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[操作系统]读BaseAdapter的一点感悟


  适配器,作为android应用层的开发中,具有很重要的作用。在诸如ListView,gallery等sdk中提供的展示批量数据的控件中,起到一个适配数据源的作用。sdk中已经为我们提供了一个简单的并且适用性很广的适配器SimpleAdapter,该类就是继承自抽象类BaseAdapter实现的一个具体的适配器,并且能接收 List<? extends Map<String, ?>>形式的数据源格式的数据。但是很多情况下,我们使用ListView等容器的时候,往往要在渲染每一条数据的时候做一些我们自己想做的事情,此时就要实现BaseAdapter来实现自己的一个Adapter。


 

  在进入正文之前,先说一下关于抽象类与接口的区别,以下摘自百度知道。

  你选择使用接口和抽象类的依据是什么? 接口和抽象类的概念不一样。接口是对动作的抽象,抽象类是对根源的抽象。抽象类表示的是,这个对象是什么。接口表示的是,这个对象能做什么。比如,男人,女人,这两个类(如果是类的话……),他们的抽象类是人。说明,他们都是人。人可以吃东西,狗也可以吃东西,你可以把“吃东西”定义成一个接口,然后让这些类去实现它.所以,在高级语言上,一个类只能继承一个类(抽象类)(正如人不可能同时是生物和非生物),但是可以实现多个接口(吃饭接口、走路接口)。第一点. 接口是抽象类的变体,接口中所有的方法都是抽象的。而抽象类是声明方法的存在而不去实现它的类。 第二点. 接口可以继承,抽象类不行 第三点. 接口定义方法,不能实现,而抽象类可以实现部分方法。 第四点. 接口中基本数据类型为static 而抽类象不是的。当你关注一个事物的本质的时候,用抽象类;当你关注一个操作的时候,用接口。 接口可以实现也可以继承,抽象类不行 抽象类的功能要远超过接口,但是,定义抽象类的代价高。因为高级语言来说(从实际设计上来说也是)每个类只能继承一个类。在这个类中,你必须继承或编写出其所有子类的所有共性。虽然接口在功能上会弱化许多,但是它只是针对一个动作的描述。而且你可以在一个类中同时实现多个接口。在设计阶段会降低难度的。

  


 

  上面说的很清楚了,接口是对动作的抽象,而抽象类是对根源的抽象。抽象类表示的是这个对象是什么,而接口表示的是这个对象能做什么。同时在我阅读BaseAdapter源码的时候也有些感悟,BaseAdapter是一个抽象类,它实现了ListAdapter和SpinnerAdapter接口,而这两个接口又实现了Adapter接口。我们在构造自己的Adapter的时候经常要必须实现的getCount()和getView()方法都是在Adapter接口中定义的。

  在大牛们写的源码中,往往都是接口与抽象类同时使用。有关于BaseAdapter中的源码,则把最基本的一些功能(能做什么)抽象为一个接口,放在最底层,而后通过继承这个接口再抽象一些子接口,当有抽象类实现了这些子接口的时候,可以选择性的(这是抽象类允许的)实现接口中定义的功能,作为一个具有统一功能的更具体的但是同样以抽象的形式而存在,最后我们要构造这种经过抽象的类型的具体类的时候,则需要将抽象类没有实现的功能全部实现(这是必须的),甚至可以重写抽象类中已经实现的某些方法(一般情况下不必这样做),这时我们可以通过实例化我们自己构造的具体类来完成一些工作了。

  这种层级关系让我体验到了从无到有的过程,也深刻的理解了什么叫抽象到具体的过程,在这种模型的基础上,我们可以从容的扩展和具象化。不必担心功能的丢失或者偏离设计此类型的初衷,由简化繁需要强大的根基,抽象类和接口的结合就是面向对象变成最强大的根基。

 

 

