Adapter.notifyDataSetChanged()源碼分析以及與ListView.setAdapter的區別

一直很好奇，notifyDataSetChanged究竟是重繪了整個ListView還是只重繪了被修改的那些Item，它與重新設置適配器即調用setAdapter的區別在哪里？所以特地追蹤了一下源碼，過程如下：

一、notifyDataSetChanged實現機制

自定義Activity中有如下調用語句：

checkoutAdapter.notifyDataSetChanged();

點擊notifyDataSetChanged()進行代碼跟蹤。首先，進入到BaseAdapter的notifyDataSetChanged方法：

public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }

我們發現其實就是DataSetObservable這個對象在發生作用，點擊notifyChanged進行追蹤。

public class DataSetObservable extends Observable<DataSetObserver> {
    /**
     * Invokes onChanged on each observer. Called when the data set being observed has
     * changed, and which when read contains the new state of the data.
     */
    public void notifyChanged() {
        synchronized(mObservers) {
            // since onChanged() is implemented by the app, it could do anything, including
            // removing itself from {@link mObservers} - and that could cause problems if
            // an iterator is used on the ArrayList {@link mObservers}.
            // to avoid such problems, just march thru the list in the reverse order.
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onChanged();
            }
        }
    }

繼續跟蹤onChanged()，我們發現DataSetObserver 是個抽象類，其派生類實例對象是在哪里指定的呢？根據經驗，我們需要回溯至adapter的構造過程。

public abstract class DataSetObserver {
    /**
     * This method is called when the entire data set has changed,
     * most likely through a call to {@link Cursor#requery()} on a {@link Cursor}.
     */
    public void onChanged() {
        // Do nothing
    }

    /**
     * This method is called when the entire data becomes invalid,
     * most likely through a call to {@link Cursor#deactivate()} or {@link Cursor#close()} on a
     * {@link Cursor}.
     */
    public void onInvalidated() {
        // Do nothing
    }
}

先看adapter的構造函數

        CheckOut_DishListViewAdapter checkoutAdapter;
// 綁定適配器
        checkoutAdapter = new CheckOut_DishListViewAdapter(
                CheckOutActivity.this, list_dish);
        list_view_dish.setAdapter(checkoutAdapter);

public class CheckOut_DishListViewAdapter extends BaseAdapter {
    private DecimalFormat df = new DecimalFormat("######0.00");// 用於double保留兩位小數
    private LayoutInflater mInflater;
    private List<HashMap<String, Object>> list;

    public CheckOut_DishListViewAdapter(Context con,
            List<HashMap<String, Object>> list) {
        mInflater = LayoutInflater.from(con);
        this.list = list;
    }

顯然沒有DataSetObserver的有關信息。

再看ListView中的setAdapter方法，我們省略其他代碼，只看與DataSetObserver相關的部分，從mDataSetObserver = new AdapterDataSetObserver();可知，AdapterDataSetObserver是DataSetObserver的實例化類。

@Override
    public void setAdapter(ListAdapter adapter) {
        ...
        if (mAdapter != null) {
            ...27 
            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

            ...46         } else {
            ...51         }

        requestLayout();
    }

查看AdapterDataSetObserver的onChanged方法：

class AdapterDataSetObserver extends DataSetObserver
  {
    private Parcelable mInstanceState = null;

    AdapterDataSetObserver() {
    }
    public void onChanged() { 
      mDataChanged = true;
      mOldItemCount = mItemCount;
      mItemCount = getAdapter().getCount();

      if ((getAdapter().hasStableIds()) && (mInstanceState != null) && (mOldItemCount == 0) && (mItemCount > 0))
      {
        onRestoreInstanceState(mInstanceState);
        mInstanceState = null;
      } else {
        rememberSyncState();
      }
      checkFocus();
      requestLayout();
    }
    //...省略不必要代碼
}

在第20行，我們看見了requestLayout()，它就是用來重繪界面的，點擊追蹤requestLayout時，無法繼續追蹤，這時通過查找系統源碼，我們發現AdapterDataSetObserver原來是抽象類AdapterView的內部類

public abstract class AdapterView<T extends Adapter> extends ViewGroup {
    ...
}

    class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;

            checkFocus();
            requestLayout();
        }

        public void clearSavedState() {
            mInstanceState = null;
        }
    }

在21行，我們又看見了requestLayout()，Ctrl+單擊該方法，進入到View類的同名方法

    /**
     * Call this when something has changed which has invalidated the
     * layout of this view. This will schedule a layout pass of the view
     * tree.
     */
    public void requestLayout() {
        if (ViewDebug.TRACE_HIERARCHY) {
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.REQUEST_LAYOUT);
        }

        mPrivateFlags |= FORCE_LAYOUT;
        mPrivateFlags |= INVALIDATED;

        if (mParent != null) {
            if (mLayoutParams != null) {
                mLayoutParams.resolveWithDirection(getResolvedLayoutDirection());
            }
            if (!mParent.isLayoutRequested()) {
                mParent.requestLayout();
            }
        }
    }

