当RecyclerView滚动时它干了什么

class RecyclerViewActivity1 : AppCompatActivity() {    private lateinit var mRecyclerView: RecyclerView    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_recycler_view1)        mRecyclerView = findViewById(R.id.recyclerview)        mRecyclerView.setHasFixedSize(true)        mRecyclerView.setItemViewCacheSize(0)        mRecyclerView.layoutManager =            LinearLayoutManager(this).apply {                orientation = LinearLayoutManager.VERTICAL                isItemPrefetchEnabled = false            }        val list: MutableList<String> =            ArrayList()        repeat(100) {            list.add("item $it")        }        mRecyclerView.adapter = MyAdapter(list)    }
    inner class MyAdapter(val mStrings: MutableList<String>) :        RecyclerView.Adapter<RecyclerView.ViewHolder>() {        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {            println("RecyclerView 场景一 onCreateViewHolder ")            val view = LayoutInflater.from(parent.context)                .inflate(R.layout.view_item, parent, false)            return object : RecyclerView.ViewHolder(view) {}        }
        override fun getItemCount(): Int {            return mStrings.size        }
        override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {            println("RecyclerView 场景一 onBindViewHolder $position ")            val textView = holder.itemView as TextView            textView.layoutParams.height = (resources.displayMetrics.density * 100).toInt()            textView.text = mStrings[position]        }
        override fun onViewRecycled(holder: RecyclerView.ViewHolder) {            println("RecyclerView 场景一 发生回收 " + (holder.itemView as TextView).text)
            super.onViewRecycled(holder)        }
    }
    fun scroll120(view: View) {        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 120).toInt())    }
    fun scroll60(view: View) {        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 60).toInt())
    }
    fun scroll40(view: View) {        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 40).toInt())
    }}

class RecyclerViewActivity2 : AppCompatActivity() {    private lateinit var mRecyclerView: RecyclerView    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_recycler_view2)        mRecyclerView = findViewById(R.id.recyclerview)        mRecyclerView.setHasFixedSize(true)        mRecyclerView.setItemViewCacheSize(0)        mRecyclerView.layoutManager =            LinearLayoutManager(this).apply {                orientation = LinearLayoutManager.VERTICAL                isItemPrefetchEnabled = false            }        val list: MutableList<String> =            ArrayList()        repeat(100) {            list.add("item $it")        }        mRecyclerView.adapter = MyAdapter(list)    }
    inner class MyAdapter(val mStrings: MutableList<String>) :        RecyclerView.Adapter<RecyclerView.ViewHolder>() {        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {            println("RecyclerView 场景二 onCreateViewHolder ")            val view = LayoutInflater.from(parent.context)                .inflate(R.layout.view_item, parent, false)            return object : RecyclerView.ViewHolder(view) {}        }
        override fun getItemCount(): Int {            return mStrings.size        }
        override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {            println("RecyclerView 场景二 onBindViewHolder $position ")            val textView = holder.itemView as TextView            textView.layoutParams.height =                if (position == 0) (resources.displayMetrics.density * 50).toInt() else (resources.displayMetrics.density * 100).toInt()            textView.text = mStrings[position]        }
        override fun onViewRecycled(holder: RecyclerView.ViewHolder) {            println("RecyclerView 场景二 发生回收 " + (holder.itemView as TextView).text)            super.onViewRecycled(holder)        }
    }
    fun scroll120(view: View) {        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 120).toInt())    }
    fun scroll60(view: View) {        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 60).toInt())
    }
    fun scroll40(view: View) {        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 40).toInt())
    }}

//LinearLayoutManager.javaint scrollBy(int delta, RecyclerView.Recycler recycler, RecyclerView.State state) {        if (getChildCount() == 0 || delta == 0) {            return 0;        }        ensureLayoutState();        mLayoutState.mRecycle = true;        final int layoutDirection = delta > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;        final int absDelta = Math.abs(delta);        updateLayoutState(layoutDirection, absDelta, true, state);        final int consumed = mLayoutState.mScrollingOffset                +                 (recycler, mLayoutState, state, false);        if (consumed < 0) {            if (DEBUG) {                Log.d(TAG, "Don't have any more elements to scroll");            }            return 0;        }        final int scrolled = absDelta > consumed ? layoutDirection * consumed : delta;        mOrientationHelper.offsetChildren(-scrolled);        if (DEBUG) {            Log.d(TAG, "scroll req: " + delta + " scrolled: " + scrolled);        }        mLayoutState.mLastScrollDelta = scrolled;        return scrolled;    }

//主要是计算private void updateLayoutState(int layoutDirection, int requiredSpace,            boolean canUseExistingSpace, RecyclerView.State state) {        // If parent provides a hint, don't measure unlimited.        mLayoutState.mInfinite = resolveIsInfinite();        mLayoutState.mLayoutDirection = layoutDirection;        mReusableIntPair[0] = 0;        mReusableIntPair[1] = 0;        calculateExtraLayoutSpace(state, mReusableIntPair);        int extraForStart = Math.max(0, mReusableIntPair[0]);        int extraForEnd = Math.max(0, mReusableIntPair[1]);        boolean layoutToEnd = layoutDirection == LayoutState.LAYOUT_END;        mLayoutState.mExtraFillSpace = layoutToEnd ? extraForEnd : extraForStart;        mLayoutState.mNoRecycleSpace = layoutToEnd ? extraForStart : extraForEnd;        int scrollingOffset;        if (layoutToEnd) {            mLayoutState.mExtraFillSpace += mOrientationHelper.getEndPadding();            // get the first child in the direction we are going            final View child = getChildClosestToEnd();            // the direction in which we are traversing children            mLayoutState.mItemDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_HEAD                    : LayoutState.ITEM_DIRECTION_TAIL;            mLayoutState.mCurrentPosition = getPosition(child) + mLayoutState.mItemDirection;            mLayoutState.mOffset = mOrientationHelper.getDecoratedEnd(child);            // calculate how much we can scroll without adding new children (independent of layout)            scrollingOffset = mOrientationHelper.getDecoratedEnd(child)                    - mOrientationHelper.getEndAfterPadding();
        } else {            final View child = getChildClosestToStart();            mLayoutState.mExtraFillSpace += mOrientationHelper.getStartAfterPadding();            mLayoutState.mItemDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_TAIL                    : LayoutState.ITEM_DIRECTION_HEAD;            mLayoutState.mCurrentPosition = getPosition(child) + mLayoutState.mItemDirection;            mLayoutState.mOffset = mOrientationHelper.getDecoratedStart(child);            scrollingOffset = -mOrientationHelper.getDecoratedStart(child)                    + mOrientationHelper.getStartAfterPadding();        }        mLayoutState.mAvailable = requiredSpace;        if (canUseExistingSpace) {            mLayoutState.mAvailable -= scrollingOffset;        }        mLayoutState.mScrollingOffset = scrollingOffset;    }

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;        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);            }            if (stopOnFocusable && layoutChunkResult.mFocusable) {                break;            }        }        if (DEBUG) {            validateChildOrder();        }        return start - layoutState.mAvailable;    }