RecyclerView无限循环效果实现及示例解析

/**
 * <p>Scroll the RecyclerView to make the position visible.</p>
 *
 * <p>RecyclerView will scroll the minimum amount that is necessary to make the
 * target position visible. If you are looking for a similar behavior to
 * {@link android.widget.ListView#setSelection(int)} or
 * {@link android.widget.ListView#setSelectionFromTop(int, int)}, use
 * {@link #scrollToPositionWithOffset(int, int)}.</p>
 *
 * <p>Note that scroll position change will not be reflected until the next layout call.</p>
 *
 * @param position Scroll to this adapter position
 * @see #scrollToPositionWithOffset(int, int)
 */
@Override
public void scrollToPosition(int position) {
    mPendingScrollPosition = position;//更新position
    mPendingScrollPositionOffset = INVALID_OFFSET; 
    if (mPendingSavedState != null) {
        mPendingSavedState.invalidateAnchor();
    }
    requestLayout();
}

@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
    // layout algorithm:
    // 1) by checking children and other variables, find an anchor coordinate and an anchor
    //  item position.
    // 2) fill towards start, stacking from bottom
    // 3) fill towards end, stacking from top
    // 4) scroll to fulfill requirements like stack from bottom.
    //..............
    // 省略，前面主要做了一些异常状态的检测、针对焦点的特殊处理、确定锚点对anchorInfo赋值、偏移量计算
    int startOffset;
    int endOffset;
    final int firstLayoutDirection;
    if (mAnchorInfo.mLayoutFromEnd) {
        // fill towards start
        updateLayoutStateToFillStart(mAnchorInfo); //根据mAnchorInfo更新layoutState
        mLayoutState.mExtraFillSpace = extraForStart;
        fill(recycler, mLayoutState, state, false);//填充
        startOffset = mLayoutState.mOffset;
        final int firstElement = mLayoutState.mCurrentPosition;
        if (mLayoutState.mAvailable > 0) {
            extraForEnd += mLayoutState.mAvailable;
        }
        // fill towards end
        updateLayoutStateToFillEnd(mAnchorInfo);//更新layoutState为fill做准备
        mLayoutState.mExtraFillSpace = extraForEnd;
        mLayoutState.mCurrentPosition += mLayoutState.mItemDirection;
        fill(recycler, mLayoutState, state, false);//填充
        endOffset = mLayoutState.mOffset;
        if (mLayoutState.mAvailable > 0) {
            // end could not consume all. add more items towards start
            extraForStart = mLayoutState.mAvailable;
            updateLayoutStateToFillStart(firstElement, startOffset);//更新layoutState为fill做准备
            mLayoutState.mExtraFillSpace = extraForStart;
            fill(recycler, mLayoutState, state, false);
            startOffset = mLayoutState.mOffset;
        }
    } else {
        //layoutFromStart 同理，省略
    }
    //try to fix gap , 省略

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
        RecyclerView.State state, boolean stopOnFocusable) {
    // max offset we should set is mFastScroll + available
    final int start = layoutState.mAvailable;
    if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
        // TODO ugly bug fix. should not happen
        if (layoutState.mAvailable < 0) {
            layoutState.mScrollingOffset += layoutState.mAvailable;
        }
        recycleByLayoutState(recycler, layoutState);
    }
    int remainingSpace = layoutState.mAvailable + layoutState.mExtraFillSpace;
    LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
    // （不限制layout个数/还有剩余空间） 并且 有剩余数据
    while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
        layoutChunkResult.resetInternal();
        if (RecyclerView.VERBOSE_TRACING) {
            TraceCompat.beginSection("LLM LayoutChunk");
        }
        layoutChunk(recycler, state, layoutState, layoutChunkResult);
        if (RecyclerView.VERBOSE_TRACING) {
            TraceCompat.endSection();
        }
        if (layoutChunkResult.mFinished) {
            break;
        }
        layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
        /**
         * Consume the available space if:
         * * layoutChunk did not request to be ignored
         * * OR we are laying out scrap children
         * * OR we are not doing pre-layout
         */
        if (!layoutChunkResult.mIgnoreConsumed || layoutState.mScrapList != null
                || !state.isPreLayout()) {
            layoutState.mAvailable -= layoutChunkResult.mConsumed;
            // we keep a separate remaining space because mAvailable is important for recycling
            remainingSpace -= layoutChunkResult.mConsumed;
        }
        if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
            layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
            if (layoutState.mAvailable < 0) {
                layoutState.mScrollingOffset += layoutState.mAvailable;
            }
            recycleByLayoutState(recycler, layoutState);//回收子view
        }
        if (stopOnFocusable && layoutChunkResult.mFocusable) {
            break;
        }
    }
    if (DEBUG) {
        validateChildOrder();
    }
    return start - layoutState.mAvailable;

//adapter关键代码
@NonNull
@Override
public DemoViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    LayoutInflater inflater = LayoutInflater.from(parent.getContext());
    Log.d("DemoAdapter","onCreateViewHolder");
    return new DemoViewHolder(inflater.inflate(R.layout.item_demo, parent, false));
}
@Override
public void onBindViewHolder(@NonNull DemoViewHolder holder, int position) {
    Log.d("DemoAdapter","onBindViewHolder： position"+position);
    String text = mData.get(position % mData.size());
    holder.bind(text);
}
@Override
public int getItemCount() {
    return Integer.MAX_VALUE;
}

RecyclerView recyclerView = findViewById(R.id.rv);
recyclerView.setAdapter(new DemoAdapter());
LinearLayoutManager layoutManager =  new LinearLayoutManager(this);
layoutManager.setOrientation(RecyclerView.VERTICAL);
recyclerView.setLayoutManager(layoutManager);
recyclerView.scrollToPosition(Integer.MAX_VALUE/2);

/**
 * The magic functions :). Fills the given layout, defined by the layoutState. This is fairly
 * independent from the rest of the {@link LinearLayoutManager}
 * and with little change, can be made publicly available as a helper class.
 */
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
        RecyclerView.State state, boolean stopOnFocusable) {

/**
 * layout具体子view
 */
private void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
                         LayoutState layoutState, LayoutChunkResult result) {
    View view = layoutState.next(recycler, state);
    if (view == null) {
        result.mFinished = true;
        return;
    }
    RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();
    // add
    if (layoutState.mLayoutDirection != LayoutState.LAYOUT_START) {
        addView(view);
    } else {
        addView(view, 0);
    }
    Rect insets = new Rect();
    calculateItemDecorationsForChild(view, insets);
    // 测量
    measureChildWithMargins(view, 0, 0);
    //布局
    layoutChild(view, result, params, layoutState, state);
    // Consume the available space if the view is not removed OR changed
    if (params.isItemRemoved() || params.isItemChanged()) {
        result.mIgnoreConsumed = true;
    }
    result.mFocusable = view.hasFocusable();
}

    boolean hasMore(RecyclerView.State state) {
        return Math.abs(mCurrentPosition) <= state.getItemCount();
    }
    View next(RecyclerView.Recycler recycler, RecyclerView.State state) {
        int itemCount = state.getItemCount();
        mCurrentPosition = mCurrentPosition % itemCount;
        if (mCurrentPosition < 0) {
            mCurrentPosition += itemCount;
        }
        final View view = recycler.getViewForPosition(mCurrentPosition);
        mCurrentPosition += mItemDirection;
        return view;
    }
}