在谷歌推出RecyclerView之後,由於其強大的解耦和定製能力等優勢,現在項目中已經基本採用RecyclerView來替代傳統的ListView了,不過RecyclerView並沒有提供之前ListView中應用非常廣泛的addHeaderView和addFooterView方法。今天首先來分析一下傳統的ListView是如何實現的,之後會基於這種思路來討論如何爲RecyclerView添加這個功能。
/**
* A class that represents a fixed view in a list, for example a header at the top
* or a footer at the bottom.
*/
public class FixedViewInfo {
/** The view to add to the list */
public View view;
/** The data backing the view. This is returned from {@link ListAdapter#getItem(int)}. */
public Object data;
/** <code>true</code> if the fixed view should be selectable in the list */
public boolean isSelectable;
}
ArrayList<FixedViewInfo> mHeaderViewInfos = Lists.newArrayList();
ArrayList<FixedViewInfo> mFooterViewInfos = Lists.newArrayList();
首先從ListView的成員變量中可以看到創建了一個專門保存頭部信息的類FixedViewInfo ,同時創建了兩個List :mHeaderViewInfos ,mFooterViewInfos 來保存對應的信息。
/**
* Add a fixed view to appear at the top of the list. If this method is
* called more than once, the views will appear in the order they were
* added. Views added using this call can take focus if they want.
* <p>
* Note: When first introduced, this method could only be called before
* setting the adapter with {@link #setAdapter(ListAdapter)}. Starting with
* {@link android.os.Build.VERSION_CODES#KITKAT}, this method may be
* called at any time. If the ListView's adapter does not extend
* {@link HeaderViewListAdapter}, it will be wrapped with a supporting
* instance of {@link WrapperListAdapter}.
*
* @param v The view to add.
* @param data Data to associate with this view
* @param isSelectable whether the item is selectable
*/
public void addHeaderView(View v, Object data, boolean isSelectable) {
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();
}
}
}
/**
* Add a fixed view to appear at the top of the list. If addHeaderView is
* called more than once, the views will appear in the order they were
* added. Views added using this call can take focus if they want.
* <p>
* Note: When first introduced, this method could only be called before
* setting the adapter with {@link #setAdapter(ListAdapter)}. Starting with
* {@link android.os.Build.VERSION_CODES#KITKAT}, this method may be
* called at any time. If the ListView's adapter does not extend
* {@link HeaderViewListAdapter}, it will be wrapped with a supporting
* instance of {@link WrapperListAdapter}.
*
* @param v The view to add.
*/
public void addHeaderView(View v) {
addHeaderView(v, null, true);
}
在addHeaderView方法內部可以看到首先創建了一個FixedViewInfo 對象,然後將其添加到mHeaderViewInfos中。然後下面的代碼非常關鍵:
// 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();
}
}
首先判斷成員變量mAdapter 是不是一個HeaderViewListAdapter ,如果不是的話就調用wrapHeaderListAdapterInternal();
創建一個HeaderViewListAdapter 。
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);
}
原來如果我們添加了header或者footer,listview內部實際上會創建一個new HeaderViewListAdapter,這是一個裝飾者模式,那相應的奧祕顯然就應該在HeaderViewListAdapter中了。
/**
* ListAdapter used when a ListView has header views. This ListAdapter
* wraps another one and also keeps track of the header views and their
* associated data objects.
*<p>This is intended as a base class; you will probably not need to
* use this class directly in your own code.
*/
public class HeaderViewListAdapter implements WrapperListAdapter, Filterable {
private final ListAdapter mAdapter;
// These two ArrayList are assumed to NOT be null.
// They are indeed created when declared in ListView and then shared.
ArrayList<ListView.FixedViewInfo> mHeaderViewInfos;
ArrayList<ListView.FixedViewInfo> mFooterViewInfos;
// Used as a placeholder in case the provided info views are indeed null.
