registerDataSetObserver:浅析Andorid ListView和Adapte

2024-08-30 14:28:32

最近由于遇到将内容分部绑定到ListView里的需求，追踪源码之后对ListView和Adapter有了点肤浅的认识，在此与大家分享。

这里用到了观察者模式，在ListView的setAdapter里注册一个AdapterDataSetObserver观察者。

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

ListAdapter是一接口，BaseAdapter继承了它

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* Notifies the attached observers that the underlying data is no longer valid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 Once invoked this adapter is no longer valid and should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;
}
}

DataSetObservable定义如下：

public class DataSetObservable extends Observable<DataSetObserver> {
public void notifyChanged() {//通知每个观察者数据已变化
synchronized(mObservers) {
for (DataSetObserver observer : mObservers) {
observer.onChanged();
}
}
}
public void notifyInvalidated() {
synchronized (mObservers) {
for (DataSetObserver observer : mObservers) {
observer.onInvalidated();
}
}
}
}

ListView里注册的观察者是AdapterDataSetObserver对象，其定义如下：

class AdapterDataSetObserver extends DataSetObserver {
private Parcelable mInstanceState = null;
@Override
public void onChanged() {
mDataChanged = true;
mOldItemCount = mItemCount;
mItemCount = getAdapter().getCount();//Adapter现有元素个数
if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
&& mOldItemCount == 0 && mItemCount > 0) {
AdapterView.this.onRestoreInstanceState(mInstanceState);
mInstanceState = null;
} else {
rememberSyncState();
}
checkFocus();
requestLayout();//更新视图
}
.................
}

AdapterDataSetObserver里的具体实现我没弄清楚，但感觉是在这里更新了ListView显示的数据，ListView里是怎样更新的没弄清楚，希望哪位高手知道的告诉我。

但是如果只在ListView里显示Adapter的部分数据，该怎么弄呢，可继承BaseAdapter，实现BaseAdapter里未实现的如下方法：

getCount()、getItem(int position)、getItemId(int position)、getView(int position, View convertView, ViewGroup parent)。

只需在getCount里返回想要的大小就行了。

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