ListView BaseAdapter中的观察者模式

太过爱你忘了你带给我的痛 2022-08-10 09:54 120阅读 0赞

Android SDK源码中用到了很多开发模式，其中BaseAdapter就属于观察者模式

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);
        }

中有DataSetObservable可被观察到的对象即被观察对象，这个对象将观察者对象加入到自己的观察者对象集中

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);
            }
        }

每次改变ListView的数据的时候，需要notifyDataSetChanged，这个实现是 mDataSetObservable.notifyChanged();

被观察者对象调用方法

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();
                }
            }
        }

然后观察者对象在onChanged方法中去处理。

还有个问题，registerDataSetObserver(DataSetObserver observer)什么时候被调用的

看源码发现原来是在ListView的setAdapter方法中调用的

@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();
                mAdapter.registerDataSetObserver(mDataSetObserver);
    
                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();
        }

看到里面初始化了一个对象AdapterDataSetObserver，这个对象在AbsListView里的内部类

class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {
            @Override
            public void onChanged() {
                super.onChanged();
                if (mFastScroll != null) {
                    mFastScroll.onSectionsChanged();
                }
            }
    
            @Override
            public void onInvalidated() {
                super.onInvalidated();
                if (mFastScroll != null) {
                    mFastScroll.onSectionsChanged();
                }
            }
        }

实现了具体的onChanged方法，好了，基本理清楚Adapter里面怎么通知更新数据了。

但是上面有个FastScroller对象，这个类找了半天没找到在哪，在GC上搜说在android.widget包下，但是并没有。

有知道是为什么得吗？麻烦请教一下！

最后补充看RecycleView的源码，里面也是观察者模式，但是整体感觉RecycleView比ListView更好！有空可以学习学习