// Currently only used by some CTS tests, which may be removed.
static final ArrayList<ListView.FixedViewInfo> EMPTY_INFO_LIST =
new ArrayList<ListView.FixedViewInfo>();
boolean mAreAllFixedViewsSelectable;
private final boolean mIsFilterable;
public HeaderViewListAdapter(ArrayList<ListView.FixedViewInfo> headerViewInfos,
ArrayList<ListView.FixedViewInfo> footerViewInfos,
ListAdapter adapter) {
mAdapter = adapter;
mIsFilterable = adapter instanceof Filterable;
if (headerViewInfos == null) {
mHeaderViewInfos = EMPTY_INFO_LIST;
} else {
mHeaderViewInfos = headerViewInfos;
}
if (footerViewInfos == null) {
mFooterViewInfos = EMPTY_INFO_LIST;
} else {
mFooterViewInfos = footerViewInfos;
}
mAreAllFixedViewsSelectable =
areAllListInfosSelectable(mHeaderViewInfos)
&& areAllListInfosSelectable(mFooterViewInfos);
}
public int getHeadersCount() {
return mHeaderViewInfos.size();
}
public int getFootersCount() {
return mFooterViewInfos.size();
}
public boolean isEmpty() {
return mAdapter == null || mAdapter.isEmpty();
}
private boolean areAllListInfosSelectable(ArrayList<ListView.FixedViewInfo> infos) {
if (infos != null) {
for (ListView.FixedViewInfo info : infos) {
if (!info.isSelectable) {
return false;
}
}
}
return true;
}
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 boolean areAllItemsEnabled() {
if (mAdapter != null) {
return mAreAllFixedViewsSelectable && mAdapter.areAllItemsEnabled();
} else {
return true;
}
}
public boolean isEnabled(int position) {
// Header (negative positions will throw an IndexOutOfBoundsException)
int numHeaders = getHeadersCount();
if (position < numHeaders) {
return mHeaderViewInfos.get(position).isSelectable;
}
// Adapter
final int adjPosition = position - numHeaders;
int adapterCount = 0;
if (mAdapter != null) {
adapterCount = mAdapter.getCount();
if (adjPosition < adapterCount) {
return mAdapter.isEnabled(adjPosition);
}
}
// Footer (off-limits positions will throw an IndexOutOfBoundsException)
return mFooterViewInfos.get(adjPosition - adapterCount).isSelectable;
}
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 long getItemId(int position) {
int numHeaders = getHeadersCount();
if (mAdapter != null && position >= numHeaders) {
int adjPosition = position - numHeaders;
int adapterCount = mAdapter.getCount();
if (adjPosition < adapterCount) {
return mAdapter.getItemId(adjPosition);
}
}
return -1;
}
public boolean hasStableIds() {
if (mAdapter != null) {
return mAdapter.hasStableIds();
}
return false;
}
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;
}
public int getItemViewType(int position) {
int numHeaders = getHeadersCount();
if (mAdapter != null && position >= numHeaders) {
int adjPosition = position - numHeaders;
int adapterCount = mAdapter.getCount();
if (adjPosition < adapterCount) {
return mAdapter.getItemViewType(adjPosition);
}
}
return AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER;
}
public int getViewTypeCount() {
if (mAdapter != null) {
return mAdapter.getViewTypeCount();
}
return 1;
}
public void registerDataSetObserver(DataSetObserver observer) {
if (mAdapter != null) {
mAdapter.registerDataSetObserver(observer);
}
}
public void unregisterDataSetObserver(DataSetObserver observer) {
if (mAdapter != null) {
mAdapter.unregisterDataSetObserver(observer);
}
}
public Filter getFilter() {
if (mIsFilterable) {
return ((Filterable) mAdapter).getFilter();
}
return null;
}
public ListAdapter getWrappedAdapter() {
return mAdapter;
}
}
其實看到這裏應該已經比較清晰了,比如getItemViewType方法,如果position是header或者footer的位置就返回AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER,反則返回內部mAdapter.getItemViewType(adjPosition),這裏計算的adjPosition是對內部mAdapter的位置校正。
public int getItemViewType(int position) {
int numHeaders = getHeadersCount();
if (mAdapter != null && position >= numHeaders) {
int adjPosition = position - numHeaders;
int adapterCount = mAdapter.getCount();
if (adjPosition < adapterCount) {
return mAdapter.getItemViewType(adjPosition);
}
}
return AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER;
}
再看listview的最重要的方法getView 方法:
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;
}
可以看到如果position是header或footer的位置,則返回mHeaderViewInfos或者mFooterViewInfos對應的view,否則返回內部的mAdapter.getView(adjPosition, convertView, parent)方法。
addFooterView的原理是類似的,就不展開分析了,可以看到通過裝飾者模式,listview會在添加頭部或者尾部的時候構造HeaderViewListAdapter,而這一切我們在使用的時候竟毫不知情。listview的實現方式讓我們在添加header和footer的時候不需要對我們自定義的adapter做任何的改變,這就是我們所需要的,下一篇我們就仿照listview的方式來爲recycle人view實現addHeaderView功能。