ListView 数据与UI更新机制之观察者模式

之前有一篇文章专门介绍观察者模式知识，当时通过EventBus来进行分析。近日在读《Android源码设计模式解析与实战》，看到书中介绍ListView中使用到观察者模式。为了加深对观察者模式的理解，以及掌握ListView的数据刷新机制，故分析其源码。

ListView数据刷新简介：

ListView使用中有一个非常重要的角色就是Adapter,其实这也是一个适配器模式的应用，外部需要增删改ListView的子元素时，常常通过Adapter的notifyDataSetChanged()方法来完成的，现在我们来看看其内部到底是如何实现。

ListView数据刷新UML类图：

image.png

源码分析：

EventBus是Android平台下用于应用内各组件或者模块间通信的一个类库，其主要实现思想为观察者模式。

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
    //被观察者对象
    private final DataSetObservable mDataSetObservable = new DataSetObservable();

    public boolean hasStableIds() {
        return false;
    }
    
    //注册观察者
    public void registerDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.registerObserver(observer);
    }
    //注销观察者
    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() {
        //数据更新时由悲观者通知所有监听的观察者
        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     * Notifies the attached observers that the underlying data is no longer valid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 Once invoked this adapter is no longer valid and should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;
    }
}

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) {
            //遍历通知所有观察者
            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();
            }
        }
    }
}

    //AbsListView中的内部类
    class AdapterDataSetObserver extends AdapterView<ListAdapter>.AdapterDataSetObserver {
        @Override
        public void onChanged() {
            super.onChanged();//父类方法调用
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }

        @Override
        public void onInvalidated() {
            super.onInvalidated();
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }
    }

    //AdapterView中的内部类，上面AdapterDataSetObserver的父类
 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();
            //请求刷新UI，实现数据变化时UI跟随变化
            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;
        }
    }

//ListView的setAdapter方法
 /**
     * Sets the data behind this ListView.
     *
     * The adapter passed to this method may be wrapped by a {@link WrapperListAdapter},
     * depending on the ListView features currently in use. For instance, adding
     * headers and/or footers will cause the adapter to be wrapped.
     *
     * @param adapter The ListAdapter which is responsible for maintaining the
     *        data backing this list and for producing a view to represent an
     *        item in that data set.
     *
     * @see #getAdapter() 
     */
    @Override
    public void setAdapter(ListAdapter adapter) {
        //若之前存在观察者，则注销
        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();
            //创建一个数据集合观察者对象，并通过Adapter进行注册
            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();
        }
       //重新请求刷新布局
        requestLayout();
    }

看到上面的源码，在使用ListView时首先会为其设置一个Adapater对象，setAdapter执行时会创建一个数据集合观察者，并通过Adapter注册监听数据集合的变化。当数据集合发生变化时，用户端调用Adapater的notifyDataSetChanged方法，此时会调用DataSetObservable类的notifyChanged方法。而notifyChanged方法内部实现就是遍历其内部所持有的观察者集合，调用观察者的onChanged方法。真正实现数据变化而导致UI刷新的方法就在AdapterView中AdapterDataSetObserver的onChanged方法，该方法内部通过调用requestLayout请求重新布局，实现数据变化与UI变化的联动。其流程图如下：

image.png

结束语

通过上述分析，相信大家对于观察者模式的印象会更深。源码比较简单，本文重点在于理解当ListView的数据集合发生变化时，如何引起UI随之变化的过程，至于底层实现没有设计，在于理解流程与设计模式的应用。