我们为什么要使用RecyclerView

boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
    final int childCount = getChildCount();
    if (childCount == 0) {
        return true;
    }

    final int firstTop = getChildAt(0).getTop();
    final int lastBottom = getChildAt(childCount - 1).getBottom();

    final Rect listPadding = mListPadding;

    // "effective padding" In this case is the amount of padding that affects
    // how much space should not be filled by items. If we don't clip to padding
    // there is no effective padding.
    int effectivePaddingTop = 0;
    int effectivePaddingBottom = 0;
    if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
        effectivePaddingTop = listPadding.top;
        effectivePaddingBottom = listPadding.bottom;
    }

     // FIXME account for grid vertical spacing too?
    final int spaceAbove = effectivePaddingTop - firstTop;
    final int end = getHeight() - effectivePaddingBottom;
    final int spaceBelow = lastBottom - end;

    final int height = getHeight() - mPaddingBottom - mPaddingTop;
    if (deltaY < 0) {
        deltaY = Math.max(-(height - 1), deltaY);
    } else {
        deltaY = Math.min(height - 1, deltaY);
    }

    if (incrementalDeltaY < 0) {
        incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
    } else {
        incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
    }

    final int firstPosition = mFirstPosition;

    // Update our guesses for where the first and last views are
    if (firstPosition == 0) {
        mFirstPositionDistanceGuess = firstTop - listPadding.top;
    } else {
        mFirstPositionDistanceGuess += incrementalDeltaY;
    }
    if (firstPosition + childCount == mItemCount) {
        mLastPositionDistanceGuess = lastBottom + listPadding.bottom;
    } else {
        mLastPositionDistanceGuess += incrementalDeltaY;
    }

    final boolean cannotScrollDown = (firstPosition == 0 &&
            firstTop >= listPadding.top && incrementalDeltaY >= 0);
    final boolean cannotScrollUp = (firstPosition + childCount == mItemCount &&
            lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0);

    if (cannotScrollDown || cannotScrollUp) {
        return incrementalDeltaY != 0;
    }

    final boolean down = incrementalDeltaY < 0;

    final boolean inTouchMode = isInTouchMode();
    if (inTouchMode) {
        hideSelector();
    }

    final int headerViewsCount = getHeaderViewsCount();
    final int footerViewsStart = mItemCount - getFooterViewsCount();

    int start = 0;
    int count = 0;

    if (down) {
        int top = -incrementalDeltaY;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            top += listPadding.top;
        }
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            if (child.getBottom() >= top) {
                break;
            } else {
                count++;
                int position = firstPosition + i;
                if (position >= headerViewsCount && position < footerViewsStart) {
                    // The view will be rebound to new data, clear any
                    // system-managed transient state.
                    child.clearAccessibilityFocus();
                    mRecycler.addScrapView(child, position);
                }
            }
        }
    } else {
        int bottom = getHeight() - incrementalDeltaY;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            bottom -= listPadding.bottom;
        }
        for (int i = childCount - 1; i >= 0; i--) {
            final View child = getChildAt(i);
            if (child.getTop() <= bottom) {
                break;
            } else {
                start = i;
                count++;
                int position = firstPosition + i;
                if (position >= headerViewsCount && position < footerViewsStart) {
                    // The view will be rebound to new data, clear any
                    // system-managed transient state.
                    child.clearAccessibilityFocus();
                    mRecycler.addScrapView(child, position);
                }
            }
        }
    }

    mMotionViewNewTop = mMotionViewOriginalTop + deltaY;

    mBlockLayoutRequests = true;

    if (count > 0) {
        detachViewsFromParent(start, count);
        mRecycler.removeSkippedScrap();
    }

    // invalidate before moving the children to avoid unnecessary invalidate
    // calls to bubble up from the children all the way to the top
    if (!awakenScrollBars()) {
       invalidate();
    }

    offsetChildrenTopAndBottom(incrementalDeltaY);

    if (down) {
        mFirstPosition += count;
    }

    final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
    if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
        fillGap(down);
    }

    if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
        final int childIndex = mSelectedPosition - mFirstPosition;
        if (childIndex >= 0 && childIndex < getChildCount()) {
            positionSelector(mSelectedPosition, getChildAt(childIndex));
        }
    } else if (mSelectorPosition != INVALID_POSITION) {
        final int childIndex = mSelectorPosition - mFirstPosition;
        if (childIndex >= 0 && childIndex < getChildCount()) {
            positionSelector(INVALID_POSITION, getChildAt(childIndex));
        }
    } else {
        mSelectorRect.setEmpty();
    }

    mBlockLayoutRequests = false;

    invokeOnItemScrollListener();

    return false;
}

final RecycleBin mRecycler = new RecycleBin();

class RecycleBin {

	........
    private ArrayList<View> mCurrentScrap;
	private ArrayList<View>[] mScrapViews;
  	........

    void addScrapView(View scrap, int position) {
            final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
            if (lp == null) {
                return;
            }

            lp.scrappedFromPosition = position;

            // Remove but don't scrap header or footer views, or views that
            // should otherwise not be recycled.
            final int viewType = lp.viewType;
            if (!shouldRecycleViewType(viewType)) {
                return;
            }

            scrap.dispatchStartTemporaryDetach();

