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
/* * Copyright (C) 2006 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package android.widget; import android.database.DataSetObserver; import android.view.View; import android.view.ViewGroup; /** * An Adapter object acts as a bridge between an {@link AdapterView} and the * underlying data for that view. The Adapter provides access to the data items. * The Adapter is also responsible for making a {@link android.view.View} for * each item in the data set. * * @see android.widget.ArrayAdapter * @see android.widget.CursorAdapter * @see android.widget.SimpleCursorAdapter */ public interface Adapter { /** * Register an observer that is called when changes happen to the data used by this adapter. * * @param observer the object that gets notified when the data set changes. */ void registerDataSetObserver(DataSetObserver observer); /** * Unregister an observer that has previously been registered with this * adapter via {@link #registerDataSetObserver}. * * @param observer the object to unregister. */ void unregisterDataSetObserver(DataSetObserver observer); /** * How many items are in the data set represented by this Adapter. * * @return Count of items. */ int getCount(); /** * Get the data item associated with the specified position in the data set. * * @param position Position of the item whose data we want within the adapter's * data set. * @return The data at the specified position. */ Object getItem(int position); /** * Get the row id associated with the specified position in the list. * * @param position The position of the item within the adapter's data set whose row id we want. * @return The id of the item at the specified position. */ long getItemId(int position); /** * Indicates whether the item ids are stable across changes to the * underlying data. * * @return True if the same id always refers to the same object. */ boolean hasStableIds(); /** * Get a View that displays the data at the specified position in the data set. You can either * create a View manually or inflate it from an XML layout file. When the View is inflated, the * parent View (GridView, ListView...) will apply default layout parameters unless you use * {@link android.view.LayoutInflater#inflate(int, android.view.ViewGroup, boolean)} * to specify a root view and to prevent attachment to the root. * * @param position The position of the item within the adapter's data set of the item whose view * we want. * @param convertView The old view to reuse, if possible. Note: You should check that this view * is non-null and of an appropriate type before using. If it is not possible to convert * this view to display the correct data, this method can create a new view. * Heterogeneous lists can specify their number of view types, so that this View is * always of the right type (see {@link #getViewTypeCount()} and * {@link #getItemViewType(int)}). * @param parent The parent that this view will eventually be attached to * @return A View corresponding to the data at the specified position. */ View getView(int position, View convertView, ViewGroup parent); /** * An item view type that causes the {@link AdapterView} to ignore the item * view. For example, this can be used if the client does not want a * particular view to be given for conversion in * {@link #getView(int, View, ViewGroup)}. * * @see #getItemViewType(int) * @see #getViewTypeCount() */ static final int IGNORE_ITEM_VIEW_TYPE = AdapterView.ITEM_VIEW_TYPE_IGNORE; /** * Get the type of View that will be created by {@link #getView} for the specified item. * * @param position The position of the item within the adapter's data set whose view type we * want. * @return An integer representing the type of View. Two views should share the same type if one * can be converted to the other in {@link #getView}. Note: Integers must be in the * range 0 to {@link #getViewTypeCount} - 1. {@link #IGNORE_ITEM_VIEW_TYPE} can * also be returned. * @see #IGNORE_ITEM_VIEW_TYPE */ int getItemViewType(int position); /** * <p> * Returns the number of types of Views that will be created by * {@link #getView}. Each type represents a set of views that can be * converted in {@link #getView}. If the adapter always returns the same * type of View for all items, this method should return 1. * </p> * <p> * This method will only be called when when the adapter is set on the * the {@link AdapterView}. * </p> * * @return The number of types of Views that will be created by this adapter */ int getViewTypeCount(); static final int NO_SELECTION = Integer.MIN_VALUE; /** * @return true if this adapter doesn't contain any data. This is used to determine * whether the empty view should be displayed. A typical implementation will return * getCount() == 0 but since getCount() includes the headers and footers, specialized * adapters might want a different behavior. */ boolean isEmpty(); }
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）
