$ListView的优化机制和滑动时数据错乱的讨论

　　Refer：http://www.myexception.cn/mobile/1612364.html

　　（一）Android ListView的基本用法

　　1、创建一个实体类Person，为其添加Getter和Setter方法，作为ListView适配器的类型：

 public class Person {
     private int imageId;
     private String name;
     private int age;

     public Person(int imageId, String name, int age) {
         this.imageId = imageId;
         this.name = name;
         this.age = age;
     }

     public int getImageId() {
         return imageId;
     }

     public String getName() {
         return name;
     }

     public int getAge() {
         return age;
     }

     public void setImageId(int imageId) {
         this.imageId = imageId;
     }

     public void setName(String name) {
         this.name = name;
     }

     public void setAge(int age) {
         this.age = age;
     }

 }

　　2、创建person_item.xml文件，其中包含一个ImageView和两个TextView：

 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/person_item_ll"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:orientation="horizontal" >

     <ImageView
         android:id="@+id/image_iv"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:src="@drawable/img" />

     <LinearLayout
         android:layout_width="0dp"
         android:layout_height="match_parent"
         android:layout_weight="1"
         android:orientation="vertical"
         android:gravity="center" >

         <TextView
             android:id="@+id/name_tv"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:gravity="center"
             android:text="Tom" />

         <TextView
             android:id="@+id/age_tv"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:gravity="center"
             android:text="20" />
     </LinearLayout>

 </LinearLayout>

　　3、创建自定义适配器类PersonAdapter，以Person类为泛型，继承自ArrayAdapter<Person>，重写父类的构造方法和getView方法，getView方法会在每个子项被滚动到屏幕内的时候调用：

 public class PersonAdapter extends ArrayAdapter<Person> {
     private int mResourceId;

     public PersonAdapter(Context context, int textViewResourceId,
             List<Person> objects) {
         super(context, textViewResourceId, objects);
         // textViewResourceId：ListView子项布局的id;objects：数据
         mResourceId = textViewResourceId;
     }

     @Override
     public View getView(int position, View convertView, ViewGroup parent) {
         // 1.获取当前项的Person实例
         Person person = getItem(position);

         // 2.为这个子项加载传入的布局
         View view = LayoutInflater.from(getContext()).inflate(mResourceId, null);

         // 3.用view的findViewById方法获取到子项布局控件的实例
         ImageView imgIv = (ImageView) view.findViewById(R.id.image_iv);
         TextView nameTv = (TextView) view.findViewById(R.id.name_tv);
         TextView ageTv = (TextView) view.findViewById(R.id.age_tv);

         // 4.设置相应控件的内容
         imgIv.setImageResource(person.getImageId());
         nameTv.setText(person.getName());
         ageTv.setText(person.getAge() + "");

         // 5.为imgIv设置点击事件，点击它的时候换图片
         final ImageView finalImgIv = imgIv;
         imgIv.setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View v) {
                 finalImgIv.setImageResource(R.drawable.another_img);
             }
         });

         // 6.返回view
         return view;
     }

 }

　　4、activity_main.xml

 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:orientation="vertical" >

     <ListView
         android:id="@+id/person_info_lv"
         android:layout_width="match_parent"
         android:layout_height="match_parent" >
     </ListView>

 </LinearLayout>

　　5、MainActivity：

 public class MainActivity extends Activity {

     private ListView personInfoLv;

     private String[] names;
     private int[] ages;
     private List<Person> persons;
     private int imageId;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

         personInfoLv = (ListView) findViewById(R.id.person_info_lv);

         names = new String[] { "AAA", "BBB", "CCC", "DDD", "EEE", "FFF", "GGG",
                 "HHH", "III", "JJJ", "KKK", "LLL", "MMM", "NNN", "OOO" };
         ages = new int[names.length];
         persons = new ArrayList<Person>();
         imageId = R.drawable.img;

         for (int i = 0; i < names.length; i++) {
             ages[i] = i + 1;
         }
         // 创建Person信息列表
         for (int i = 0; i < names.length; i++) {
             Person person = new Person(imageId, names[i], ages[i]);
             persons.add(person);
         }
         // 创建adapter
         PersonAdapter adapter = new PersonAdapter(MainActivity.this,
                 R.layout.person_item, persons);

         // 设置adapter
         personInfoLv.setAdapter(adapter);

     }
 }

　　运行效果：

　

　　（二）ListView的性能优化及滑动时数据显示错乱问题解决

　　1、在adapter的getView方法中，每次都将布局重新加载一遍，当快速滚动屏幕时候就会带来性能问题；此外，View的findViewById方法对性能的影响也比较大。为此要做一些优化，主要使用缓存和ViewHolder两种策略。缓存机制如下图，可以实现item的复用（假设一屏可以容纳7个item）

　　2、假设现在有两个新需求：一个是把列表的前三项的背景颜色设置成蓝色的，另一个是在每次点击每一item的图片时，不仅要修改图片，还要把修改后的图片id存到列表对象中去，这个可以用控件的setTag方法来实现。加上实现优化策略，最终修改原adapter如下：

 public class PersonAdapter extends ArrayAdapter<Person> {
     private int mResourceId;

     public PersonAdapter(Context context, int textViewResourceId,
             List<Person> objects) {
         super(context, textViewResourceId, objects);
         mResourceId = textViewResourceId;
     }