  下面贴出源码来具体看

  既然上面说到了由简化繁,那么就从最基本的接口开始

  1、Adapter

 1 package android.widget; 2  3 import android.database.DataSetObserver; 4 import android.view.View; 5 import android.view.ViewGroup; 6  7 /** 8  * 。。。。。。 9 */ 10 public interface Adapter { 11   /** 12    * Register an observer that is called when changes happen to the data used by this adapter. 13    * 14    * @param observer the object that gets notified when the data set changes. 15   */ 16   void registerDataSetObserver(DataSetObserver observer); 17  18   /** 19    * Unregister an observer that has previously been registered with this 20    * adapter via {@link #registerDataSetObserver}. 21    * 22    * @param observer the object to unregister. 23   */ 24   void unregisterDataSetObserver(DataSetObserver observer); 25  26   /** 27    * How many items are in the data set represented by this Adapter. 28    *  29    * @return Count of items. 30   */ 31   int getCount();   32    33   /** 34    * Get the data item associated with the specified position in the data set. 35    *  36    * @param position Position of the item whose data we want within the adapter's  37    * data set. 38    * @return The data at the specified position. 39   */ 40   Object getItem(int position); 41    42   /** 43    * Get the row id associated with the specified position in the list. 44    *  45    * @param position The position of the item within the adapter's data set whose row id we want. 46    * @return The id of the item at the specified position. 47   */ 48   long getItemId(int position); 49    50   /** 51    * Indicates whether the item ids are stable across changes to the 52    * underlying data. 53    *  54    * @return True if the same id always refers to the same object. 55   */ 56   boolean hasStableIds(); 57    58   /** 59    * 。。。。。。 60   */ 61   View getView(int position, View convertView, ViewGroup parent); 62  63   /** 64    * An item view type that causes the {@link AdapterView} to ignore the item 65    * view. For example, this can be used if the client does not want a 66    * particular view to be given for conversion in 67    * {@link #getView(int, View, ViewGroup)}. 68    *  69    * @see #getItemViewType(int) 70    * @see #getViewTypeCount() 71   */ 72   static final int IGNORE_ITEM_VIEW_TYPE = AdapterView.ITEM_VIEW_TYPE_IGNORE; 73    74   /** 75    * Get the type of View that will be created by {@link #getView} for the specified item. 76    *  77    * @param position The position of the item within the adapter's data set whose view type we 78    *    want. 79    * @return An integer representing the type of View. Two views should share the same type if one 80    *     can be converted to the other in {@link #getView}. Note: Integers must be in the 81    *     range 0 to {@link #getViewTypeCount} - 1. {@link #IGNORE_ITEM_VIEW_TYPE} can 82    *     also be returned. 83    * @see #IGNORE_ITEM_VIEW_TYPE 84   */ 85   int getItemViewType(int position); 86    87   /** 88    * 。。。。。。 89   */ 90   int getViewTypeCount(); 91    92   static final int NO_SELECTION = Integer.MIN_VALUE; 93  94   /** 95    * @return true if this adapter doesn't contain any data. This is used to determine 96    * whether the empty view should be displayed. A typical implementation will return 97    * getCount() == 0 but since getCount() includes the headers and footers, specialized 98    * adapters might want a different behavior. 99    */100   boolean isEmpty();101 }

 

  然后是继承自这个接口的两个接口

  2、ListAdapter、SpinnerAdapter

  (由于内容非常少,则粘在一起)

  

 1 package android.widget; 2  3 /** 4  * 。。。。。。 5 */ 6 public interface ListAdapter extends Adapter { 7  8   /** 9    * 。。。。。。10   */11   public boolean areAllItemsEnabled();12 13   /**14    * 。。。。。。15   */16   boolean isEnabled(int position);17 }18 19 20 21 22 package android.widget;23 24 import android.view.View;25 import android.view.ViewGroup;26 27 /**28  * Extended {@link Adapter} that is the bridge between a29  * {@link android.widget.Spinner} and its data. A spinner adapter allows to30  * define two different views: one that shows the data in the spinner itself and31  * one that shows the data in the drop down list when the spinner is pressed.</p>32 */33 public interface SpinnerAdapter extends Adapter {34   /**35    * 。。。。。。36   */37   public View getDropDownView(int position, View convertView, ViewGroup parent);38 }

 

 

 

  这里BaseAdapter用到的是观察者模式,在ListView的setAdapter里注册一个AdapterDataSetObserver观察者,相当于在使用按钮时候,对按钮set一个OnClickListener。这里的AdapterDataSetObserver是一个内部类,这个类继承自DataSetObserver这个抽象类,并且重写了onChanged()和onInvalidated()方法,而OnClickListener则是一个接口的子类,实现了这个接口的匿名类或者是实体类,重写了OnClick()方法。

  3、BaseAdapter

  

