兩個控件
1. ListView
基本使用示例
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);
}
}
數組中的數據是無法直接傳遞給ListView的,我們還需要藉助適配器來完成。Android中提供了很多適配器的實現類,其中常用的就是ArrayAdapter。它可以通過泛型來指定要適配的數據類型,然後在構造函數中把要適配的數據傳入。ArrayAdapter有多個構造函數的重 載,應該根據實際情況選擇最合適的一種。這裏由於我們提供的數據都是字符串,因此將 ArrayAdapter的泛型指定爲String ,然後在ArrayAdapter的構造函數中依次傳入當前上下文、 ListView子項佈局的id,以及要適配的數據。注意,我們使用了 android.R.layout.simple_list_item_1 作爲ListView子項佈局的id,這是一個Android 內置的佈局文件,裏面只有一個TextView,可用於簡單地顯示一段文本。這樣適配器對象就構建好了。 最後,還需要調用ListView的setAdapter() 方法,將構建好的適配器對象傳遞進去,這樣 ListView和數據之間的關聯就建立完成了。
定製ListView的界面
- 新建實體類Fruit,作爲ListView適配器的適配類型。
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;
}
}
- 爲ListView的子項指定一個我們自定義的佈局,在layout目錄下新建fruit_item.xml:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<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"/>
</LinearLayout>
- 創建一個自定義的適配器(新建類FruitAdapter),這個適配器繼承自ArrayAdapter,並將泛型指定爲Fruit 類:
public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId;
//構造方法
public FruitAdapter(Context context,int textViewResourceId,List<Fruit> objects) {
super(context, textViewResourceId, objects);
resourceId = textViewResourceId;
}
//重寫getView方法,這個方法在每個子項被滾動到屏幕內的時候會被調用
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);//獲取當前項的Fruit實例
//用LayoutInflater 來爲這個子項加載我們傳入的佈局。
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;
}
}
- 修改MainActivity中的代碼
public class MainActivity extends AppCompatActivity {
private List<Fruit> fruitList = new ArrayList<>();//水果實例列表
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits(); // 初始化水果數據
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() {
//創建水果實例,並添加到水果實例列表
for (int i = 0; i < 5; i++) {
Fruit apple = new Fruit("Apple", R.drawable.apple_pic);
fruitList.add(apple);
Fruit banana = new Fruit("Banana", R.drawable.apple_pic);
fruitList.add(banana);
Fruit orange = new Fruit("Orange", R.drawable.apple_pic);
fruitList.add(orange);
}
}
}
提升ListView的運行效率
- 在FruitAdapter 的getView() 方法 中,每次都將佈局重新加載了一遍,當ListView快速滾動的時候,這就會成爲性能的瓶頸。getView() 方法中還有一個convertView 參數,這個參數用於將之前加載 好的佈局進行緩存,以便之後可以進行重用。修改FruitAdapter 中的代碼,如下所示:
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);//獲取當前項的Fruit示例
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;
}
- 每次在getView() 方法中還是會調用View 的findViewById() 方法來獲取一次控件的實 例。我們可以藉助一個ViewHolder 來對這部分性能進行優化,修改FruitAdapter 中的代 碼,如下所示:
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);//獲取當前項的Fruit示例
View view;
ViewHolder viewHolder;
if (convertView == null){
view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
viewHolder = new ViewHolder();
viewHolder.fruitName = (TextView) view.findViewById(R.id.fruit_name);
viewHolder.fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
view.setTag(viewHolder);// 將ViewHolder存儲在View中
}else {
view = convertView;
viewHolder = (ViewHolder) view.getTag();// 重新獲取ViewHolder
}
viewHolder.fruitImage.setImageResource(fruit.getImageId());
viewHolder.fruitName.setText(fruit.getName());
return view;
}
class ViewHolder {
ImageView fruitImage;
TextView fruitName;
}
新增了一個內部類ViewHolder ,用於對控件的實例進行緩存。當convertView 爲null 的時候,創建一個ViewHolder 對象,並將控件的實例都存放在ViewHolder 裏,然後調 用View 的setTag() 方法,將ViewHolder 對象存儲在View 中。當convertView 不 爲null 的時候,則調用View 的getTag() 方法,把ViewHolder 重新取出。這樣所有控件的 實例都緩存在了ViewHolder 裏,就沒有必要每次都通過findViewById() 方法來獲取控件 實例了。
ListView的點擊事件
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int position, long l) {
Fruit fruit = fruitList.get(position);
Toast.makeText(MainActivity.this,fruit.getName(),Toast.LENGTH_SHORT).show();
}
});
2. RecyclerView
基本用法
- 打開app/build.gradle文件,在dependencies 閉包中添加語句並同步:
implementation ‘com.android.support:recyclerview-v7:28.0.0’ - 修改activity_main.xml中的代碼:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/recycler_view"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
- 新建FruitAdapter 類(適配器),繼承自 RecyclerView.Adapter ,並將泛型指定爲FruitAdapter.ViewHolder。ViewHolder 是在FruitAdapter 中定義的一個內部類,代碼:
public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {
private List<Fruit> mFruitList;
//先定義了一個內部類ViewHolder ,ViewHolder 要繼承自RecyclerView.ViewHolder 。
//然後ViewHolder 的構造函數中要傳入一個View 參數,這個參數通常就是RecyclerView子項的最外層佈局,
//就可以通過findViewById()方法來獲取到佈局中的ImageView和 TextView的實例了。
static class ViewHolder extends RecyclerView.ViewHolder {
ImageView fruitImage;
TextView fruitName;
public ViewHolder(View view) {
super(view);
fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
