ListView(2)——自定义Adapter
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之前介绍了三种adapter:ArrayAdapter<T>、SimpleAdapter和SimpleCursorAdapter。
使用android提供的adapter来绘制列表的话,列表的每一项的显示都是一样的。为了实现ListView的单双行不同颜色显示,需要自定义adapter的子类。adapter的常用子类有BaseAdapter、ArrayAdapter、SimpleAdapter等,下面介绍自定义BaseAdapter和ArrayAdapter的实现。
1.自定义BaseAdapter
效果图如下:
在上一篇《ListView(1)——各种Adapter的使用》的基础上,为了实现ListView的单双行不同颜色显示,需要自定义adapter的子类,而activity_main.xml文件和list_item.xml文件都不需要更改,MainActivity.java代码修改如下:
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.activity_main);
-
-
- int[] images = new int[] { R.drawable.item_img_a,
- R.drawable.item_img_b, R.drawable.item_img_c,
- R.drawable.item_img_d, R.drawable.item_img_e };
-
- List<HashMap<String, Object>> data = new ArrayList<HashMap<String, Object>>();
- for (int i = 0; i < 5; i++) {
- HashMap<String, Object> map = new HashMap<String, Object>();
- map.put("ItemImage", images[i]);
- map.put("ItemTitle", "This is Title " + i);
- map.put("ItemText", "This is text " + i);
- data.add(map);
- }
-
-
- ListView listView = (ListView) findViewById(R.id.list);
-
- MyBaseAdapter adapter = new MyBaseAdapter(MainActivity.this, data);
- listView.setAdapter(adapter);
- }
下面我们实现自定义的MyBaseAdapter类。MyBaseAdapter类继承自BaseAdapter类,BaseAdapter为抽象类,继承它需要实现如下方法,因此具有较高的灵活性。
- public class MyBaseAdapter extends BaseAdapter {
-
- @Override
- public int getCount() {
-
- return 0;
- }
-
- @Override
- public Object getItem(int arg0) {
-
- return null;
- }
-
- @Override
- public long getItemId(int position) {
-
- return 0;
- }
-
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
-
- return null;
- }
-
- }
ListView在绘制时首先会调用getCount()方法得到绘制次数,然后通过getView()方法一层一层进行绘制,所以我们可以在getView()方法中根据position(当前绘制的ID)来的修改绘制内容。而getItem()和getItemId()则在需要处理和取得Adapter中的数据时调用。
- public class MyBaseAdapter extends BaseAdapter {
- private int[] colors = new int[] { 0xff3cb371, 0xffa0a0a0 };
- private Context mContext;
- private List<HashMap<String, Object>> dataList;
-
- public MyBaseAdapter(Context context, List<HashMap<String, Object>> dataList) {
- this.mContext = context;
- this.dataList = dataList;
- }
-
- @Override
- public int getCount() {
- return dataList.size();
- }
-
- @Override
- public HashMap<String, Object> getItem(int position) {
- return dataList.get(position);
- }
-
- @Override
- public long getItemId(int position) {
- return position;
- }
-
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
- ViewHolder holder = null;
- if (convertView == null) {
- holder = new ViewHolder();
- convertView = LayoutInflater.from(mContext).inflate(
- R.layout.list_item, null);
- holder.image = (ImageView) convertView.findViewById(R.id.ItemImage);
- holder.title = (TextView) convertView.findViewById(R.id.ItemTitle);
- holder.text = (TextView) convertView.findViewById(R.id.ItemText);
-
-
- convertView.setTag(holder);
- } else {
- holder = (ViewHolder) convertView.getTag();
- }
-
-
- holder.image.setImageResource((Integer) getItem(position).get(
- "ItemImage"));
- holder.title.setText((String) getItem(position).get("ItemTitle"));
- holder.text.setText((String) getItem(position).get("ItemText"));
-
- int colorPos = position % colors.length;
- convertView.setBackgroundColor(colors[colorPos]);
-
- return convertView;
- }
-
-
-
-
- final class ViewHolder {
- ImageView image;
