android ListView 分析(一)

需要了解的内容

1. listview中的getItemAtPosition与Adapter的getItem的position的区别 

        listView中的getItemAtPosition的源码实现:

 /**
 * Gets the data associated with the specified position in the list.
 *
 * @param position Which data to get
 * @return The data associated with the specified position in the list
 */
 public Object getItemAtPosition(int position) {
 T adapter = getAdapter();
 return (adapter == null || position < 0) ? null : adapter.getItem(position);
 }

 public long getItemIdAtPosition(int position) {
 T adapter = getAdapter();
 return (adapter == null || position < 0) ? INVALID_ROW_ID : adapter.getItemId(position);
 }

 

如果在adapter部位null并且position不是负数的时候,那么就会直接调用listView中真正的adapter的getItem()方法

 

因而我们自己定义的adapter,其实和listView中定义的getItemAtPosition()存在的差异就是在有headView和footView的时候

 

但是在我们自己的adapter中还是应该用有效的position

而在外部想直接调用的时候,应该让listView.getAdapter().getItem(position),如果调用到了headView或者footView的时候,那么就是返回为空的,因为在包装的

那么就是调用的headViewInfo中的data或者mFooterViewInfos.data

 public Object getItem(int position) {
 // Header (negative positions will throw an ArrayIndexOutOfBoundsException)
 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 ArrayIndexOutOfBoundsException)
 return mFooterViewInfos.get(adjPosition - adapterCount).data;
 }
 这个算是简单的View的包装器
 /**
 * 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;
 }

  

2. listview中如何传递position

    在listView中获得的position是经过包装的,因而在我们的adapter中需要转换

3. onItemClick的parent是否与listview为同一个view --> 是的

4. 在获取类型时,到底应该使用那个类型 

   在使用类型的时候,即getViewTypeCount()一般都是我们的adapter的getViewTypeCount()    

 

5.adapter中 getItemViewType()方法的作用

        我们使用的都是正数,而且是从0开始的,但是系统中的headView和footView的ViewType是负数

6.listView的优化

7.listView的扩展

        7.1 listView实现下拉刷新,下拉加载

        7.2 listView实现其他特效

        7.3 listView实现动态的更改headView和footView的大小

 

......

 

8.listView相关知识: adapter的体系结构以及特点

 

 

ListView 在 存在headView或者footView的时候,listView设置的adapter并不是我们设置的adapter

 

查看源码便得知

 /**
 * 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) {
 if (null != mAdapter) {
 mAdapter.unregisterDataSetObserver(mDataSetObserver);
 }

 resetList();
 mRecycler.clear();
 //关键代码 ListView 在 存在headView或者footView的时候,listView设置的adapter并不是我们设置的adapter,而是包装过的HeaderViewListAdapter

 if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
 mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
 } else {
 mAdapter = adapter;
 }

 ......
 }

既然如此设计,那么在使用的时候,我们便需要注意一些细节

listView中获取的getItemAtPosition中的position并不一定和我们的adapter的position相同

 

因而在设置position的时候,需要将position进行处理

例如

onItemClick中,传递而来的position有可能是包裹后的,因而我们考虑手工去除或者让系统来去除,即将包裹后的position转换为我们的adapter的position

 

1.手工方式

 listView.setOnItemClickListener(new OnItemClickListener() {

 @Override
 public void onItemClick(AdapterView<?> parent, View view,
 int position, long id) {
 int realPosition = position - listView.getHeaderViewsCount();
 }
 });

2. 调用系统api的方式(推荐使用的方式)

 

在设置listView的相关监听的时候,会提供AdapterView<?> parent这个参数,这个其实就是我们的listView

调用他的getAdapter()获取到实际的adapter,然后调用adapter的获取数据的方式

 

 @Override
 public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
 doSomething(parent.getAdapter().getItem(position));
 }

 扩展内容,包裹的adapter --> HeaderViewListAdapter

 ......
 import java.util.ArrayList;

 /**
 * 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>();
 ......

 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();
 }

 ......

 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 ArrayIndexOutOfBoundsException)
 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 ArrayIndexOutOfBoundsException)
 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 View getView(int position, View convertView, ViewGroup parent) {
 // Header (negative positions will throw an ArrayIndexOutOfBoundsException)
 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 ArrayIndexOutOfBoundsException)
 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 Filter getFilter() {
 if (mIsFilterable) {
 return ((Filterable) mAdapter).getFilter();
 }
 return null;
 }
 //获取被包装的adapter,即我们设置的listView
 public ListAdapter getWrappedAdapter() {
 return mAdapter;
 }
 }

 

要获取到我们设置的adapter,这个方法是在接口中实现的,直接获取到即可

package android.widget;

 

 /**
 * List adapter that wraps another list adapter. The wrapped adapter can be retrieved
 * by calling {@link #getWrappedAdapter()}.
 *
 * @see ListView
 */
 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();
 }

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