     @Override
     public View getView(int position, View convertView, ViewGroup parent) {
         Person person = getItem(position);

         View view;
         ViewHolder viewHolder;

         if (null == convertView) {
             view = LayoutInflater.from(getContext()).inflate(
                     R.layout.person_item, null);

             viewHolder = new ViewHolder();
             viewHolder.imageIv = (ImageView) view.findViewById(R.id.image_iv);
             viewHolder.nameTv = (TextView) view.findViewById(R.id.name_tv);
             viewHolder.ageTv = (TextView) view.findViewById(R.id.age_tv);

             // 点击图片的时候更换图片，并更改列表对象中的imageId的值
             final ViewHolder finalViewHolder = viewHolder;
             viewHolder.imageIv.setOnClickListener(new OnClickListener() {

                 @Override
                 public void onClick(View v) {
                     Person p = (Person) finalViewHolder.imageIv.getTag();

                     int currentImageId;
                     if (p.getImageId() == R.drawable.img) {
                         finalViewHolder.imageIv
                                 .setImageResource(R.drawable.another_img);
                         currentImageId = R.drawable.another_img;
                     } else {
                         finalViewHolder.imageIv
                                 .setImageResource(R.drawable.img);
                         currentImageId = R.drawable.img;
                     }

                     p.setImageId(currentImageId);
                 }
             });

             view.setTag(viewHolder);
             viewHolder.imageIv.setTag(person);
         } else {
             view = convertView;
             viewHolder = (ViewHolder) view.getTag();
             viewHolder.imageIv.setTag(person);
         }

         viewHolder.imageIv.setImageResource(person.getImageId());
         viewHolder.nameTv.setText(person.getName());
         viewHolder.ageTv.setText(person.getAge() + "");

         // 为前三个item设置背景颜色为蓝色
         if (position < 3) {
             view.setBackgroundColor(0xFF0000FF);
         } 

         return view;
     }

     class ViewHolder {
         ImageView imageIv;
         TextView nameTv;
         TextView ageTv;
     }

 }

　　3、这时发现在滑动ListView后，不仅是前三个item的背景颜色是蓝色的，而且后面有些项的背景颜色也变成了蓝色的，而且毫无规律可循。滑动几次后的效果如下图所示：

　　

　　这主要是因为缓存复用引起的问题，只需要在原来代码的64行后面添加else判断即可，将不是前三行的item的背景颜色设置成默认的白色的。最终代码如下：

 public class PersonAdapter extends ArrayAdapter<Person> {
     private int mResourceId;

     public PersonAdapter(Context context, int textViewResourceId,
             List<Person> objects) {
         super(context, textViewResourceId, objects);
         mResourceId = textViewResourceId;
     }

     @Override
     public View getView(int position, View convertView, ViewGroup parent) {
         Person person = getItem(position);

         View view;
         ViewHolder viewHolder;

         if (null == convertView) {
             view = LayoutInflater.from(getContext()).inflate(
                     R.layout.person_item, null);

             viewHolder = new ViewHolder();
             viewHolder.imageIv = (ImageView) view.findViewById(R.id.image_iv);
             viewHolder.nameTv = (TextView) view.findViewById(R.id.name_tv);
             viewHolder.ageTv = (TextView) view.findViewById(R.id.age_tv);

             // 点击图片的时候更换图片，并更改列表对象中的imageId的值
             final ViewHolder finalViewHolder = viewHolder;
             viewHolder.imageIv.setOnClickListener(new OnClickListener() {

                 @Override
                 public void onClick(View v) {
                     Person p = (Person) finalViewHolder.imageIv.getTag();

                     int currentImageId;
                     if (p.getImageId() == R.drawable.img) {
                         finalViewHolder.imageIv
                                 .setImageResource(R.drawable.another_img);
                         currentImageId = R.drawable.another_img;
                     } else {
                         finalViewHolder.imageIv
                                 .setImageResource(R.drawable.img);
                         currentImageId = R.drawable.img;
                     }

                     p.setImageId(currentImageId);
                 }
             });

             view.setTag(viewHolder);
             viewHolder.imageIv.setTag(person);
         } else {
             view = convertView;
             viewHolder = (ViewHolder) view.getTag();
             viewHolder.imageIv.setTag(person);
         }

         viewHolder.imageIv.setImageResource(person.getImageId());
         viewHolder.nameTv.setText(person.getName());
         viewHolder.ageTv.setText(person.getAge() + "");

         // 为前三个item设置背景颜色为蓝色
         if (position < 3) {
             view.setBackgroundColor(0xFF0000FF);
         } else {
             view.setBackgroundColor(0xFFFFFFFF);
         }

         return view;
     }

     class ViewHolder {
         ImageView imageIv;
         TextView nameTv;
         TextView ageTv;
     }

 }

　　总结：总之防止错乱关键就是一句话：哪里对控件有修改，另外的地方就要把它改回来。