- TextView title;
- TextView text;
- }
- }
getView()方法用来获得绘制每个item的View对象,如果每次getView()被执行都new出一个View对象,长此以往会产生很大的消耗,特别当item中还有Bitmap等,甚至会造成OOM的错误导致程序崩溃。从上面的代码可以看到getView()有一个convertView参数,这个参数用来缓存View对象。当ListView滑动的过程中,会有item被滑出屏幕而不再被使用,这时候Android会回收这个item的view,这个view也就是这里的convertView。这样如果convertView不为null,就不用new出一个新的View对象,只用往convertView中填充新的item,这样就省去了new View的大量开销。在上面的代码中,在缓存convertView减少new View开销的同时,通过setTag()方法将数据结构ViewHolder绑定到convertView,从而利用ViewHolder存储convertView中控件对象的引用,这样避免每次调用findViewById()方法。
2.自定义ArrayAdapter<T>
在开发中需要将对象显示在listview中,这时候使用ArrayAdapter<T>来显示指定对象类型。下面自定义ArrayAdapter<T>实现上一节中自定义BaseAdapter实现的同样的效果,首先定义要显示的对象,代码如下ItemBean.java。
- public class ItemBean {
- private int image;
- private String title;
- private String text;
-
- public ItemBean(int image, String title, String text) {
- this.image = image;
- this.title = title;
- this.text = text;
- }
-
- public int getImage() {
- return image;
- }
-
- public String getTitle() {
- return title;
- }
-
- public String getText() {
- return text;
- }
- }
和上一节一样,为了实现ListView的单双行不同颜色显示,需要自定义adapter的子类,而activity_main.xml文件和list_item.xml文件都不需要更改,MainActivity.java代码修改如下:
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.activity_main);
-
-
- int[] images = new int[] { R.drawable.item_img_a,
- R.drawable.item_img_b, R.drawable.item_img_c,
- R.drawable.item_img_d, R.drawable.item_img_e };
-
- ArrayList<ItemBean> data = new ArrayList<ItemBean>();
- for (int i = 0; i < 5; i++) {
- ItemBean itemBean = new ItemBean(images[i], "This is Title " + i,
- "This is text " + i);
- data.add(itemBean);
- }
-
-
- ListView listView = (ListView) findViewById(R.id.list);
-
- MyArrayAdapter adapter = new MyArrayAdapter(MainActivity.this,
- R.layout.list_item, data);
- listView.setAdapter(adapter);
- }
接下来自定义继承自ArrayAdapter<ItemBean>的MyArrayAdapter类,继承ArrayAdapter<ItemBean>只需要重写getView()方法就可以实现与上一节相同的效果,并且不用保存List<ItemBean>对象引用。
- public class MyArrayAdapter extends ArrayAdapter<ItemBean> {
- private int[] colors = new int[] { 0xff3cb371, 0xffa0a0a0 };
- private Context mContext;
- private int resource;
-
- public MyArrayAdapter(Context context, int resource, List<ItemBean> data) {
- super(context, resource, data);
- this.mContext = context;
- this.resource = resource;
- }
-
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
- ViewHolder holder = null;
- if (convertView == null) {
- holder = new ViewHolder();
- convertView = LayoutInflater.from(mContext).inflate(resource, null);
- holder.image = (ImageView) convertView.findViewById(R.id.ItemImage);
- holder.title = (TextView) convertView.findViewById(R.id.ItemTitle);
- holder.text = (TextView) convertView.findViewById(R.id.ItemText);
-
-
- convertView.setTag(holder);
- } else {
- holder = (ViewHolder) convertView.getTag();
- }
-
-
- holder.image.setImageResource(getItem(position).getImage());
- holder.title.setText(getItem(position).getTitle());
- holder.text.setText(getItem(position).getText());
-
- int colorPos = position % colors.length;
- convertView.setBackgroundColor(colors[colorPos]);
-
- return convertView;
- }
-
-
-
-
- final class ViewHolder {
- ImageView image;
- TextView title;
- TextView text;
- }
- }
参考文章:
http://www.open-open.com/lib/view/open1339485728006.html
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