建议先了解《Lifecycle原理、源码解析》
- LiveData是一种具有生命周期感知能力的可观察数据持有类
- LiveData可以保证屏幕上的显示内容和数据一直保持同步
- 特点：
- 1.LiveData了解UI界面的状态，如果activity不在屏幕上显示，livedata不会触发没必要的界面更新，如果activity已经被销毁，会自动清空与observer的连接，意外的调用就不会发生
- 2.LiveData是一个LifecycleOwner,他可以直接感知activity或fragment的生命周期
使用
- 项目中livedata一般都存放在ＶiewModel中，以保证app配置变更时，数据不会丢失
- 使用注意：
setValue只能在主线程运行
postValue只能在子线程中运行
方式一
- 定义一个viewmodel保存名字
package com.example.livedatademo;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;
public class NameViewModel extends ViewModel {
private MutableLiveData<String> currentName;
public MutableLiveData<String> getCurrentName(){
if(currentName==null){
currentName=new MutableLiveData<>();
}
return currentName;
}
}
- 在activity里面监听名字变化然后刷新数据
package com.example.livedatademo;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
public class NameActivity extends AppCompatActivity {
private NameViewModel model;
private TextView nameTextView;
private Button btn;
private int i=0;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_name);
nameTextView=findViewById(R.id.tvText);
btn=findViewById(R.id.btn);
//需要一个观察者来观察数据
Observer observer=new Observer<String>(){
@Override
public void onChanged(String s) {
nameTextView.setText(s);
}
};
//获取到viewmodel
model= ViewModelProviders.of(this).get(NameViewModel.class);
//取出livedata完成订阅
model.getCurrentName().observe(this,observer);
btn.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v) {
String anotherName="Gavin"+(i++);
model.getCurrentName().setValue(anotherName);
}
});
}
}
方式二
- 定义一个总线LiveDataBus，统一处理
package com.example.livedatademo.databus;
import androidx.lifecycle.MutableLiveData;
import java.util.HashMap;
import java.util.Map;
public class LiveDataBus {
//存放订阅者
private Map<String, MutableLiveData<Object>> bus;
private static LiveDataBus liveDataBus = new LiveDataBus();
private LiveDataBus() {
bus = new HashMap();
}
public static LiveDataBus getInstance() {
return liveDataBus;
}
//注册订阅者
public synchronized <T> MutableLiveData<T> with(String key, Class<T> type) {
if(!bus.containsKey(key)){
bus.put(key,new MutableLiveData<Object>());
}
return (MutableLiveData<T>)bus.get(key);
}
}
- 在TestLiveDataBusActivity里面监听数据变化
package com.example.livedatademo;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import androidx.appcompat.app.AppCompatActivity;
import com.example.livedatademo.databus.LiveDataBus;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void click(View view) {
if (view.getId() == R.id.btn) {
Intent intent = new Intent(this, NameActivity.class);
startActivity(intent);
} else if (view.getId() == R.id.btn2) {
Intent intent = new Intent(this, TestLiveDataBusActivity.class);
startActivity(intent);
new Thread(){
@Override
public void run() {
for (int i = 0; i < 10; i++) {
//发送消息
LiveDataBus.getInstance().with("data", String.class).postValue("Gavin");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}.start();
}
}
}
- TestLiveDataBusActivity
package com.example.livedatademo;
import android.os.Bundle;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import com.example.livedatademo.databus.LiveDataBus;
public class TestLiveDataBusActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_livedatabus);
LiveDataBus.getInstance().with("data", String.class)
.observe(this, new Observer<String>() {
@Override
public void onChanged(String s) {
if(s!=null)
Toast.makeText(TestLiveDataBusActivity.this, s, Toast.LENGTH_SHORT).show();
}
});
}
}
什么是LiveData的粘性事件（数据倒灌）？
- 在上面的使用方式二中，让R.id.btn2的点击事件里面先更新livedata的数据，然后再创建TestLiveDataBusActivity，会发现TestLiveDataBusActivity还是收到了旧数据通知（还没创建activity前的数据）
- 修改后的代码如下
Intent intent = new Intent(this, TestLiveDataBusActivity.class);
//先更新数据
new Thread(){
@Override
public void run() {
for (int i = 0; i < 10; i++) {
//发送消息
LiveDataBus.getInstance().with("data", String.class).postValue("Gavin");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}.start();
//再创建activity
startActivity(intent);
