ListView 和 Adapter 的基础

工作原理:

ListView 针对List中每个item,要求 adapter “给我一个视图” (getView)。
一个新的视图被返回并显示
如果我们有上亿个项目要显示怎么办?为每个项目创建一个新视图?NO!这不可能!

实际上Android为你缓存了视图。

Android中有个叫做Recycler的构件,下图是他的工作原理:



如果你有10亿个项目(item),其中只有可见的项目存在内存中,其他的在Recycler中。
ListView先请求一个type1视图(getView)然后请求其他可见的项目。convertView在getView中是空(null)的。
当item1滚出屏幕,并且一个新的项目从屏幕低端上来时,ListView再请求一个type1视图。convertView此时不是空值了,它的值是item1。你只需设定新的数据然后返回convertView,不必重新创建一个视图。
请看下面的示例代码,这里在getView中使用了System.out进行输出

public class MultipleItemsList extends ListActivity {

private MyCustomAdapter mAdapter;

@Override
public void onCreate(Bundle savedInstanceState) {
super .onCreate(savedInstanceState);
mAdapter = new MyCustomAdapter();
for ( int i = 0 ; i < 50 ; i++) {
mAdapter.addItem( "item " + i);
}
setListAdapter(mAdapter);
}

private class MyCustomAdapter extends BaseAdapter {

private ArrayList mData = new ArrayList();
private LayoutInflater mInflater;

public MyCustomAdapter() {
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}

public void addItem( final String item) {
mData.add(item);
notifyDataSetChanged();
}

@Override
public int getCount() {
return mData.size();
}

@Override
public String getItem( int position) {
return mData.get(position);
}

@Override
public long getItemId( int position) {
return position;
}

@Override
public View getView( int position, View convertView, ViewGroup parent) {
System.out.println( "getView " + position + " " + convertView);
ViewHolder holder = null ;
if (convertView == null ) {
convertView = mInflater.inflate(R.layout.item1, null );
holder = new ViewHolder();
holder.textView = (TextView)convertView.findViewById(R.id.text);
convertView.setTag(holder);
} else {
holder = (ViewHolder)convertView.getTag();
}
holder.textView.setText(mData.get(position));
return convertView;
}

}

public static class ViewHolder {
public TextView textView;
}
}


执行程序,然后在Logcat中查看日志





getView 被调用 9 次 ,convertView 对于所有的可见项目是空值(如下)



02 - 05 13 : 47 : 32.559 : INFO/System.out( 947 ): getView 0 null
02 - 05 13 : 47 : 32.570 : INFO/System.out( 947 ): getView 1 null
02 - 05 13 : 47 : 32.589 : INFO/System.out( 947 ): getView 2 null
02 - 05 13 : 47 : 32.599 : INFO/System.out( 947 ): getView 3 null
02 - 05 13 : 47 : 32.619 : INFO/System.out( 947 ): getView 4 null
02 - 05 13 : 47 : 32.629 : INFO/System.out( 947 ): getView 5 null
02 - 05 13 : 47 : 32.708 : INFO/System.out( 947 ): getView 6 null
02 - 05 13 : 47 : 32.719 : INFO/System.out( 947 ): getView 7 null
02 - 05 13 : 47 : 32.729 : INFO/System.out( 947 ): getView 8 null


然后稍微向下滚动List,直到item10出现:





convertView仍然是空值,因为recycler中没有视图(item1的边缘仍然可见,在顶端)



02 - 05 13 : 48 : 25.169 : INFO/System.out( 947 ): getView 9 null


再滚动List





convertView不是空值了!item1离开屏幕到Recycler中去了,然后item11被创建



02 - 05 13 : 48 : 42.879 : INFO/System.out( 947 ): getView 10 android.widget.LinearLayout @437430f8


再滚动:

02 - 05 14 : 01 : 31.069 : INFO/System.out( 947 ): getView 11 android.widget.LinearLayout @437447d0
02 - 05 14 : 01 : 31.142 : INFO/System.out( 947 ): getView 12 android.widget.LinearLayout @43744ff8
02 - 05 14 : 01 : 31.279 : INFO/System.out( 947 ): getView 13 android.widget.LinearLayout @43743fa8
02 - 05 14 : 01 : 31.350 : INFO/System.out( 947 ): getView 14 android.widget.LinearLayout @43745820
02 - 05 14 : 01 : 31.429 : INFO/System.out( 947 ): getView 15 android.widget.LinearLayout @43746048
02 - 05 14 : 01 : 31.550 : INFO/System.out( 947 ): getView 16 android.widget.LinearLayout @43746870
02 - 05 14 : 01 : 31.669 : INFO/System.out( 947 ): getView 17 android.widget.LinearLayout @43747098
02 - 05 14 : 01 : 31.839 : INFO/System.out( 947 ): getView 18 android.widget.LinearLayout @437478c0
02 - 05 14 : 03 : 30.900 : INFO/System.out( 947 ): getView 19 android.widget.LinearLayout @43748df0
02 - 05 14 : 03 : 32.069 : INFO/System.out( 947 ): getView 20 android.widget.LinearLayout @437430f8
convertView 如我们所期待的非空了,在item11离开屏幕之后,它的视图(@437430f8)作为convertView容纳item21了

不同的项目布局(item layout)
我们再举一个稍微复杂的例子,在上例的list中加入一些分隔线

你需要做这些:

