Android 适配器(Adapter)、观察者(Observer) 模式

适配器(Adapter)详述：http://blog.csdn.net/jjwwmlp456/article/details/39893723

观察者(Observer)详述：http://blog.csdn.net/jjwwmlp456/article/details/39967021

AdapterView 体系



  AdapterView中会使用Adapter

Adapter 体系



  BaseAdapter 实现了 SpinnerAdapter、ListAdapter

        这样的形式，就是 适配器模式的应用， 这里是采用的是  接口适配器

观察者(Observer) 模式

在Adapter中定义了注册和反注册DataSetObserver的方法

public interface Adapter {

void registerDataSetObserver(DataSetObserver observer);

void unregisterDataSetObserver(DataSetObserver observer);

...
}

public abstract class DataSetObserver {

public void onChanged() {
// Do nothing
}

public void onInvalidated() {
// Do nothing
}
}

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
private final DataSetObservable mDataSetObservable = new DataSetObservable();

public void registerDataSetObserver(DataSetObserver observer) {
mDataSetObservable.registerObserver(observer);
}

public void unregisterDataSetObserver(DataSetObserver observer) {
mDataSetObservable.unregisterObserver(observer);
}

public void notifyDataSetChanged() {
mDataSetObservable.notifyChanged();
}

public void notifyDataSetInvalidated() {
mDataSetObservable.notifyInvalidated();
}
...
}

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();
}
}
}
}

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();
}
}
}

DataSetObserver  观察者（观察数据集：发生变化onChanged、无效onInvalidated）

Observable 处理观察者（添加、移除、注册、反注册）

DataSetObservable extends Observable<DataSetObserver>   添加了通知数据改变和无效的 两个方法

在AdapterView中

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;
}
}

   自定义了AdapterDataSetObserver，在两个实现方法中都调用了requestLayout()，重新设置layout  

整个观察数据变化导致界面改变的流程：
1.  在AdapterView体系中 自定义 AdapterDataSetObserver，并注册(即添加observer到observable)。

2.  在调用adapter.notifyDataSetChanged()后，内部执行observable的notifyChanged()，

       将执行每一个observer的onChanged()，也就达到了更新界面的效果。

    notifyDataSetInvalidated同理