.max(offScreenLeft, childRight - parentRight);
        } else {
            dx = offScreenLeft != 0 ? offScreenLeft
                    : Math.min(childLeft - parentLeft, offScreenRight);
        }

        // Favor bringing the top into view over the bottom. If top is already visible and
        // we should scroll to make bottom visible, make sure top does not go out of bounds.
        final int dy = offScreenTop != 0 ? offScreenTop
                : Math.min(childTop - parentTop, offScreenBottom);

        if (dx != 0 || dy != 0) {
            if (immediate) {
                scrollBy(dx, dy);
            } else {
                smoothScrollBy(dx, dy);
            }
            return true;
        }
        return false;
    }

这里要注意的是，为了适配v7，需要自定义LayoutManager，不然RecyclerView的requestChildRectangleOnScreen可能无法执行。

public class V7LinearLayoutManager extends LinearLayoutManager {
    public V7LinearLayoutManager(Context context) {
        super(context);
    }

    public V7LinearLayoutManager(Context context, int orientation, boolean reverseLayout) {
        super(context, orientation, reverseLayout);
    }

    public V7LinearLayoutManager(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr, 0);
    }

    @Override
    public boolean requestChildRectangleOnScreen(RecyclerView parent, View child, Rect rect, boolean immediate, boolean focusedChildVisible) {
        if(parent instanceof TvRecyclerView) {
            return parent.requestChildRectangleOnScreen(child, rect, immediate);
        }
        return super.requestChildRectangleOnScreen(parent, child, rect, immediate, focusedChildVisible);
    }
}

系列文章总结

想要改变焦点查找规则，可以关注focusSearch的过程。
想要监听焦点变化的回调，可以关注requestFocus的过程。

如果想要实现一套通用焦点框架，个人想法是在Android原生焦点机制的基础上做一些定制化的操作，或许并不需要完全自己去实现一套焦点框架。

TV端焦点问题的比较复杂的根本问题我认为有两点：

主观因素可能是我们对Android原生的焦点机制还没有特别的清楚，所以不知道如何下手去处理一些不符合预期的现象。其实这些现象如果跟着源码去看的话，会发现它的实现都是有一定道理的。
客观因素是某些的UI交互比较复杂，Andorid原生的焦点机制只是采用了比较折中的处理方案。没有什么语言是完美的，也没有什么框架是完美的，能满足我们需求才是最好的。所以我认为焦点问题的处理应该建立在我们有一套统一的UI交互的基础上，然后我们在去基于Android原生焦点机制做一些定制化的操作，具体如何定制化，基本上问题都可以在文中提到的几个回调接口中去处理。

android tv常见问题（四）焦点变化时，Recyclerview是如何进行滚动的