Android 通过ViewHolder优化适配器的实现方法(必看)

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;
  }

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