RecyclerView焦点跳转BUG优化的方法

我们把RecyclerView写成GridView样式，并把RecyclerView的item写成focusable并且有焦点框的时候，我们用焦点滚动RecyclerView的时候会发现RecyclerView的焦点跳转有bug，跟我们想要的焦点跳转规则不一致，会出现的BUG如下图：

黑色方框代表屏幕，我们从左上角的一个item往下按焦点的时候，当需要加载新的一行的时候焦点却跑到了新的一行的最后一个item上面了，（如图，本来是item1获得焦点的，结果跑到item2上面了）。

这是RecyclerView的一个BUG，记得RecyclerView刚出来的时候滚动都还有点卡顿，到了现在滚动起来还是非常流畅的，比较一个全新的艺术般的空间是需要时间来沉淀的，这个BUG我们可以重写GridLayoutManger来解决。直接看代码：

package com.wasu.cs.widget; 

import android.content.Context;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;
import android.view.View; 

/**
 * 自定义GridLayoutManager，修改RecyelerView焦点乱跳的BUG
 * Created by Danxingxi on 2016/4/1.
 */
public class FocusGridLayoutManager extends GridLayoutManager { 

  /**
   * Constructor used when layout manager is set in XML by RecyclerView attribute
   * "layoutManager". If spanCount is not specified in the XML, it defaults to a
   * single column.
   *
   * @param context
   * @param attrs
   * @param defStyleAttr
   * @param defStyleRes
   * @attr ref android.support.v7.recyclerview.R.styleable#RecyclerView_spanCount
   */
  public FocusGridLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    super(context, attrs, defStyleAttr, defStyleRes);
  } 

  /**
   * Creates a vertical GridLayoutManager
   *
   * @param context  Current context, will be used to access resources.
   * @param spanCount The number of columns in the grid
   */
  public FocusGridLayoutManager(Context context, int spanCount) {
    super(context, spanCount);
  } 

  /**
   * @param context    Current context, will be used to access resources.
   * @param spanCount   The number of columns or rows in the grid
   * @param orientation  Layout orientation. Should be {@link #HORIZONTAL} or {@link
   *           #VERTICAL}.
   * @param reverseLayout When set to true, layouts from end to start.
   */
  public FocusGridLayoutManager(Context context, int spanCount, int orientation, boolean reverseLayout) {
    super(context, spanCount, orientation, reverseLayout);
  } 

  /**
   * Return the current number of child views attached to the parent RecyclerView.
   * This does not include child views that were temporarily detached and/or scrapped.
   *
   * @return Number of attached children
   */
  @Override
  public int getChildCount() {
    return super.getChildCount();
  } 

  /**
   * Return the child view at the given index
   *
   * @param index Index of child to return
   * @return Child view at index
   */
  @Override
  public View getChildAt(int index) {
    return super.getChildAt(index);
  } 

  /**
   * Returns the number of items in the adapter bound to the parent RecyclerView.
   * @return The number of items in the bound adapter
   */
  @Override
  public int getItemCount() {
    return super.getItemCount();
  } 

  /**
   * Returns the item View which has or contains focus.
   *
   * @return A direct child of RecyclerView which has focus or contains the focused child.
   */
  @Override
  public View getFocusedChild() {
    return super.getFocusedChild();
  } 

  /**
   * Returns the adapter position of the item represented by the given View. This does not
   * contain any adapter changes that might have happened after the last layout.
   *
   * @param view The view to query
   * @return The adapter position of the item which is rendered by this View.
   */
  @Override
  public int getPosition(View view) {
    return super.getPosition(view);
  } 

  /**
   * 获取列数
   * @return
   */
  @Override
  public int getSpanCount() {
    return super.getSpanCount();
  } 

  /**
   * Called when searching for a focusable view in the given direction has failed for the current content of the RecyclerView.
   * This is the LayoutManager's opportunity to populate views in the given direction to fulfill the request if it can.
   * The LayoutManager should attach and return the view to be focused. The default implementation returns null.
   * 防止当recyclerview上下滚动的时候焦点乱跳
   * @param focused
   * @param focusDirection
   * @param recycler
   * @param state
   * @return
   */
  @Override
  public View onFocusSearchFailed(View focused, int focusDirection, RecyclerView.Recycler recycler, RecyclerView.State state) { 

    // Need to be called in order to layout new row/column
    View nextFocus = super.onFocusSearchFailed(focused, focusDirection, recycler, state); 

    if (nextFocus == null) {
      return null;
    }
    /**
     * 获取当前焦点的位置
     */
    int fromPos = getPosition(focused);
    /**
     * 获取我们希望的下一个焦点的位置
     */
    int nextPos = getNextViewPos(fromPos, focusDirection); 

    return findViewByPosition(nextPos); 

  } 

  /**
   * Manually detect next view to focus.
   *
   * @param fromPos from what position start to seek.
   * @param direction in what direction start to seek. Your regular {@code View.FOCUS_*}.
   * @return adapter position of next view to focus. May be equal to {@code fromPos}.
   */
  protected int getNextViewPos(int fromPos, int direction) {
    int offset = calcOffsetToNextView(direction); 

    if (hitBorder(fromPos, offset)) {
      return fromPos;
    } 

    return fromPos + offset;
  } 

  /**
   * Calculates position offset.
   *
   * @param direction regular {@code View.FOCUS_*}.
   * @return position offset according to {@code direction}.
   */
  protected int calcOffsetToNextView(int direction) {
    int spanCount = getSpanCount();
    int orientation = getOrientation(); 

    if (orientation == VERTICAL) {
      switch (direction) {
        case View.FOCUS_DOWN:
          return spanCount;
        case View.FOCUS_UP:
          return -spanCount;
        case View.FOCUS_RIGHT:
          return 1;
        case View.FOCUS_LEFT:
          return -1;
      }
    } else if (orientation == HORIZONTAL) {
      switch (direction) {
        case View.FOCUS_DOWN:
          return 1;
        case View.FOCUS_UP:
          return -1;
        case View.FOCUS_RIGHT:
          return spanCount;
        case View.FOCUS_LEFT:
          return -spanCount;
      }
    } 

    return 0;
  } 

  /**
   * Checks if we hit borders.
   *
   * @param from from what position.
   * @param offset offset to new position.
   * @return {@code true} if we hit border.
   */
  private boolean hitBorder(int from, int offset) {
    int spanCount = getSpanCount(); 

    if (Math.abs(offset) == 1) {
      int spanIndex = from % spanCount;
      int newSpanIndex = spanIndex + offset;
      return newSpanIndex < 0 || newSpanIndex >= spanCount;
    } else {
      int newPos = from + offset;
      return newPos < 0 && newPos >= spanCount;
    }
  }
} 

分析：在我们从第五行往下按的时候，第六行的view是重新加载的，当新的一行的item还没有加载出来的时候，去找焦点是找不到的，找不到焦点就会调用mLayout.onFocusSearchFailed()方法，

onFocusSearchFailed方法是LayoutManager的方法，默认是返回null的，我们在自定义GridLayoutManager的时候重写此方法即可，具体的处理步骤请看到代码。在RecyclerView源代码中，onFocusSearchFailed是内部抽象类LayoutManager的一个成员方法，默认返回null。

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