ListView
ListView 简单用法
- width和height是match_parent
- 数据可以从数据库,网上下载.也可以是数组
- 数组中的数据是 无法直接传递给ListView的,要借助适配器,其中ArrayAdapter最好用
- ArrayAdapter可以用泛型指定数据类型,在构造函数中吧要适配的数据传入
- ArrayAdapter有多个构造函数重载,要选择最适合的一种.因为传入数据是字符串,所以
ArrayAdapter<String> - 构造函数传入(上下文,ListView子项布局id,适配数据)
- 最后调用ListView的setAdapter()方法,将构建好的适配器对象传递进去 ,这样ListView和数据之间的关联就建立完成 了
<ListView android:id="@+id/list_view" android:layout_width="match_parent" android:layout_height="match_parent"></ListView>public class MainActivity extends AppCompatActivity { private String[] data = {"Apple","Banana","Orange","Watermelon","Pear","Grape", "Pineapple","Strawberry","Cherry","Mango","Apple", "Banana","Orange","Watermelon","Pear","Grape", "Pineapple","Strawberry","Cherry","Mango"}; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); ArrayAdapter<String> adapter = new ArrayAdapter<String>( MainActivity.this,android.R.layout.simple_list_item_1,data); ListView listView = (ListView) findViewById(R.id.list_view); listView.setAdapter(adapter); } }
定制ListView的界面
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定义一个实体类,作为ListView适配器的适配类型,新建Fruit
public class Fruit { private String name; private int imageId; public Fruit(String name, int imageId) { this.name = name; this.imageId = imageId; } public String getName() { return name; } public int getImageId() { return imageId; } } -
自定义布局 LinearLayout 要 TextVIew垂直布局居中
<ImageView android:id="@+id/fruit_image" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <TextView android:id="@+id/fruit_name" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_gravity="center_vertical" android:layout_marginLeft="10dp" /> -
创建适配器
继承自ArrayAdapter 泛型Fruit 新建类FruitAdapter
Fruit重写一组父类的构造函数,用于将 上下文,ListView子项布局的id和数据传递进来
重写getView()方法,这个方法每个子项被滚动到屏幕内的时候会被调用.
在getView()方法中,首先通过getItem()方法得到当前项的Fruit实例,然后使用LayoutInflater 来为这个子项加载传入的布局
inflate()接收三个参数 ()
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调用View的findViewById() 获取ImageView和TextView的实例,并分别调用他们的setImageResource()和setText()方法来设置显示的图片和文字,最后布局返回
public class FruitAdapter extends ArrayAdapter { private int resourceId; public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects) { super(context, textViewResourceId, objects); resourceId = textViewResourceId; } @NonNull @Override public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { Fruit fruit = (Fruit) getItem(position); // 获取当前项的Fruit实例 View view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false); ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image); TextView fruitName = (TextView) view.findViewById(R.id.fruit_name); fruitImage.setImageResource(fruit.getImageId()); fruitName.setText(fruit.getName()); return view; } } initFruit()方法 初始化所有水果数据
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onCreat() 方法中创建FruitAdapter对象,并将FruitAdapter作为适配器传递给ListVIew,这样定制ListView界面的任务就完成了
public class MainActivity extends AppCompatActivity { private List<Fruit> fruitList = new ArrayList<>(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); initFruits(); // 初始化水果数据 FruitAdapter adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList); ListView listView = (ListView) findViewById(R.id.list_view); listView.setAdapter(adapter); } private void initFruits() { for (int i = 0; i < 2; i++) { Fruit apple = new Fruit("Apple", R.drawable.apple_pic); fruitList.add(apple); Fruit banana = new Fruit("banana", R.drawable.banana_pic); Fruit orange = new