package android.widget; /** * Extended {@link Adapter} that is the bridge between a {@link ListView} * and the data that backs the list. Frequently that data comes from a Cursor, * but that is not * required. The ListView can display any data provided that it is wrapped in a * ListAdapter. */ public interface ListAdapter extends Adapter { /** * Indicates whether all the items in this adapter are enabled. If the * value returned by this method changes over time, there is no guarantee * it will take effect. If true, it means all items are selectable and * clickable (there is no separator.) * * @return True if all items are enabled, false otherwise. * * @see #isEnabled(int) */ public boolean areAllItemsEnabled(); /** * Returns true if the item at the specified position is not a separator. * (A separator is a non-selectable, non-clickable item). * * The result is unspecified if position is invalid. An {@link ArrayIndexOutOfBoundsException} * should be thrown in that case for fast failure. * * @param position Index of the item * * @return True if the item is not a separator * * @see #areAllItemsEnabled() */ boolean isEnabled(int position); }
import android.view.View; import android.view.ViewGroup; /** * Extended {@link Adapter} that is the bridge between a * {@link android.widget.Spinner} and its data. A spinner adapter allows to * define two different views: one that shows the data in the spinner itself * and one that shows the data in the drop down list when the spinner is * pressed. */ public interface SpinnerAdapter extends Adapter { /** * Gets a {@link android.view.View} that displays in the drop down popup * the data at the specified position in the data set. * * @param position index of the item whose view we want. * @param convertView the old view to reuse, if possible. Note: You should * check that this view is non-null and of an appropriate type before * using. If it is not possible to convert this view to display the * correct data, this method can create a new view. * @param parent the parent that this view will eventually be attached to * @return a {@link android.view.View} corresponding to the data at the * specified position. */ public View getDropDownView(int position, View convertView, ViewGroup parent); }
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
import android.database.DataSetObservable; import android.database.DataSetObserver; import android.view.View; import android.view.ViewGroup; /** * Common base class of common implementation for an {@link Adapter} that can be * used in both {@link ListView} (by implementing the specialized * {@link ListAdapter} interface) and {@link Spinner} (by implementing the * specialized {@link SpinnerAdapter} interface). */ public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter { private final DataSetObservable mDataSetObservable = new DataSetObservable(); public boolean hasStableIds() { return false; } public void registerDataSetObserver(DataSetObserver observer) { mDataSetObservable.registerObserver(observer); } public void unregisterDataSetObserver(DataSetObserver observer) { mDataSetObservable.unregisterObserver(observer); } /** * Notifies the attached observers that the underlying data has been changed * and any View reflecting the data set should refresh itself. */ public void notifyDataSetChanged() { mDataSetObservable.notifyChanged(); } /** * Notifies the attached observers that the underlying data is no longer valid * or available. Once invoked this adapter is no longer valid and should * not report further data set changes. */ public void notifyDataSetInvalidated() { mDataSetObservable.notifyInvalidated(); } public boolean areAllItemsEnabled() { return true; } public boolean isEnabled(int position) { return true; } public View getDropDownView(int position, View convertView, ViewGroup parent) { return getView(position, convertView, parent); } public int getItemViewType(int position) { return 0; } public int getViewTypeCount() { return 1; } public boolean isEmpty() { return getCount() == 0; } }
4、观察者抽象类DataSetObserver
package android.database; /** * Receives call backs when a data set has been changed, or made invalid. The typically data sets * that are observed are {@link Cursor}s or {@link android.widget.Adapter}s. * DataSetObserver must be implemented by objects which are added to a DataSetObservable. */ 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 } }
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。
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; } }
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。
package android.database; /** * A specialization of {@link Observable} for {@link DataSetObserver} * that provides methods for sending notifications to a list of * {@link DataSetObserver} objects. */ public class DataSetObservable extends Observable<DataSetObserver> { /** * Invokes {@link DataSetObserver#onChanged} on each observer. * Called when the contents of the data set have changed. The recipient * will obtain the new contents the next time it queries the data set. */ 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(); } } } /** * Invokes {@link DataSetObserver#onInvalidated} on each observer. * Called when the data set is no longer valid and cannot be queried again, * such as when the data set has been closed. */ public void notifyInvalidated() { synchronized (mObservers) { for (int i = mObservers.size() - 1; i >= 0; i--) { mObservers.get(i).onInvalidated(); } } } }
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 }