 1 public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter { 2   // 这里实例化一个观察者操作类 3   private final DataSetObservable mDataSetObservable = new DataSetObservable(); 4   5   public boolean hasStableIds() { 6     return false; 7   } 8     9   // 这里DataSetObserver是一个观察者的抽象类型 通过参数传入具体的观察者10   public void registerDataSetObserver(DataSetObserver observer) {11     mDataSetObservable.registerObserver(observer);12   }13  14   public void unregisterDataSetObserver(DataSetObserver observer) {15     mDataSetObservable.unregisterObserver(observer);16   }17    18   /**19    * Notifies the attached observers that the underlying data has been changed20    * and any View reflecting the data set should refresh itself.21   */22   public void notifyDataSetChanged() {23     // 通过观察者操作类回调注册的观察者中用户编写的OnChanged()方法24     mDataSetObservable.notifyChanged();25   }26  27   /**28    * Notifies the attached observers that the underlying data is no longer valid29    * or available. Once invoked this adapter is no longer valid and should30    * not report further data set changes.31   */32   public void notifyDataSetInvalidated() {33     mDataSetObservable.notifyInvalidated();34   }35  36   public boolean areAllItemsEnabled() {37     return true;38   }39  40   public boolean isEnabled(int position) {41     return true;42   }43  44   public View getDropDownView(int position, View convertView, ViewGroup parent) {45     return getView(position, convertView, parent);46   }47  48   public int getItemViewType(int position) {49     return 0;50   }51  52   public int getViewTypeCount() {53     return 1;54   }55    56   public boolean isEmpty() {57     return getCount() == 0;58   }59 }

 

  4、观察者抽象类DataSetObserver

  

 1 /* 2  * Copyright (C) 2007 The Android Open Source Project 3  * 4  * Licensed under the Apache License, Version 2.0 (the "License"); 5  * you may not use this file except in compliance with the License. 6  * You may obtain a copy of the License at 7  * 8  *   http://www.apache.org/licenses/LICENSE-2.0 9  *10  * Unless required by applicable law or agreed to in writing, software11  * distributed under the License is distributed on an "AS IS" BASIS,12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.13  * See the License for the specific language governing permissions and14  * limitations under the License.15 */16 17 package android.database;18 19 /**20  * Receives call backs when a data set has been changed, or made invalid. The typically data sets21  * that are observed are {@link Cursor}s or {@link android.widget.Adapter}s.22  * DataSetObserver must be implemented by objects which are added to a DataSetObservable.23 */24 public abstract class DataSetObserver {25   /**26    * This method is called when the entire data set has changed,27    * most likely through a call to {@link Cursor#requery()} on a {@link Cursor}.28   */29   public void onChanged() {30     // Do nothing31   }32 33   /**34    * This method is called when the entire data becomes invalid,35    * most likely through a call to {@link Cursor#deactivate()} or {@link Cursor#close()} on a36    * {@link Cursor}.37   */38   public void onInvalidated() {39     // Do nothing40   }41 }

  5、实现了功能的具体观察者AdapterDataSetObserver----实则是抽象类AdapterView的一个内部类,而ListView类继承自抽象类AbsListView,AbsListView继承自抽象类AdapterView,其中这两个抽象类的其他实现我们不关注,只关注有关于ListView的地方。

  

 1 class AdapterDataSetObserver extends DataSetObserver { 2  3     private Parcelable mInstanceState = null; 4  5     @Override 6     public void onChanged() { 7       mDataChanged = true; 8       mOldItemCount = mItemCount; 9       mItemCount = getAdapter().getCount();10 11       // Detect the case where a cursor that was previously invalidated has12       // been repopulated with new data.13       if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null14           && mOldItemCount == 0 && mItemCount > 0) {15         AdapterView.this.onRestoreInstanceState(mInstanceState);16         mInstanceState = null;17       } else {18         rememberSyncState();19       }20       checkFocus();21       requestLayout();22     }23 24     @Override25     public void onInvalidated() {26       mDataChanged = true;27 28       if (AdapterView.this.getAdapter().hasStableIds()) {29         // Remember the current state for the case where our hosting activity is being30         // stopped and later restarted31         mInstanceState = AdapterView.this.onSaveInstanceState();32       }33 34       // Data is invalid so we should reset our state35       mOldItemCount = mItemCount;36       mItemCount = 0;37       mSelectedPosition = INVALID_POSITION;38       mSelectedRowId = INVALID_ROW_ID;39       mNextSelectedPosition = INVALID_POSITION;40       mNextSelectedRowId = INVALID_ROW_ID;41       mNeedSync = false;42 43       checkFocus();44       requestLayout();45     }46 47     public void clearSavedState() {48       mInstanceState = null;49     }50   }