Fruit("orange", R.drawable.orange_pic); fruitList.add(orange); Fruit watermelon = new Fruit("watermelon", R.drawable.watermelon_pic); fruitList.add(watermelon); Fruit pear = new Fruit("pear", R.drawable.pear_pic); fruitList.add(pear); Fruit grape = new Fruit("grape", R.drawable.grape_pic); fruitList.add(grape); Fruit pineapple = new Fruit("pineapple", R.drawable.pineapple_pic); fruitList.add(pineapple); Fruit strawberry = new Fruit("strawberry", R.drawable.strawberry_pic); fruitList.add(strawberry); Fruit cherry = new Fruit("cherry", R.drawable.cherry_pic); fruitList.add(cherry); Fruit mango = new Fruit("mango", R.drawable.mango_pic); fruitList.add(mango); } } }
提升ListView运行效率 修改FruitAdapter
在getView() 方法中还有 convertView参数,用于将之前加载好的布局进行缓存,以后可重用
- 对getView()方法进行判断,如果convertView为null,则使用LayoutInflater 加载布局,不为null则直接对convertView进行重用
public class FruitAdapter extends ArrayAdapter { ... @Override public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { Fruit fruit = (Fruit) getItem(position); // 获取当前项的Fruit实例 View view; if(convertView == null){ view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false); }else{ view = convertView; } ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image); TextView fruitName = (TextView) view.findViewById(R.id.fruit_name); fruitImage.setImageResource(fruit.getImageId()); fruitName.setText(fruit.getName()); return view; } }
继续优化 借助ViewHolder来对部分性能进行优化 修改FruitAdapter 📖P120
- 新建一个内部类ViewHolder,用于对控件的实例进行缓存
- 当converView为null时,创建一个ViewHolder对象,并将控件的实例都存放在ViewHolder里
- 然后调用View的setTag() 方法,将ViewHolder对象存储在View中.
- 当convertView不为null时,则调用View的getTag()方法,吧ViewHolder重新取出.
- 这样所有空间的实例都缓存在了ViewHolder里,就没有必要每次都通过findViewById() 方法来获取控件实例
public class FruitAdapter extends ArrayAdapter { ... @Override public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { Fruit fruit = (Fruit) getItem(position); // 获取当前项的Fruit实例 View view; ViewHoler viewHolder; if(convertView == null){ view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false); viewHolder = new ViewHoler(); viewHolder.fruitImage = (ImageView) view.findViewById(R.id.fruit_image); viewHolder.fruitName = (TextView) view.findViewById(R.id.fruit_name); view.setTag(viewHolder); // 将ViewHolder存储在View中 }else{ view = convertView; viewHolder = (ViewHoler) view.getTag(); // 重新获取ViewHolder } viewHolder.fruitImage.setImageResource(fruit.getImageId()); viewHolder.fruitName.setText(fruit.getName()); return view; } class ViewHoler{ ImageView fruitImage; TextView fruitName; } }
为ListView添加点击事件
- 在MainActivity 添加 点击事件 Toast
- 使用setOnItemClickListener() 方法为ListView 注册一个监听器
- 点击任何一个子项是,回调onItemClick() 方法
- 这个方法可以通过position 参数判断用户点击的是哪个子项,然后获取相应的水果
- 通过Toast将水果的名字显示出来
public class MainActivity extends AppCompatActivity { private List<Fruit> fruitList = new ArrayList<>(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); initFruits(); // 初始化水果数据 FruitAdapter adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList); ListView listView = (ListView) findViewById(R.id.list_view); listView.setAdapter(adapter); listView.setOnItemClickListener(new AdapterView.OnItemClickListener() { @Override public void onItemClick(AdapterView<?> parent, View view, int position, long id) { Fruit fruit = fruitList.get(position); Toast.makeText(MainActivity.this,fruit.getName(),Toast.LENGTH_SHORT).show(); } }); } ... }
这是我学习Android的笔记,参考书<<第一行代码-第二版>>,感谢郭霖大神
如有不足之处,可以一起讨论
附上郭霖的blog http://blog.csdn.net/guolin_blog
