装饰设计模式在Android开发中的应用

1. 模式定义

装饰设计模式也称包装设计模式,用来动态的扩展对象的功能,也是继承关系的的一种替代方案之一。在不使用的继承的方式下,采用装饰设计模式可以扩展一个对象的功能,可以使一个对象变得越来越强大。

2. Android 源码的学习和思考

相信用过 ListView 的朋友都知道,ListView 有自带的方法可以添加 header 和 footer,那么 ListView 是怎样做到的呢?

ListView.java 的源码

ArrayList mHeaderViewInfos = Lists.newArrayList();
ArrayList mFooterViewInfos = Lists.newArrayList();

public void addHeaderView(View v, Object data, boolean isSelectable) {
        if (v.getParent() != null && v.getParent() != this) {
            if (Log.isLoggable(TAG, Log.WARN)) {
                Log.w(TAG, "The specified child already has a parent. "
                           + "You must call removeView() on the child's parent first.");
            }
        }
        final FixedViewInfo info = new FixedViewInfo();
        info.view = v;
        info.data = data;
        info.isSelectable = isSelectable;
        mHeaderViewInfos.add(info);
        mAreAllItemsSelectable &= isSelectable;

        // Wrap the adapter if it wasn't already wrapped.
        if (mAdapter != null) {
            if (!(mAdapter instanceof HeaderViewListAdapter)) {
                wrapHeaderListAdapterInternal();
            }

            // In the case of re-adding a header view, or adding one later on,
            // we need to notify the observer.
            if (mDataSetObserver != null) {
                mDataSetObserver.onChanged();
            }
        }
    }
    
protected void wrapHeaderListAdapterInternal() {
        mAdapter = wrapHeaderListAdapterInternal(mHeaderViewInfos, mFooterViewInfos, mAdapter);
    }
    
protected HeaderViewListAdapter wrapHeaderListAdapterInternal(
            ArrayList headerViewInfos,
            ArrayList footerViewInfos,
            ListAdapter adapter) {
        return new HeaderViewListAdapter(headerViewInfos, footerViewInfos, adapter);
    }

类关系

public interface WrapperListAdapter extends ListAdapter {
    /**
     * Returns the adapter wrapped by this list adapter.
     *
     * @return The {@link android.widget.ListAdapter} wrapped by this adapter.
     */
    public ListAdapter getWrappedAdapter();
}

-----------------------------------------------

public class HeaderViewListAdapter implements WrapperListAdapter, Filterable {
    ...
}

可以看到 adapter 是 HeaderViewListAdapter 的子类的时候,会最终将 headerView 和 footerView 配置到 HeaderViewListAdapter 中实现。那么 HeaderViewListAdapter 是怎样实现的呢?

HeaderViewListAdapter.java 主要源码

public class HeaderViewListAdapter implements WrapperListAdapter, Filterable {

    private final ListAdapter mAdapter;

    ArrayList mHeaderViewInfos;
    ArrayList mFooterViewInfos;

    static final ArrayList EMPTY_INFO_LIST =
        new ArrayList();
        
    public HeaderViewListAdapter(ArrayList headerViewInfos,
                                 ArrayList footerViewInfos,
                                 ListAdapter adapter) {
        mAdapter = adapter;
        ...
        if (headerViewInfos == null) {
            mHeaderViewInfos = EMPTY_INFO_LIST;
        } else {
            mHeaderViewInfos = headerViewInfos;
        }

        if (footerViewInfos == null) {
            mFooterViewInfos = EMPTY_INFO_LIST;
        } else {
            mFooterViewInfos = footerViewInfos;
        }
        ...
    }
    
    public boolean removeHeader(View v) {
        for (int i = 0; i < mHeaderViewInfos.size(); i++) {
            ListView.FixedViewInfo info = mHeaderViewInfos.get(i);
            if (info.view == v) {
                mHeaderViewInfos.remove(i);

                mAreAllFixedViewsSelectable =
                        areAllListInfosSelectable(mHeaderViewInfos)
                        && areAllListInfosSelectable(mFooterViewInfos);

