android 开发技巧(14)--通过ViewHolder优化适配器

Adapter类的定义：

Adapter对象是AdapterView和底层数据见的桥梁。Adapter用于访问数据项，并且负责为数据项生成视图

AdapterView是一个抽象类，用于那些需要通过Adapter填充自身的视图，其常见子类是ListView。显示AdapterView时会调用Adapter的getView()方法创建并添加每个子条目的视图。Adapter的getView()方法就是用来创建这些视图的，Adapter并不会为每行数据都创建一个新视图，而是提供了回收旧视图的方法。运行机制简单说就是当getView()方法被调用是，如果convertView参数不为null,就使用convertView，不用新建视图，通过convertView.findViewById()方法获取每个UI控件的引用，然后使用与当前项的位置绑定的数据来填充视图

为了优化，使用ViewHolder模式，ViewHolder是一个静态类，可以用于保存每行的视图以避免每次调用getView时都会调用findViewById()

public class Hack25Activity extends ListActivity {    private static final int MODEL_COUNT = 30;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setListAdapter(new ModelAdapter(this, 0, buildModels()));    }    private List<Model> buildModels() {        final ArrayList<Model> ret = new ArrayList<Model>(MODEL_COUNT);        for (int i = 0; i < MODEL_COUNT; i++) {            final Model model = new Model();            model.setImage(R.mipmap.ic_launcher);            model.setText1("Name " + i);            model.setText2("Description " + i);            ret.add(model);        }        return ret;    }}

Model.java

public class Model {  private String mText1;  private String mText2;  private int mImageResId;  public String getText1() {    return mText1;  }  public void setText1(String text1) {    mText1 = text1;  }  public String getText2() {    return mText2;  }  public void setText2(String text2) {    mText2 = text2;  }  public int getImage() {    return mImageResId;  }  public void setImage(int imageResId) {    mImageResId = imageResId;  }}

ModelAdapter.java

public class ModelAdapter extends ArrayAdapter<Model> {  private LayoutInflater mInflater;  public ModelAdapter(Context context, int textViewResourceId,      List<Model> objects) {    super(context, textViewResourceId, objects);    mInflater = LayoutInflater.from(context);  }  @Override  public View getView(int position, View convertView, ViewGroup parent) {    final ViewHolder viewHolder;    if (convertView == null) {      convertView = mInflater.inflate(R.layout.row_layout, parent,          false);      viewHolder = new ViewHolder();      viewHolder.imageView = (ImageView) convertView          .findViewById(R.id.image);      viewHolder.text1 = (TextView) convertView          .findViewById(R.id.text1);      viewHolder.text2 = (TextView) convertView          .findViewById(R.id.text2);      convertView.setTag(viewHolder);    } else {      viewHolder = (ViewHolder) convertView.getTag();    }    Model model = getItem(position);    viewHolder.imageView.setImageResource(model.getImage());    viewHolder.text1.setText(model.getText1());    viewHolder.text2.setText(model.getText2());    return convertView;  }  private static class ViewHolder {    public ImageView imageView;    public TextView text1;    public TextView text2;  }}

row_layout.xml

<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="fill_parent"    android:layout_height="wrap_content" >    <ImageView        android:id="@+id/image"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_alignParentLeft="true"        android:layout_centerVertical="true" />    <LinearLayout        android:layout_width="fill_parent"        android:layout_height="wrap_content"        android:layout_centerVertical="true"        android:layout_toRightOf="@id/image"        android:orientation="vertical" >        <TextView            android:id="@+id/text1"            android:layout_width="fill_parent"            android:layout_height="wrap_content" />        <TextView            android:id="@+id/text2"            android:layout_width="fill_parent"            android:layout_height="wrap_content" />    </LinearLayout></RelativeLayout>

以上代码就基本实现了功能
不过每次都自定义ViewHolder比较繁琐，在网上找了一个工具类，共享一下
ViewHolder.java

public class ViewHolder {    public static  <T extends View> T get(View view, int id) {        SparseArray<View> viewHolder = (SparseArray<View>) view.getTag();        if (viewHolder == null) {             viewHolder = new SparseArray<View>();             view.setTag(viewHolder);         }         View childView = viewHolder.get(id);         if (childView == null) {             childView = view.findViewById(id);             viewHolder.put(id, childView);         }         return (T) childView;     } } 

使用方法：

    @Override      public View getView(int position, View convertView, ViewGroup parent) {          if (convertView == null) {              convertView = LayoutInflater.from(context)                .inflate(R.layout.banana_phone, parent, false);          }          ImageView bananaView = ViewHolder.get(convertView, R.id.banana);          TextView phoneView = ViewHolder.get(convertView, R.id.phone);          BananaPhone bananaPhone = getItem(position);          phoneView.setText(bananaPhone.getPhone());          bananaView.setImageResource(bananaPhone.getBanana());          return convertView;      }  

参考资料
【工具类】ViewHolder模式超简洁写法