在第19行，我們發現該方法將requestLayout()任務上拋至其mParent，因此我們需要追蹤mParent，先來看看誰為它賦值：

    /*
     * Caller is responsible for calling requestLayout if necessary.
     * (This allows addViewInLayout to not request a new layout.)
     */
    void assignParent(ViewParent parent) {
        if (mParent == null) {
            mParent = parent;
        } else if (parent == null) {
            mParent = null;
        } else {
            throw new RuntimeException("view " + this + " being added, but"
                    + " it already has a parent");
        }
    }

原來是assignParent，因此在構造子view的過程中，子view一定有assignParent的操作。根據View Tree的層級關系，我們可以猜測，這樣一層層的上拋請求，最后應該上拋至Activity的根View，這個根View是誰？根據我們對Activity加載布局流程的理解，這個根View其實就是DecorView，那么我們先來看看DecorView中是否有requestLayout方法的具體實現。

我們知道DecorView是PhoneWindow的內部類，進入DecorView類，

private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {

發現DecorView繼承自FrameLayout ，也即間接繼承自View，但DecorView中並未重寫requestLayout方法，說明DecorView並不是requestLayout的最終執行者，DecorView存在mParent，要想弄清楚DecorView的mParent是誰，我們有必要回顧一下DecorView是如何裝載到Activity的。

我們按照流程圖一級一級的找，在WindowManagerImpl中找到addView方法，發現新建了一個ViewRootImpl對象，並在最后調用ViewRootImpl的setView方法，接下來我們繼續跟進setView方法。

private void addView(View view, ViewGroup.LayoutParams params,
            CompatibilityInfoHolder cih, boolean nest) {
            ...
        
            ViewRootImpl root;
            ...
            
            root = new ViewRootImpl(view.getContext());
            ...
            root.setView(view, wparams, panelParentView);
    } 

在ViewRootImpl的setView方法中找到如下代碼：view.assignParent(this);也即將DecorView的mParent指定為ViewRootImpl實例，並且在第6行發現調用了requestLayout方法。

     public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
         synchronized (this) {
             if (mView == null) {
                 mView = view;
                 ...
　　　　　　　　　　requestLayout();
　　　　　　　　　　...

                 view.assignParent(this);
                 ...
     } 

進入到ViewRootImpl的requestLayout方法：

    public void requestLayout() {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }

之后的流程參考從ViewRootImpl類分析View繪制的流程一文。

從以上分析可知，每一次notifyDataSetChange()都會引起界面的重繪，重繪的最終實現是在ViewRootImpl.java中。

二、notifyDataSetChanged與setAdapter區別

仔細閱讀ListView的setAdapter方法，當ListView之前綁定過adapter信息時，在這里會清除原有Adapter和數據集觀察者等信息，重置了ListView當前選中項等信息，並在方法的最后一句調用requestLayout進行界面的重繪。

public void setAdapter(ListAdapter adapter) {
        // 與原有觀察者解綁定
        if (mAdapter != null && mDataSetObserver != null) {
            mAdapter.unregisterDataSetObserver(mDataSetObserver);
        }

        resetList();
        mRecycler.clear();

        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
            mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
        } else {
            mAdapter = adapter;
        }
        
        mOldSelectedPosition = INVALID_POSITION;
        mOldSelectedRowId = INVALID_ROW_ID;

        // AbsListView#setAdapter will update choice mode states.
        super.setAdapter(adapter);

        if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
            checkFocus();
            // 重新綁定新的數據集觀察者
            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

            mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

            int position;
            if (mStackFromBottom) {
                position = lookForSelectablePosition(mItemCount - 1, false);
            } else {
                position = lookForSelectablePosition(0, true);
            }
            setSelectedPositionInt(position);
            setNextSelectedPositionInt(position);

            if (mItemCount == 0) {
                // Nothing selected
                checkSelectionChanged();
            }
        } else {
            mAreAllItemsSelectable = true;
            checkFocus();
            // Nothing selected
            checkSelectionChanged();
        }
        // 重繪
        requestLayout();
    }

由此可知，調用adapter.notifyDataSetChanged與listView.setAdapter函數都會引起界面重繪，區別是前者會保留原有位置、數據信息，后者是回到初始狀態。

 

 注：以上過程純屬個人探索，如有錯誤敬請批評指正。

參考文獻：

1.從ViewRootImpl類分析View繪制的流程（http://blog.csdn.net/feiduclear_up/article/details/46772477）

2.從源代碼的角度分析--在BaseAdapter調用notifyDataSetChanged()之后發生了什么（http://www.cnblogs.com/kissazi2/p/3721941.html ）