Android 通過ViewHolder優(yōu)化適配器的實(shí)現(xiàn)方法(必看)

2019-12-12 03:04:20

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供稿：網(wǎng)友

Adapter類的定義：

Adapter對象是AdapterView和底層數(shù)據(jù)見的橋梁。Adapter用于訪問數(shù)據(jù)項(xiàng)，并且負(fù)責(zé)為數(shù)據(jù)項(xiàng)生成視圖

AdapterView是一個抽象類，用于那些需要通過Adapter填充自身的視圖，其常見子類是ListView。顯示AdapterView時會調(diào)用Adapter的getView()方法創(chuàng)建并添加每個子條目的視圖。Adapter的getView()方法就是用來創(chuàng)建這些視圖的，Adapter并不會為每行數(shù)據(jù)都創(chuàng)建一個新視圖，而是提供了回收舊視圖的方法。運(yùn)行機(jī)制簡單說就是當(dāng)getView()方法被調(diào)用是，如果convertView參數(shù)不為null,就使用convertView，不用新建視圖，通過convertView.findViewById()方法獲取每個UI控件的引用，然后使用與當(dāng)前項(xiàng)的位置綁定的數(shù)據(jù)來填充視圖

為了優(yōu)化，使用ViewHolder模式，ViewHolder是一個靜態(tài)類，可以用于保存每行的視圖以避免每次調(diào)用getView時都會調(diào)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>

以上代碼就基本實(shí)現(xiàn)了功能

不過每次都自定義ViewHolder比較繁瑣，在網(wǎng)上找了一個工具類，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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