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<FixedViewInfo> mHeaderViewInfos = Lists.newArrayList();
ArrayList<FixedViewInfo> 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<ListView.FixedViewInfo> headerViewInfos,
ArrayList<ListView.FixedViewInfo> 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<ListView.FixedViewInfo> mHeaderViewInfos;
ArrayList<ListView.FixedViewInfo> mFooterViewInfos;
static final ArrayList<ListView.FixedViewInfo> EMPTY_INFO_LIST =
new ArrayList<ListView.FixedViewInfo>();
public HeaderViewListAdapter(ArrayList<ListView.FixedViewInfo> headerViewInfos,
ArrayList<ListView.FixedViewInfo> 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<RecyclerView.ViewHolder>{
// 装饰 RecyclerView.Adapter
private final RecyclerView.Adapter mRealAdapter;
ArrayList<View> mHeaderViews;
ArrayList<View> 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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