一、LiveData的基本使用

1. 使用方式一

MyLiveData.kt

package com.example.mylivedata.simple1import androidx.lifecycle.MutableLiveDataobject MyLiveData {  // 单例// 懒加载val info : MutableLiveData<String> by lazy { MutableLiveData<String>() }
}

MainActivity.kt

package com.example.mylivedata.simple1import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import androidx.lifecycle.Observer
import com.example.mylivedata.R
import kotlin.concurrent.threadclass MainActivity : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)val textView : TextView = findViewById(R.id.tv_textView)// TODO 1.观察者 环节MyLiveData.info.observe(this, Observer<String> { t ->textView.text = t // 更新UI})// 完整写法  new Observer 重写onChange方法MyLiveData.info.observe(this, object : Observer<String> {override fun onChanged(t: String?) {textView.text = t  // 更新UI}})// TODO 2.触发数据改变  环节MyLiveData.info.value = "default"  // setValue  主线程thread {Thread.sleep(3000)MyLiveData.info.postValue("三秒钟后，修改了哦")  // postValue 子线程}thread {Thread.sleep(6000)MyLiveData.info.postValue("六秒钟后，修改了哦")  // postValue 子线程}}
}

2. 使用方式二

MyLiveData.kt

package com.example.mylivedata.simple2import androidx.lifecycle.MutableLiveDataobject MyLiveData {  // 单例// 这里为data的MutableLiveData 懒加载初始化（懒加载：用到时才加载）val data : MutableLiveData<String> by lazy { MutableLiveData<String>()}init {// data.value = "dafault" // 违背 在子线程setValue(SetValue在主线程中执行)data.postValue("test")  // 子线程 执行postValue(postValue在子线程中执行)}}

MyServer.kt

package com.example.mylivedata.simple2import android.app.Service
import android.content.Intent
import android.os.IBinder
import android.util.Log
import kotlin.concurrent.thread// 模拟后台推送
class MyServer : Service() {override fun onBind(intent: Intent?): IBinder? = nulloverride fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {thread {for ( x in 1 .. 1000) {Log.d("MyServer", "服务器给你推送消息(叮咚声响),消息内容是:${x}")MyLiveData.data.postValue("服务器给你推送消息啦，消息内容是:${x}")Thread.sleep(2000)  // 2秒推送一次}}return super.onStartCommand(intent, flags, startId)}
}

MainActivity2.kt

package com.example.mylivedata.simple2import android.content.Intent
import android.os.Bundle
import android.util.Log
import android.widget.Button
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import com.example.mylivedata.R
import java.util.*class MainActivity2 : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main2)// 启动服务val button = findViewById<Button>(R.id.bt)button.setOnClickListener {startService(Intent(this, MyServer::class.java))Toast.makeText(this, "推送服务器启动成功", Toast.LENGTH_SHORT).show()}// 观察者 界面可见的情况下，才能做事情MyLiveData.data.observe(this, androidx.lifecycle.Observer {Log.d("MyServer", "界面可见，说明用户在查看列表界面啦，更新信息列表UI界面:${it}")Toast.makeText(this, "更新消息列表UI界面成功:${it}", Toast.LENGTH_SHORT).show()})}
}

3. 使用方式三

该方式使用的是黏性数据
MyLiveData.kt

package com.example.mylivedata.simple3import androidx.lifecycle.MutableLiveDataobject MyLiveData {// 这里的data的MutableLiveData 懒加载初始化（懒加载：用到时才加载）val data : MutableLiveData<String> by lazy { MutableLiveData<String>() }}

MainActivity3.kt

package com.example.mylivedata.simple3import android.content.Intent
import android.os.Bundle
import android.widget.Button
import androidx.appcompat.app.AppCompatActivity
import com.example.mylivedata.Rclass MainActivity3 : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main3)val button = findViewById<Button>(R.id.button)button.setOnClickListener {MyLiveData.data.value = "我就是我，不一样的烟火"startActivity(Intent(this, MainActivity4::class.java))}}
}