重(@Override)写 getViewTypeCount() – 返回你有多少个不同的布局
重写 getItemViewType(int) – 由position返回view type id
根据view item的类型,在getView中创建正确的convertView
以下是代码:



public class MultipleItemsList extends ListActivity {

private MyCustomAdapter mAdapter;

@Override
public void onCreate(Bundle savedInstanceState) {
super .onCreate(savedInstanceState);
mAdapter = new MyCustomAdapter();
for ( int i = 1 ; i < 50 ; i++) {
mAdapter.addItem( "item " + i);
if (i % 4 == 0 ) {
mAdapter.addSeparatorItem( "separator " + i);
}
}
setListAdapter(mAdapter);
}

private class MyCustomAdapter extends BaseAdapter {

private static final int TYPE_ITEM = 0 ;
private static final int TYPE_SEPARATOR = 1 ;
private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1 ;

private ArrayList mData = new ArrayList();
private LayoutInflater mInflater;

private TreeSet mSeparatorsSet = new TreeSet();

public MyCustomAdapter() {
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}

public void addItem( final String item) {
mData.add(item);
notifyDataSetChanged();
}

public void addSeparatorItem( final String item) {
mData.add(item);
// save separator position
mSeparatorsSet.add(mData.size() - 1 );
notifyDataSetChanged();
}

@Override
public int getItemViewType( int position) {
return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM;
}

@Override
public int getViewTypeCount() {
return TYPE_MAX_COUNT;
}

@Override
public int getCount() {
return mData.size();
}

@Override
public String getItem( int position) {
return mData.get(position);
}

@Override
public long getItemId( int position) {
return position;
}

@Override
public View getView( int position, View convertView, ViewGroup parent) {
ViewHolder holder = null ;
int type = getItemViewType(position);
System.out.println( "getView " + position + " " + convertView + " type = " + type);
if (convertView == null ) {
holder = new ViewHolder();
switch (type) {
case TYPE_ITEM:
convertView = mInflater.inflate(R.layout.item1, null );
holder.textView = (TextView)convertView.findViewById(R.id.text);
break ;
case TYPE_SEPARATOR:
convertView = mInflater.inflate(R.layout.item2, null );
holder.textView = (TextView)convertView.findViewById(R.id.textSeparator);
break ;
}
convertView.setTag(holder);
} else {
holder = (ViewHolder)convertView.getTag();
}
holder.textView.setText(mData.get(position));
return convertView;
}

}

public static class ViewHolder {
public TextView textView;
}
}


运行程序,你会看到每4个item一个分割线



看看日志,无异常,所有的convertView都是空的



02 - 05 15 : 19 : 03.080 : INFO/System.out( 1035 ): getView 0 null type = 0
02 - 05 15 : 19 : 03.112 : INFO/System.out( 1035 ): getView 1 null type = 0
02 - 05 15 : 19 : 03.130 : INFO/System.out( 1035 ): getView 2 null type = 0
02 - 05 15 : 19 : 03.141 : INFO/System.out( 1035 ): getView 3 null type = 0
02 - 05 15 : 19 : 03.160 : INFO/System.out( 1035 ): getView 4 null type = 1
02 - 05 15 : 19 : 03.170 : INFO/System.out( 1035 ): getView 5 null type = 0
02 - 05 15 : 19 : 03.180 : INFO/System.out( 1035 ): getView 6 null type = 0
02 - 05 15 : 19 : 03.190 : INFO/System.out( 1035 ): getView 7 null type = 0
02 - 05 15 : 19 : 03.210 : INFO/System.out( 1035 ): getView 8 null type = 0
02 - 05 15 : 19 : 03.210 : INFO/System.out( 1035 ): getView 9 null type = 1
滚动list:

02 - 05 15 : 19 : 54.160 : INFO/System.out( 1035 ): getView 10 null type = 0
02 - 05 15 : 19 : 57.440 : INFO/System.out( 1035 ): getView 11 android.widget.LinearLayout @43744528 type = 0
02 - 05 15 : 20 : 01.310 : INFO/System.out( 1035 ): getView 12 android.widget.LinearLayout @43744eb0 type = 0
02 - 05 15 : 20 : 01.880 : INFO/System.out( 1035 ): getView 13 android.widget.LinearLayout @437456d8 type = 0
02 - 05 15 : 20 : 02.869 : INFO/System.out( 1035 ): getView 14 null type = 1
02 - 05 15 : 20 : 06.489 : INFO/System.out( 1035 ): getView 15 android.widget.LinearLayout @43745f00 type = 0
02 - 05 15 : 20 : 07.749 : INFO/System.out( 1035 ): getView 16 android.widget.LinearLayout @43747170 type = 0
02 - 05 15 : 20 : 10.250 : INFO/System.out( 1035 ): getView 17 android.widget.LinearLayout @43747998 type = 0
02 - 05 15 : 20 : 11.661 : INFO/System.out( 1035 ): getView 18 android.widget.LinearLayout @437481c0 type = 0
02 - 05 15 : 20 : 13.180 : INFO/System.out( 1035 ): getView 19 android.widget.LinearLayout @437468a0 type = 1
02 - 05 15 : 20 : 16.900 : INFO/System.out( 1035 ): getView 20 android.widget.LinearLayout @437489e8 type = 0
02 - 05 15 : 20 : 25.690 : INFO/System.out( 1035 ): getView 21 android.widget.LinearLayout @4374a8d8 type = 0
convertView对于分割线是空的,直到第一个分割线可见,当其离开屏幕,视图去到Recycler并且convertView开始起作用。