ListView与Adapter之间的观察者模式

本文用文字叙述不太好描述，主要通过下面这张图来表示： 这张图体现出了主要的关系

• 首先创建一个MyAdapter继承BaseAdapter

public class MyAdapter extends BaseAdapter {private List<String> list;public MyAdapter(List<String> list) {this.list = list;}@Overridepublic int getCount() {return list.size();}@Overridepublic Object getItem(int position) {return null;}@Overridepublic long getItemId(int position) {return 0;}@Overridepublic View getView(int position, View convertView, ViewGroup parent) {return null;}
}
复制代码

• 给ListView设置适配器

    MyAdapter adapter =  new MyAdapter();listView.setAdapter(adapter);
复制代码

• 通过ListView、BaseAdapter以及相关继承关系的类的源码进行分析

有注释的地方为主要分析部分（主要代码）

  //分析主要代码/************* ListView.class 中* class ListView extends AbsListView*/
public class ListView extends AbsListView{@Overridepublic void setAdapter(ListAdapter adapter) {  //传入一个adaptersuper.setAdapter(adapter);if (mAdapter != null) { // 适配器不为空时      //分析1mDataSetObserver = new AdapterDataSetObserver();//在这里实例化一个适配器观察者集合//分析2mAdapter.registerDataSetObserver(mDataSetObserver);//给适配器注册一个数据适配器观察者}}}/************AbsListView.class 中    */public abstract class AbsListView extends AdapterView{//这两个引用会被ListView继承以及实例化和调用AdapterDataSetObserver mDataSetObserver;ListAdapter mAdapter;//AdapterDataSetObserver为AbsListView的内部类// "分析1 "作用：实现onChanged（） onInvalidated（）两个方法class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {@Overridepublic void onChanged() {super.onChanged();if (mFastScroll != null) {mFastScroll.onSectionsChanged();}}@Overridepublic void onInvalidated() {super.onInvalidated();if (mFastScroll != null) {mFastScroll.onSectionsChanged();}}}}/*********************** Adapter 的相关类
*BaseAdapter.class 中
*/
public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {//实例化DataSetObservable（）private final DataSetObservable mDataSetObservable = new DataSetObservable();private CharSequence[] mAutofillOptions;public boolean hasStableIds() {return false;} //注册观察者//"分析2" 时调用public void registerDataSetObserver(DataSetObserver observer) {mDataSetObservable.registerObserver(observer);}//注销观察者public void unregisterDataSetObserver(DataSetObserver observer) {mDataSetObservable.unregisterObserver(observer);}//notify 通知数据改变public void notifyDataSetChanged() {mDataSetObservable.notifyChanged();}}
/*****ListAdapter.class 中
*/public interface ListAdapter extends Adapter {public boolean areAllItemsEnabled();boolean isEnabled(int position);}/***********************DataSetObservable.class 中是Observable的子类可以调用mObservers集合*/public class DataSetObservable extends Observable<DataSetObserver> {public void notifyChanged() {synchronized(mObservers) {   for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onChanged();}}}public void notifyInvalidated() {synchronized (mObservers) {for (int i = mObservers.size() - 1; i >= 0; i--) {mObservers.get(i).onInvalidated();}}}
}/*********Observable.class 中
*/
public abstract class Observable<T> {//观察者集合 list，用于存放所有的观察者，会被子类调用protected final ArrayList<T> mObservers = new ArrayList<T>();//注册观察者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.");}//添加进list中mObservers.add(observer);}}//注销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.");}//从list中移除mObservers.remove(index);}}public void unregisterAll() {synchronized(mObservers) {mObservers.clear();}}
}
复制代码