一赔癌、概述

觀察者模式在Android中應(yīng)用非常廣泛，這篇文章就以這個(gè)模式的角度來(lái)看一看Google開(kāi)發(fā)者是怎么在ListView的源碼設(shè)計(jì)中運(yùn)用這一模式的。先看一張圖：

觀察者模式.PNG

帶著結(jié)論，我們一起看看源碼

二裹匙、源碼分析

BaseAdapter持有觀察者的集合

private final DataSetObservable mDataSetObservable = new DataSetObservable();

看一下DataSetObservable

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();
            }
        }
    }

    /**
     * Invokes {@link DataSetObserver#onInvalidated} on each observer.
     * Called when the data set is no longer valid and cannot be queried again,
     * such as when the data set has been closed.
     */
    public void notifyInvalidated() {
        synchronized (mObservers) {
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onInvalidated();
            }
        }
    }
}

這個(gè)類(lèi)繼承自系統(tǒng)的Observable瑞凑，看Observable中的代碼：

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

這個(gè)類(lèi)，里面有mObservers 集合概页，用來(lái)裝觀察者籽御，用泛型T表示這里就是DataSetObserver。
ListView作為觀察者應(yīng)該是持有DataSetObserver的 我們看關(guān)鍵源碼

public void setAdapter(ListAdapter adapter) {
        ....省略....
        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();
    }

mDataSetObserver = new AdapterDataSetObserver()這句代碼惰匙，就是在listView調(diào)用setAdapter之后就會(huì)把觀察者mDataSetObserver 實(shí)例化出來(lái)技掏，mDataSetObserver這個(gè)成員變量在其父類(lèi)AbsListView中 看看代碼

 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();
            }
        }
    }

其繼承自AdapterDataSetObserver而這個(gè)AdapterDataSetObserver繼承自DataSetObserver 這樣就是listView在setAdapter的時(shí)候 就實(shí)例化了觀察者mDataSetObserver，對(duì)BaseAdapter數(shù)據(jù)改變進(jìn)行監(jiān)聽(tīng)徽曲。

而當(dāng)listview數(shù)據(jù)改變時(shí)零截，用調(diào)用 notifyDataSetChanged方法，看源碼

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

adapter中的觀察者集合mDataSetObservable就會(huì)調(diào)用notifyChanged()方法
再看notifyChanged方法：

 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();
            }
        }
    }

用for循環(huán)調(diào)用觀察者的onChange方法,我們接著看onChanged()源碼：

public abstract class DataSetObserver {
    /**
     * This method is called when the entire data set has changed,
     * most likely through a call to {@link Cursor#requery()} on a {@link Cursor}.
     */
    public void onChanged() {
        // Do nothing
    }
   ...省略...
}

空方法秃臣，我們看他的實(shí)現(xiàn)類(lèi)涧衙，也就是上面看過(guò)的AdapterDataSetObserver

 class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {
        @Override
        public void onChanged() {
            super.onChanged();
           ...省略...
        }
 }

這里調(diào)用了AbsListView的父類(lèi)AdapterView的onChanged()方法 super.onChanged();

再看其具體代碼：

    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();
        }

這里就會(huì)調(diào)用 requestLayout(); 方法開(kāi)始listView的繪制，分析完畢奥此，整個(gè)過(guò)程就是一個(gè)觀察者模式很好的運(yùn)用弧哎。