轉載:從源代碼的角度分析--在BaseAdapter調用notifyDataSetChanged()之後發生了什麽
利用Adapter作為ListView的適配器,為ListView提供數據。選中某一項後,要讓這一項變成選中狀態,也就是背景圖片要換一下。下面我就用一個小例子來模擬。重點不在於實現,而是了解Adapter中notifyDataSetChanged()背後的運行機制。
我們先做一個小Demo(文中涉及的Demo在文章末尾),功能是選中某一項後,背景顏色會變紅。代碼非常簡單,這裏就不解釋了。值得註意的是,當我們需要ListView進行刷新的時候,我們需要調用Adapter.notifyDataSetChanged()來讓界面刷新。
1 public class MainActivity extends Activity { 2 @Override 3 protected void onCreate(Bundle savedInstanceState) { 4 5 super.onCreate(savedInstanceState); 6 setContentView(R.layout.activity_main); 7 8 ListView main_list = (ListView)this.findViewById(R.id.main_list); 9 MyArrayAdapter mArrayList=new MyArrayAdapter(this,R.layout.list_item,getData()); 10 main_list.setAdapter(mArrayList); 11 main_list.setOnItemClickListener(mArrayList); 12 } 13 14 private String[] getData() { 15 return new String[]{"測試數據1","測試數據2","測試數據3","測試數據4"}; 16 } 17 }
適配器MyArrayAdapter代碼:
1 public class MyArrayAdapter extends ArrayAdapter<String> implements 2 OnItemClickListener { 3 4 private int itemClicked; 5 6 public MyArrayAdapter(Context context, int textViewResourceId, 7 String[] objects) { 8 super(context, textViewResourceId, objects); 9 10 } 11 @Override 12 public View getView(int position, View convertView, ViewGroup parent) { 13 14 convertView=super.getView(position, convertView, parent); 15 //如果是被點擊的項,變換顏色 16 if (position==this.itemClicked) { 17 convertView.setBackgroundColor(Color.RED); 18 }else { 19 convertView.setBackgroundColor(Color.WHITE); 20 } 21 return convertView; 22 } 23 @Override 24 public void onItemClick(AdapterView<?> parent, View view, int position, 25 long id) { 26 //設置某項被點擊 27 itemClicked=position; 28 this.notifyDataSetChanged(); 29 } 30 31 }
下面就讓我們跟進去MyArrayAdapter.notifyDataSetChange()中看看。在本文中,我所查看的Android源代碼是4.4.0的,不同版本可能有所出入。
1 public void notifyDataSetChanged() { 2 super.notifyDataSetChanged(); 3 mNotifyOnChange = true; 4 }
源代碼就簡單兩句話,那麽繼續看看super是什麽?
public class ArrayAdapter<T> extends BaseAdapter implements Filterable
從類的聲明中,父類就是ArrayAdapter,而ArrayList的父類是BaseAdapter。我們跟進BaseAdapter中看看。
1 public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter { 2 private final DataSetObservable mDataSetObservable = new DataSetObservable(); 3 //...省略不必要的代碼 4 public void registerDataSetObserver(DataSetObserver observer) { 5 mDataSetObservable.registerObserver(observer); 6 } 7 8 public void unregisterDataSetObserver(DataSetObserver observer) { 9 mDataSetObservable.unregisterObserver(observer); 10 } 11 12 public void notifyDataSetChanged() { 13 mDataSetObservable.notifyChanged(); 14 } 15 16 public void notifyDataSetInvalidated() { 17 mDataSetObservable.notifyInvalidated(); 18 } 19 //...省略不必要的代碼 20 }
我們發現其實就是DataSetObservable這個對象在發生作用,但是DataSetObservable這個對象估計就是一個簡單的觀察者的實現,Android框架的編寫者不大可能將業務邏輯放在這裏面,不過我們還是要確認是不是跟我們所想的一樣。
1 public class DataSetObservable extends Observable<DataSetObserver> { 2 /** 3 * Invokes onChanged on each observer. Called when the data set being observed has 4 * changed, and which when read contains the new state of the data. 5 */ 6 public void notifyChanged() { 7 synchronized(mObservers) { 8 for (DataSetObserver observer : mObservers) { 9 observer.onChanged(); 10 } 11 } 12 } 13 14 /** 15 * Invokes onInvalidated on each observer. Called when the data set being monitored 16 * has changed such that it is no longer valid. 17 */ 18 public void notifyInvalidated() { 19 synchronized (mObservers) { 20 for (DataSetObserver observer : mObservers) { 21 observer.onInvalidated(); 22 } 23 } 24 } 25 }
果然,跟預想的一樣,它只是簡單地調用了綁定在它身上的回調接口。那麽BaseAdapter.notifyDataSetChange()的接口具體是在哪裏綁定的呢?很有可能在構造函數中綁定,我們跟進ArrayListAdapter看看。