MainActivity4.kt

package com.example.mylivedata.simple3import android.os.Bundle
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import com.example.mylivedata.Rclass MainActivity4 : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main4)// 后观察数据，居然能够收到 前面修改的数据，这就是 数据黏性/*MyLiveData.data.observe(this, Observer {Toast.makeText(this, "观察者数据变化:${it}", Toast.LENGTH_SHORT).show()})*/MyLiveData.data.observe(this, object : Observer<String> {override fun onChanged(t: String?) {Toast.makeText(this@MainActivity4, "观察者数据变化:$t", Toast.LENGTH_SHORT).show()}})// 此观察者 和handler没有区别，一股脑的执行（极端的情况，可以用）// 但是需要手动考虑释放工作//MyLiveData.data.observeForever {  }}override fun onDestroy() {super.onDestroy()// 手动释放//MyLiveData.data.removeObserver()}
}

二、LiveData 去除黏性数据的方法

1. 去除黏性的Java版本

OkLiveDataBusJava.java

package com.example.mylivedata.simple4;import androidx.annotation.NonNull;
import androidx.arch.core.internal.SafeIterableMap;
import androidx.lifecycle.LifecycleOwner;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.Observer;import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;/*** 单例模式 去掉粘性事件  Java版本*/
public class OkLiveDataBusJava {// 存放订阅者private Map<String, BusMutableLiveData<Object>> bus;private static OkLiveDataBusJava liveDataBus = new OkLiveDataBusJava();private OkLiveDataBusJava() {bus = new HashMap<>();}public static OkLiveDataBusJava getInstance() {return liveDataBus;}// 注册订阅者public synchronized <T> BusMutableLiveData<T> with(String key, Class<T> type) {if (bus.containsKey(key)) {bus.put(key, new BusMutableLiveData<>());}return (BusMutableLiveData<T>) bus.get(key);}public static class BusMutableLiveData<T> extends MutableLiveData<T> {@Overridepublic void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {super.observe(owner, observer);  // 启用系统的功能，不写就破坏了hook(observer);}private void hook(Observer<? super T> observer) {try {// TODO 1.得到mLastVersion// 获取到LiveData类中的mObservers对象Class<LiveData> liveDataClass = LiveData.class;Field mObserversField = liveDataClass.getDeclaredField("mObservers");mObserversField.setAccessible(true);// 获取到这个成员变量的对象Object mObserversObject = mObserversField.get(this);// 得到map对象的class对象   private SafeIterableMap<Observer<? super T>, ObserverWrapper> mObservers = new SafeIterableMap<>();Class<?> mObserversClass = mObserversObject.getClass();// 获取到mObservers对象的get方法Method get = mObserversClass.getDeclaredMethod("get", Object.class);get.setAccessible(true);// 执行get方法Object invokeEntry = get.invoke(mObserversObject, observer);// 取到entry中的valueObject observerWrapper = null;if (invokeEntry != null && invokeEntry instanceof Map.Entry) {observerWrapper = ((Map.Entry) invokeEntry).getValue();}if (observerWrapper == null) {throw new NullPointerException("observerWrapper is null");}// 得到observerWrapper的类对象// observerWrapper.getClass() 获取的是LifecycleBoundObserver类对象// observerWrapper类是LifecycleBoundObserver类的父类Class<?> supperClass = observerWrapper.getClass().getSuperclass();Field mLastVersion = supperClass.getDeclaredField("mLastVersion");mLastVersion.setAccessible(true);// TODO 2.得到mVersionField mVersion = liveDataClass.getDeclaredField("mVersion");mVersion.setAccessible(true);// TODO 3.mLastVersion = mVersionObject mVersionValue = mVersion.get(this);mLastVersion.set(observerWrapper, mVersionValue);} catch (Exception e) {e.printStackTrace();}}}
}

