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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