【转】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

/*
 * 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
    }