DataSetObserver猜想观察者模式

frameworks/base/core/java/android/database/DataSetObserver.java

此类的所有代码为：

public abstract class DataSetObserver{

    public void onChanged(){}

    public void onInvalidated(){}

}

我在想，谁调用onChanged呢？对于观察者，当数据变动时调用onChanged()吧，那应该有数据变动的地方，insert / update /delete

frameworks/base/core/java/android/widget/AdapterView.java内部类AdapterDataSetObserver继承了此类，但它也只是重写了此类中的两个方法，没有写调用关系。

 

frameworks/base/core/java/android/database/DataSetObserverable.java

此类的所有代码

DataSetObserverable

public class DataSetObservable extends Observable<DataSetObserver> {

    /** 

     * Invokes onChanged on each observer. Called when the data set being observed has

     * changed, and which when read contains the new state of the data.

     */

    public void notifyChanged() {

        synchronized(mObservers) {

            for (DataSetObserver observer : mObservers) {

                observer.onChanged();

            }   

        }   

    }   



    /** 

     * Invokes onInvalidated on each observer. Called when the data set being monitored

     * has changed such that it is no longer valid.

     */

    public void notifyInvalidated() {

        synchronized (mObservers) {

            for (DataSetObserver observer : mObservers) {

                observer.onInvalidated();

            }   

        }   

    }

}

 

Observable

public abstract class Observable<T> {

    /**

     * The list of observers.  An observer can be in the list at most

     * once and will never be null.

     */

    protected final ArrayList<T> mObservers = new ArrayList<T>();



    /**

     * Adds an observer to the list. The observer cannot be null and it must not already

     * be registered.

     * @param observer the observer to register

     * @throws IllegalArgumentException the observer is null

     * @throws IllegalStateException the observer is already registered

     */

    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);

        }

...

}

 

BaseAdapter

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 View that the underlying data has been changed

     * and it should refresh itself.

     */

    public void notifyDataSetChanged() {

        mDataSetObservable.notifyChanged();

    }

    

    public void notifyDataSetInvalidated() {

        mDataSetObservable.notifyInvalidated();

    }

...

}

从类BaseAdapter中看，这里的注册和通知动作都是通过DataSetObservable对象的，而DataSetObservable继承了Observable<DataSetObserver>。

 因此，要查找那些继承自BaseAdapter，又重写了DataSetObserver的onChanged的函数。