1 public ArrayAdapter(Context context, int textViewResourceId, List<T> objects) { 2 init(context, textViewResourceId, 0, objects); 3 } 4 5 private void init(Context context, int resource, int textViewResourceId, List<T> objects) { 6 mContext = context; 7 mInflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); 8 mResource = mDropDownResource = resource; 9 mObjects = objects; 10 mFieldId = textViewResourceId; 11 }
ArrayListAdapter中有很多構造函數,但是幾經輾轉全部都會轉到init()函數中,很遺憾,我們撲空了。那麽還在哪裏可能綁定notifyDataSetChange()回調函數呢?其實從MainActivity中Adapter的初始化過程中,基本上只能鎖定在MainActivity第十行中setAdapter函數中。接下去看看 public void setAdapter(ListAdapter adapter)這個函數。
1 public void setAdapter(ListAdapter adapter) { 2 if (null != mAdapter) { 3 mAdapter.unregisterDataSetObserver(mDataSetObserver); 4 } 5 6 resetList(); 7 mRecycler.clear(); 8 9 if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) { 10 mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter); 11 } else { 12 mAdapter = adapter; 13 } 14 15 mOldSelectedPosition = INVALID_POSITION; 16 mOldSelectedRowId = INVALID_ROW_ID; 17 if (mAdapter != null) { 18 mAreAllItemsSelectable = mAdapter.areAllItemsEnabled(); 19 mOldItemCount = mItemCount; 20 mItemCount = mAdapter.getCount(); 21 checkFocus(); 22 23 mDataSetObserver = new AdapterDataSetObserver(); 24 mAdapter.registerDataSetObserver(mDataSetObserver); 25 26 mRecycler.setViewTypeCount(mAdapter.getViewTypeCount()); 27 28 int position; 29 if (mStackFromBottom) { 30 position = lookForSelectablePosition(mItemCount - 1, false); 31 } else { 32 position = lookForSelectablePosition(0, true); 33 } 34 setSelectedPositionInt(position); 35 setNextSelectedPositionInt(position); 36 37 if (mItemCount == 0) { 38 // Nothing selected 39 checkSelectionChanged(); 40 } 41 42 if (mChoiceMode != CHOICE_MODE_NONE && 43 mAdapter.hasStableIds() && 44 mCheckedIdStates == null) { 45 mCheckedIdStates = new LongSparseArray<Boolean>(); 46 } 47 48 } else { 49 mAreAllItemsSelectable = true; 50 checkFocus(); 51 // Nothing selected 52 checkSelectionChanged(); 53 } 54 55 if (mCheckStates != null) { 56 mCheckStates.clear(); 57 } 58 59 if (mCheckedIdStates != null) { 60 mCheckedIdStates.clear(); 61 } 62 63 requestLayout(); 64 }
setAdapter(...)這個函數有點長,不過我們只需要關註跟notifiDataSetChange()有關的實現,也就是第23、24行。不過這裏另一個值得關註的點就是第63行,requestLayout()這個函數,它主要就是用來刷新界面,讓界面重新繪制的。在23,、24行,綁定了一個AdapterDataSetObserver對象,下面我們就跟進去看看。從前面DataSetObservable的實現中,我們知道了它在notifyDataSetChange()的時候會調用DataSetObserver的onChange()。
1 class AdapterDataSetObserver extends DataSetObserver 2 { 3 private Parcelable mInstanceState = null; 4 5 AdapterDataSetObserver() { 6 } 7 public void onChanged() { mDataChanged = true; 8 mOldItemCount = mItemCount; 9 mItemCount = getAdapter().getCount(); 10 11 if ((getAdapter().hasStableIds()) && (mInstanceState != null) && (mOldItemCount == 0) && (mItemCount > 0)) 12 { 13 onRestoreInstanceState(mInstanceState); 14 mInstanceState = null; 15 } else { 16 rememberSyncState(); 17 } 18 checkFocus(); 19 requestLayout(); 20 } 21 //...省略不必要代碼 22 }
終於,在第19行,我們看見了requestLayout(),它就是用來重繪界面的,它在ViewRootImpl.java中有具體的實現。
1 public void requestLayout() { 2 checkThread(); 3 mLayoutRequested = true; 4 scheduleTraversals(); 5 }
關於scheduleTraversals()的實現,涉及到Android中View的繪制流程,感興趣的可以看看《Android視圖狀態及重繪流程分析,帶你一步步深入了解View(三)》。
到了這裏,我們就清楚了notifyDataSetChange()背後的實現機制了,在不知不覺之間Android框架幫我們幹了很多事情,不過需要提醒的時,每一次notifyDataSetChange()都會引起界面的重繪。當需要修改界面上View的相關屬性的時候,最後先設置完成再調用notifyDataSetChange()來重繪界面。
轉載鏈接:https://www.cnblogs.com/kissazi2/p/3721941.html
轉載:從源代碼的角度分析--在BaseAdapter調用notifyDataSetChanged()之後發生了什麽