  6、观察者操作类的抽象类Observable<T>

  为了方便阅读,删去了注释,只看方法名比较容易理解,这里主要实现的是观察者的注册,实则是在一个ArrayList中不断的添加观察者DataSetObserver实体

package android.database;import java.util.ArrayList;public abstract class Observable<T> {  protected final ArrayList<T> mObservers = new ArrayList<T>();  public void registerObserver(T observer) {    if (observer == null) {      throw new IllegalArgumentException("The observer is null.");    }    synchronized(mObservers) {      if (mObservers.contains(observer)) {        throw new IllegalStateException("Observer " + observer + " is already registered.");      }      mObservers.add(observer);    }  }  public void unregisterObserver(T observer) {    if (observer == null) {      throw new IllegalArgumentException("The observer is null.");    }    synchronized(mObservers) {      int index = mObservers.indexOf(observer);      if (index == -1) {        throw new IllegalStateException("Observer " + observer + " was not registered.");      }      mObservers.remove(index);    }  }  public void unregisterAll() {    synchronized(mObservers) {      mObservers.clear();    }  }}

  7、观察者操作类的具体实现DataSetObservable

  可以看到,这里定义了notifyChanged方法的实现,通过同步代码块逐条的对集合中的每一条数据进行OnChanged方法调用。

 1 /* 2  * Copyright (C) 2007 The Android Open Source Project 3  * 4  * Licensed under the Apache License, Version 2.0 (the "License"); 5  * you may not use this file except in compliance with the License. 6  * You may obtain a copy of the License at 7  * 8  *   http://www.apache.org/licenses/LICENSE-2.0 9  *10  * Unless required by applicable law or agreed to in writing, software11  * distributed under the License is distributed on an "AS IS" BASIS,12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.13  * See the License for the specific language governing permissions and14  * limitations under the License.15 */16 17 package android.database;18 19 /**20  * A specialization of {@link Observable} for {@link DataSetObserver}21  * that provides methods for sending notifications to a list of22  * {@link DataSetObserver} objects.23 */24 public class DataSetObservable extends Observable<DataSetObserver> {25   /**26    * Invokes {@link DataSetObserver#onChanged} on each observer.27    * Called when the contents of the data set have changed. The recipient28    * will obtain the new contents the next time it queries the data set.29   */30   public void notifyChanged() {31     synchronized(mObservers) {32       // since onChanged() is implemented by the app, it could do anything, including33       // removing itself from {@link mObservers} - and that could cause problems if34       // an iterator is used on the ArrayList {@link mObservers}.35       // to avoid such problems, just march thru the list in the reverse order.36       for (int i = mObservers.size() - 1; i >= 0; i--) {37         mObservers.get(i).onChanged();38       }39     }40   }41 42   /**43    * Invokes {@link DataSetObserver#onInvalidated} on each observer.44    * Called when the data set is no longer valid and cannot be queried again,45    * such as when the data set has been closed.46   */47   public void notifyInvalidated() {48     synchronized (mObservers) {49       for (int i = mObservers.size() - 1; i >= 0; i--) {50         mObservers.get(i).onInvalidated();51       }52     }53   }54 }

  8、最后就是ListView中setAdapter方法的实现了,小小的setAdapter方法中涉及到了BaseAdapter的引用,DataSetObserver的引用。

  这里实际上是通过入参传入了我们自己实现的Adapter,一个继承自BaseAdapter的Adapter,并且在方法中已经实例化了一个上文提到的AdapterDataSetObserver,一个继承自DataSetObserver的具体观察者。

  

@Override  public void setAdapter(ListAdapter adapter) {    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();      mDataSetObserver = new AdapterDataSetObserver();//AdapterDataSetObserver是ListView的基类AdapterView的内部类      mAdapter.registerDataSetObserver(mDataSetObserver);//注册一个观察者      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();    }    requestLayout();//更新视图  }