Android面试题目之（13） ListView的设计原理

1.ListView的目标

ListView希望通过复用看不见的View来达到目的。其中最最重要的就是这个

2.RecycleBin--回收站.

回收站并不是垃圾站。
    最重要的是两个成员变量 mScrapViews 和mActiveViews， mScrapViews 表示回收站的View. mActiveViews表示激活的View，激活的View是可以直接拿来显示的，不想要重新调用getView填充数据。激活的View（mActiveViews）和废弃的View（mScrapViews） 不同，激活的View不想要调用getView填充数据，而废弃的View需要。

      

 /**
         * Unsorted views that can be used by the adapter as a convert view.
         */
        private ArrayList<View>[] mScrapViews;

        private View[] mActiveViews = new View[0];

  看不见的View都会放进这里面。主要是通过以下方法 addScrapView 加进去的
   

    /**
         * Put a view into the ScapViews list. These views are unordered.
         *
         * @param scrap The view to add
         */
        void addScrapView(View scrap, int position) {
            AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
            if (lp == null) {
                return;
            }

            // Don't put header or footer views or views that should be ignored
            // into the scrap heap
            int viewType = lp.viewType;
            if (!shouldRecycleViewType(viewType)) {
                if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                    removeDetachedView(scrap, false);
                }
                return;
            }

            lp.scrappedFromPosition = position;

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

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

3.回收站算法

makeAndAddView 这个方法显示了回收站的使用算法。如果没有dadachange，那么就直接从getActiveView 方法取得那个View，如果成功，就调整其位置。如果datachange，就需要重新绑定数据到这个View上。这里的 重新绑定数据 方法obtainView 同时考虑是否使用回收站的View.

    

 private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
            boolean selected) {
        View child;


        if (!mDataChanged) {
            // Try to use an existing view for this position
            child = mRecycler.getActiveView(position);
            if (child != null) {
                if (ViewDebug.TRACE_RECYCLER) {
                    ViewDebug.trace(child, ViewDebug.RecyclerTraceType.RECYCLE_FROM_ACTIVE_HEAP,
                            position, getChildCount());
                }

                // Found it -- we're using an existing child
                // This just needs to be positioned
                setupChild(child, position, y, flow, childrenLeft, selected, true);

                return child;
            }
        }

        // Make a new view for this position, or convert an unused view if possible
        child = obtainView(position, mIsScrap);

        // This needs to be positioned and measured
        setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

        return child;
}

4.通过ObtainView方法绑定数据

其实也很简单。主要任务是复用mScrapViews。

   

View obtainView(int position, boolean[] isScrap) {
        isScrap[0] = false;
        View scrapView;

        scrapView = mRecycler.getScrapView(position);

        View child;
        if (scrapView != null) {
            if (ViewDebug.TRACE_RECYCLER) {
                ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.RECYCLE_FROM_SCRAP_HEAP,
                        position, -1);
            }

            child = mAdapter.getView(position, scrapView, this);

            if (ViewDebug.TRACE_RECYCLER) {
                ViewDebug.trace(child, ViewDebug.RecyclerTraceType.BIND_VIEW,
                        position, getChildCount());
            }

            if (child != scrapView) {
                mRecycler.addScrapView(scrapView, position);
                if (mCacheColorHint != 0) {
                    child.setDrawingCacheBackgroundColor(mCacheColorHint);
                }
                if (ViewDebug.TRACE_RECYCLER) {
                    ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP,
                            position, -1);
                }
            } else {
                isScrap[0] = true;
                child.dispatchFinishTemporaryDetach();
            }
        } else {
            child = mAdapter.getView(position, null, this);
            if (mCacheColorHint != 0) {
                child.setDrawingCacheBackgroundColor(mCacheColorHint);
            }
            if (ViewDebug.TRACE_RECYCLER) {
                ViewDebug.trace(child, ViewDebug.RecyclerTraceType.NEW_VIEW,
                        position, getChildCount());
            }
        }

        return child;
    }

5.布局listView

fillDown 方法用得最多，其目的就是从pos这个index和nextTop这个View top坐标，向下填充Views直到底部。

    /**
     * Fills the list from pos down to the end of the list view.
     *
     * @param pos The first position to put in the list
     *
     * @param nextTop The location where the top of the item associated with pos
     *        should be drawn
     *
     * @return The view that is currently selected, if it happens to be in the
     *         range that we draw.
     */
    private View fillDown(int pos, int nextTop) {
        View selectedView = null;

        int end = (mBottom - mTop);
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            end -= mListPadding.bottom;
        }

        while (nextTop < end && pos < mItemCount) {
            // is this the selected item?
            boolean selected = pos == mSelectedPosition;
            View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);

            nextTop = child.getBottom() + mDividerHeight;
            if (selected) {
                selectedView = child;
            }
            pos++;
        }

        return selectedView;
    }

6.触发fillDown的切入点

layoutChildren
trackMotionScroll


7.不得不说的LayoutParams

    /**
     * AbsListView extends LayoutParams to provide a place to hold the view type.
     */
    public static class LayoutParams extends ViewGroup.LayoutParams {
        /**
         * View type for this view, as returned by
         * {@link android.widget.Adapter#getItemViewType(int) }
         */
        @ViewDebug.ExportedProperty(category = "list", mapping = {
            @ViewDebug.IntToString(from = ITEM_VIEW_TYPE_IGNORE, to = "ITEM_VIEW_TYPE_IGNORE"),
            @ViewDebug.IntToString(from = ITEM_VIEW_TYPE_HEADER_OR_FOOTER, to = "ITEM_VIEW_TYPE_HEADER_OR_FOOTER")
        })
        int viewType;

        /**
         * When this boolean is set, the view has been added to the AbsListView
         * at least once. It is used to know whether headers/footers have already
         * been added to the list view and whether they should be treated as
         * recycled views or not.
         */
        @ViewDebug.ExportedProperty(category = "list")
        boolean recycledHeaderFooter;

        /**
         * When an AbsListView is measured with an AT_MOST measure spec, it needs
         * to obtain children views to measure itself. When doing so, the children
         * are not attached to the window, but put in the recycler which assumes
         * they've been attached before. Setting this flag will force the reused
         * view to be attached to the window rather than just attached to the
         * parent.
         */
        @ViewDebug.ExportedProperty(category = "list")
        boolean forceAdd;

        /**
         * The position the view was removed from when pulled out of the
         * scrap heap.
         * @hide
         */
        int scrappedFromPosition;

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }

        public LayoutParams(int w, int h) {
            super(w, h);
        }

        public LayoutParams(int w, int h, int viewType) {
            super(w, h);
            this.viewType = viewType;
        }

        public LayoutParams(ViewGroup.LayoutParams source) {
            super(source);
        }
    }

8.提高布局性能的函数 offsetLeftAndRight和 offsetTopAndBottom

在setupChild函数 中可以看到如下两行

      if (needToMeasure) {
            final int childRight = childrenLeft + w;
            final int childBottom = childTop + h;
            child.layout(childrenLeft, childTop, childRight, childBottom);
        } else {
            child.offsetLeftAndRight(childrenLeft - child.getLeft());
            child.offsetTopAndBottom(childTop - child.getTop());
 }

9. 提高性能的函数addViewInLayout

        if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
                p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
            attachViewToParent(child, flowDown ? -1 : 0, p);
        } else {
            p.forceAdd = false;
            if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                p.recycledHeaderFooter = true;
            }
            addViewInLayout(child, flowDown ? -1 : 0, p, true);
        }