            // The the accessibility state of the view may change while temporary
            // detached and we do not allow detached views to fire accessibility
            // events. So we are announcing that the subtree changed giving a chance
            // to clients holding on to a view in this subtree to refresh it.
            notifyViewAccessibilityStateChangedIfNeeded(
                    AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);

            // Don't scrap views that have transient state.
            final boolean scrapHasTransientState = scrap.hasTransientState();
            if (scrapHasTransientState) {
                if (mAdapter != null && mAdapterHasStableIds) {
                    // If the adapter has stable IDs, we can reuse the view for
                    // the same data.
                    if (mTransientStateViewsById == null) {
                        mTransientStateViewsById = new LongSparseArray<View>();
                    }
                    mTransientStateViewsById.put(lp.itemId, scrap);
                } else if (!mDataChanged) {
                    // If the data hasn't changed, we can reuse the views at
                    // their old positions.
                    if (mTransientStateViews == null) {
                        mTransientStateViews = new SparseArray<View>();
                    }
                    mTransientStateViews.put(position, scrap);
                } else {
                    // Otherwise, we'll have to remove the view and start over.
                    if (mSkippedScrap == null) {
                        mSkippedScrap = new ArrayList<View>();
                    }
                    mSkippedScrap.add(scrap);
                }
            } else {
                if (mViewTypeCount == 1) {
                    mCurrentScrap.add(scrap);
                } else {
                    mScrapViews[viewType].add(scrap);
                }

                if (mRecyclerListener != null) {
                    mRecyclerListener.onMovedToScrapHeap(scrap);
                }
            }
        }
}

if (mViewTypeCount == 1) {
    mCurrentScrap.add(scrap);
} else {
    mScrapViews[viewType].add(scrap);
}

abstract void fillGap(boolean down);

void fillGap(boolean down) {
    final int count = getChildCount();
    if (down) {
        int paddingTop = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            paddingTop = getListPaddingTop();
        }
        final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
                paddingTop;
        fillDown(mFirstPosition + count, startOffset);
        correctTooHigh(getChildCount());
    } else {
        int paddingBottom = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            paddingBottom = getListPaddingBottom();
        }
        final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
                getHeight() - paddingBottom;
        fillUp(mFirstPosition - 1, startOffset);
        correctTooLow(getChildCount());
    }
}

View obtainView(int position, boolean[] isScrap) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

    isScrap[0] = false;

    // Check whether we have a transient state view. Attempt to re-bind the
    // data and discard the view if we fail.
    final View transientView = mRecycler.getTransientStateView(position);
    if (transientView != null) {
        final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

        // If the view type hasn't changed, attempt to re-bind the data.
        if (params.viewType == mAdapter.getItemViewType(position)) {
            final View updatedView = mAdapter.getView(position, transientView, this);

            // If we failed to re-bind the data, scrap the obtained view.
            if (updatedView != transientView) {
                setItemViewLayoutParams(updatedView, position);
                mRecycler.addScrapView(updatedView, position);
            }
        }

        // Scrap view implies temporary detachment.
        isScrap[0] = true;
        return transientView;
    }

    final View scrapView = mRecycler.getScrapView(position);
    final View child = mAdapter.getView(position, scrapView, this);
    if (scrapView != null) {
        if (child != scrapView) {
            // Failed to re-bind the data, return scrap to the heap.
            mRecycler.addScrapView(scrapView, position);
        } else {
            isScrap[0] = true;

            child.dispatchFinishTemporaryDetach();
        }
    }

    if (mCacheColorHint != 0) {
        child.setDrawingCacheBackgroundColor(mCacheColorHint);
    }

    if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
        child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
    }

    setItemViewLayoutParams(child, position);

    if (AccessibilityManager.getInstance(mContext).isEnabled()) {
        if (mAccessibilityDelegate == null) {
            mAccessibilityDelegate = new ListItemAccessibilityDelegate();
        }
        if (child.getAccessibilityDelegate() == null) {
            child.setAccessibilityDelegate(mAccessibilityDelegate);
        }
    }

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);

    return child;
}

int scrollBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
    if (getChildCount() == 0 || dy == 0) {
        return 0;
    }
    mLayoutState.mRecycle = true;
    ensureLayoutState();
    final int layoutDirection = dy > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;
    final int absDy = Math.abs(dy);
    updateLayoutState(layoutDirection, absDy, true, state);
    final int freeScroll = mLayoutState.mScrollingOffset;
    final int consumed = freeScroll + fill(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 = absDy > consumed ? layoutDirection * consumed : dy;
    mOrientationHelper.offsetChildren(-scrolled);
    if (DEBUG) {
        Log.d(TAG, "scroll req: " + dy + " scrolled: " + scrolled);
    }
    mLayoutState.mLastScrollDelta = scrolled;
    return scrolled;
}

public final class Recycler {

	..........

    final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<ViewHolder>();
    private ArrayList<ViewHolder> mChangedScrap = null;
    final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();
    private final List<ViewHolder> mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap);
    private int mViewCacheMax = DEFAULT_CACHE_SIZE;
    private RecycledViewPool mRecyclerPool;
    private ViewCacheExtension mViewCacheExtension;
    private static final int DEFAULT_CACHE_SIZE = 2;

    ..........