                return true;
            }
        }

        return false;
    }

    public boolean removeFooter(View v) {
        for (int i = 0; i < mFooterViewInfos.size(); i++) {
            ListView.FixedViewInfo info = mFooterViewInfos.get(i);
            if (info.view == v) {
                mFooterViewInfos.remove(i);

                mAreAllFixedViewsSelectable =
                        areAllListInfosSelectable(mHeaderViewInfos)
                        && areAllListInfosSelectable(mFooterViewInfos);

                return true;
            }
        }

        return false;
    }
    
    public int getCount() {
        if (mAdapter != null) {
            return getFootersCount() + getHeadersCount() + mAdapter.getCount();
        } else {
            return getFootersCount() + getHeadersCount();
        }
    }
    
    public Object getItem(int position) {
        // Header (negative positions will throw an IndexOutOfBoundsException)
        int numHeaders = getHeadersCount();
        if (position < numHeaders) {
            return mHeaderViewInfos.get(position).data;
        }

        // Adapter
        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mAdapter != null) {
            adapterCount = mAdapter.getCount();
            if (adjPosition < adapterCount) {
                return mAdapter.getItem(adjPosition);
            }
        }

        // Footer (off-limits positions will throw an IndexOutOfBoundsException)
        return mFooterViewInfos.get(adjPosition - adapterCount).data;
    }
    
    public View getView(int position, View convertView, ViewGroup parent) {
        // Header (negative positions will throw an IndexOutOfBoundsException)
        int numHeaders = getHeadersCount();
        if (position < numHeaders) {
            return mHeaderViewInfos.get(position).view;
        }

        // Adapter
        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mAdapter != null) {
            adapterCount = mAdapter.getCount();
            if (adjPosition < adapterCount) {
                return mAdapter.getView(adjPosition, convertView, parent);
            }
        }

        // Footer (off-limits positions will throw an IndexOutOfBoundsException)
        return mFooterViewInfos.get(adjPosition - adapterCount).view;
    }

}

可以看到,HeaderViewListAdapter 在 ListAdapter 的基础上,且不修改 原有开发者自行定义的 Adapter下,对 ListView 提供了支持 header 和 footer 的添加。所以, ListView 的源码是通过装饰设计模式让其具有添加 header 和 footer 的能力。

RecycleView 没有相关的方法,应该怎样才可以 添加 header 和 footer?这时,通过学习 ListView 的源码后,不就可以使用装饰设计模式轻松解决问题了吗?!

2. 使用装饰设计模式让 RecycleView 也可添加 header 和 footer

好,那么我们从装饰 Adapter 入手,鉴于本文是介绍如何使用装饰设计模式解决问题,此处实例仅供参考学习,如实际应用有问题,应自行完善和拓展

自己动手写一个 HeaderViewListAdapter.java

public class HeaderViewListAdapter extends RecyclerView.Adapter{
    // 装饰 RecyclerView.Adapter
    private final RecyclerView.Adapter mRealAdapter;
    ArrayList mHeaderViews;
    ArrayList mFooterViews;

    public HeaderViewListAdapter(RecyclerView.Adapter realAdapter){
        this.mRealAdapter = realAdapter;

        mRealAdapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
            @Override
            public void onChanged() {
                notifyDataSetChanged();
            }
        });

        mHeaderViews = new ArrayList<>();
        mFooterViews = new ArrayList<>();
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int position) {
        // 头部返回 头部的ViewHolder
        int numHeaders = getHeadersCount();
        if (position < numHeaders) {
            return createHeaderFooterViewHolder(mHeaderViews.get(position));
        }
        // mRealAdapter 返回 mRealAdapter的ViewHolder
        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mRealAdapter != null) {
            adapterCount = mRealAdapter.getItemCount();
            if (adjPosition < adapterCount) {
                // 直接传 position ,不兼容 万能适配多布局条目
                return mRealAdapter.onCreateViewHolder(parent,mRealAdapter.getItemViewType(adjPosition));
            }
        }
        // 底部返回 底部的ViewHolder
        // Footer (off-limits positions will throw an IndexOutOfBoundsException)
        return createHeaderFooterViewHolder(mFooterViews.get(adjPosition - adapterCount));
    }

    private RecyclerView.ViewHolder createHeaderFooterViewHolder(View view) {
        return new RecyclerView.ViewHolder(view){};
    }

    public int getHeadersCount() {
        return mHeaderViews.size();
    }

    public int getFootersCount() {
        return mFooterViews.size();
    }

    @Override
    public int getItemViewType(int position) {
        // 把位置作为 viewType
        return position;
    }