fruitName = (TextView) view.findViewById(R.id.fruit_name);
}
}
//自己的構造方法
public FruitAdapter(List<Fruit> fruitList) {
mFruitList = fruitList;
}
//用於創建ViewHolder 實例的,將fruit_item 佈局加載進來,
// 然後創建一個ViewHolder 實例,並把加載出來的佈局傳入到構造函數當中,
// 最後將ViewHolder 的實例返回。
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()) .inflate(R.layout.fruit_item, parent, false);
ViewHolder holder = new ViewHolder(view);
return holder;
}
//用於對 RecyclerView子項的數據進行賦值的,會在每個子項被滾動到
// 屏幕內的時候執行,這裏我們通過 position 參數得到當前項的Fruit 實例,
// 然後再將數據設置到ViewHolder 的ImageView和 TextView當中即可。
public void onBindViewHolder(ViewHolder holder, int position) {
Fruit fruit = mFruitList.get(position);
holder.fruitImage.setImageResource(fruit.getImageId());
holder.fruitName.setText(fruit.getName());
}
//用於告訴RecyclerView一共有多少子項,返回數據源的長度。
public int getItemCount() {
return mFruitList.size();
}
}
- 修改MainActivity中的代碼:
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(); // 初始化水果數據
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.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);
fruitList.add(banana);
Fruit orange = new Fruit("Orange", R.drawable.orange_pic);
fruitList.add(orange);
}
}
}
橫向滾動
- 把fruit_item 裏的元素改成垂直排列,即修改fruit_item.xml中的代碼:
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="100dp"
android:layout_height="wrap_content" >
<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal" />
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:layout_marginTop="10dp" />
</LinearLayout>
- 修改MainActivity中的代碼:
······
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
添加:layoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
recyclerView.setLayoutManager(layoutManager);
······
瀑布流佈局 & 網格佈局
以瀑布流佈局爲例
- 修改一下fruit_item.xml中的代碼:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="5dp" >
<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"/>
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="left"
android:layout_marginTop="10dp" />
</LinearLayout>
- 修改fruit_item.xml中的代碼:
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(); // 初始化水果數據
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(3, StaggeredGridLayoutManager.VERTICAL);//主要語句,3列瀑布流佈局
recyclerView.setLayoutManager(layoutManager);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.setAdapter(adapter);
}
private void initFruits() {
for (int i = 0; i < 50; i++) {
Fruit apple = new Fruit("Apple", R.drawable.apple_pic);
fruitList.add(apple);
Fruit banana = new Fruit("Banana", R.drawable.apple_pic);
fruitList.add(banana);
Fruit orange = new Fruit("Orange", R.drawable.apple_pic);
fruitList.add(orange);
Fruit watermelon = new Fruit("Watermelon", R.drawable.apple_pic);
}
}
}
RecyclerView的點擊事件
- 修改FruitAdapter 中的代碼:
public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {
private List<Fruit> mFruitList;
static class ViewHolder extends RecyclerView.ViewHolder {
View fruitView;//新添加代碼
ImageView fruitImage;
TextView fruitName;
public ViewHolder(View view) {
super(view);
fruitView = view;//新添加代碼
fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
fruitName = (TextView) view.findViewById(R.id.fruit_name);
}
}
public FruitAdapter(List<Fruit> fruitList) {
mFruitList = fruitList;
}
//用於創建ViewHolder 實例的,將fruit_item 佈局加載進來,
// 然後創建一個ViewHolder 實例,並把加載出來的佈局傳入到構造函數當中,
// 最後將ViewHolder 的實例返回。
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()) .inflate(R.layout.fruit_item, parent, false);
final ViewHolder holder = new ViewHolder(view);
//新添加代碼
holder.fruitView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position = holder.getAdapterPosition();
Fruit fruit = mFruitList.get(position);
Toast.makeText(v.getContext(), "you clicked view " + fruit.getName(), Toast.LENGTH_SHORT).show();
}
});
//新添加代碼
holder.fruitImage.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position = holder.getAdapterPosition();
Fruit fruit = mFruitList.get(position);
Toast.makeText(v.getContext(),"you clicked image "+fruit.getName(),Toast.LENGTH_SHORT).show();
}
});
return holder;
}
//用於對 RecyclerView子項的數據進行賦值的,會在每個子項被滾動到
// 屏幕內的時候執行,這裏我們通過 position 參數得到當前項的Fruit 實例,
// 然後再將數據設置到ViewHolder 的ImageView和 TextView當中即可。
public void onBindViewHolder(ViewHolder holder, int position) {
Fruit fruit = mFruitList.get(position);
holder.fruitImage.setImageResource(fruit.getImageId());
holder.fruitName.setText(fruit.getName());
}
//用於告訴RecyclerView一共有多少子項,返回數據源的長度。
public int getItemCount() {
return mFruitList.size();
}
}