    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
        // 这个方法先不写,测试一下
        // 头部和底部是都不需要做处理的,只要 mRealAdapter 要去做处理
        int numHeaders = getHeadersCount();
        if (position < numHeaders) {
            return ;
        }

        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mRealAdapter != null) {
            adapterCount = mRealAdapter.getItemCount();
            if (adjPosition < adapterCount) {
                mRealAdapter.onBindViewHolder(holder,position);
            }
        }
    }

    @Override
    public int getItemCount() { // 总共返回多少条 = 底部条数+头部条数+真实的Adapter条数
        return mFooterViews.size()+mHeaderViews.size()+mRealAdapter.getItemCount();
    }

    /**
     * 添加头部
     * @param view
     */
    public void addHeaderView(View view){
        if(!mHeaderViews.contains(view)){
            mHeaderViews.add(view);
            notifyDataSetChanged();
        }
    }

    /**
     * 添加底部
     * @param view
     */
    public void addFooterView(View view){
        if(!mFooterViews.contains(view)){
            mFooterViews.add(view);
            notifyDataSetChanged();
        }
    }

    /**
     * 移除头部
     * @param view
     */
    public void removeHeaderView(View view){
        if(mHeaderViews.contains(view)){
            mHeaderViews.remove(view);
            notifyDataSetChanged();
        }
    }

    /**
     * 移除底部
     * @param view
     */
    public void removeFooterView(View view){
        if(mFooterViews.contains(view)){
            mFooterViews.remove(view);
            notifyDataSetChanged();
        }
    }
}

ListView 中直接提供了方法添加 header 和 footer,那我们需要自定义拓展 RecyclerView 来添加相关的方法

HeaderViewRecyclerView.java

public class HeaderViewRecyclerView extends RecyclerView{

    private HeaderViewListAdapter mAdapter;

    public HeaderViewRecyclerView(Context context) {
        super(context);
    }

    public HeaderViewRecyclerView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public HeaderViewRecyclerView(Context context, @Nullable AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    public void setAdapter(Adapter adapter) {
        mAdapter = new HeaderViewListAdapter(adapter);
        super.setAdapter(mAdapter);
    }

    /**
     * 添加头部
     * @param view
     */
    public void addHeaderView(View view){
        // 必须要设置 Adapter 之后才能添加头部和底部
        if(mAdapter != null){
            mAdapter.addHeaderView(view);
        }
    }

    /**
     * 添加底部
     * @param view
     */
    public void addFooterView(View view){
        if(mAdapter != null){
            mAdapter.addFooterView(view);
        }
    }

    /**
     * 移除头部
     * @param view
     */
    public void removeHeaderView(View view){
        if(mAdapter != null){
            mAdapter.removeHeaderView(view);
        }
    }

    /**
     * 移除底部
     * @param view
     */
    public void removeFooterView(View view){
        if(mAdapter != null){
            mAdapter.removeFooterView(view);
        }
    }
}
3. 使用示例
        mRecyclerView.setAdapter(new RecyclerAdapter());
        View headerView = LayoutInflater.from(this).inflate(R.layout.layout_header_view,mRecyclerView,false);
        mRecyclerView.addHeaderView(headerView);
        View footerView = LayoutInflater.from(this).inflate(R.layout.layout_header_view,mRecyclerView,false);
        mRecyclerView.addFooterView(headerView);

其实,就是正常操作使用 RecyclerView,addHeaderView 和 addFooterView 需在 setAdapter 后。

希望读者能从中感受到到 装饰设计模式 的魅力。

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