2. 去除黏性的Kotlin版本

OKLiveDataBusKT.kt

package com.example.mylivedata.simple4import android.util.Log
import androidx.lifecycle.LifecycleOwner
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.Observer
import java.lang.NullPointerException
import java.lang.reflect.Field
import java.lang.reflect.Method/*** 单例模式 去掉黏性事件（有关闭黏性的开关）KT版本*/
object OKLiveDataBusKT {// 存放订阅者private val bus : MutableMap<String, BusMutableLiveData<Any>> by lazy { HashMap<String, BusMutableLiveData<Any>>() }// 暴露一个函数，给外界注册，订阅者关系fun <T> with(key : String, type : Class<T>, isStack : Boolean = true) : BusMutableLiveData<T> {if (!bus.containsKey(key)) {bus[key] = BusMutableLiveData(isStack)}return bus[key] as BusMutableLiveData<T>}class BusMutableLiveData<T> private constructor() : MutableLiveData<T>() {var isStack : Boolean = false// 次构造constructor(isStack: Boolean) : this() {this.isStack = isStack}override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {super.observe(owner, observer)if (!isStack) {hook(observer)Log.d("abc", "Kotlin版本 不启用黏性")} else {Log.d("abc", "Kotlin版本 启用黏性")}}private fun hook(observer: Observer<in T>) {// TODO 1.得到mLastVersion// 获取到LiveData的类中的mObservers对象val liveDataClass = LiveData::class.javaval mObserversField = liveDataClass.getDeclaredField("mObservers")mObserversField.isAccessible = true  // 私有修饰可以访问// 获取到这个成员变量的对象 Kotlin Any == Java Objectval mObserversObject : Any = mObserversField.get(this)// 得到map对象的class对象  private SafeIterableMap<Observer<? super T>, ObserverWrapper> mObservers = new SafeIterableMap<>();val mObserversClass : Class<*> = mObserversObject.javaClass// 获取到mObservers对象的get方法， protected Entry<K, V> get(K k)val get : Method = mObserversClass.getDeclaredMethod("get", Any::class.java)get.isAccessible = true  // 私有修饰也可以访问// 执行get方法val invokeEntry : Any = get.invoke(mObserversObject, observer)// 取到entry中的valuevar observerWrapper : Any? = nullif (invokeEntry != null && invokeEntry is Map.Entry<*, *>) {observerWrapper = invokeEntry.value}if (observerWrapper == null) {throw NullPointerException("observerWrapper is null")}// 得到observerWrapper的类对象val supperClass : Class<*> = observerWrapper.javaClass.superclassval mLastVersion : Field = supperClass.getDeclaredField("mLastVersion")mLastVersion.isAccessible = true// TODO 2.得到mVersionval mVersion : Field = liveDataClass.getDeclaredField("mVersion")mVersion.isAccessible = true// TODO 3.mLastVersion = mVersionval mVersionValue = mVersion.get(this)mLastVersion.set(observerWrapper, mVersionValue)}}
}

MainActivity.kt

package com.example.mylivedata.simple4import android.content.Intent
import android.os.Bundle
import android.widget.Button
import androidx.appcompat.app.AppCompatActivity
import com.example.mylivedata.Rclass MainActivity : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)// LiveData发生消息通知所有的观察者数据变化了// KT版本 旧数据  黏性数据OKLiveDataBusKT.with("data1", String::class.java).value = "liveData  kotlin数据"// Java版本OkLiveDataBusJava.getInstance().with("data2", String::class.java).value = "livaData java数据"// 点击事件，跳转到下一个Activityval button = findViewById<Button>(R.id.button)button.setOnClickListener {startActivity(Intent(this, MainActivity2::class.java))}}
}

MainActivity2.kt

package com.example.mylivedata.simple4import android.os.Bundle
import android.util.Log
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import com.example.mylivedata.R
import kotlin.concurrent.threadclass MainActivity2 : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main2)// Kotlin版本订阅观察者OKLiveDataBusKT.with("data1", String::class.java).observe(this, Observer(){Toast.makeText(this, "Kotlin版本的观察者:${it}", Toast.LENGTH_SHORT).show()Log.d("abc", "Kotlin版本的观察者:${it}")})// Java版本订阅观察者（Java是模拟剔除黏性的）OkLiveDataBusJava.getInstance().with("data2", String::class.java).observe(this, Observer() {Toast.makeText(this, "Java版本的观察者:${it}", Toast.LENGTH_SHORT).show()Log.d("abc", "Java版本的观察者:${it}")})thread {Thread.sleep(2000)OKLiveDataBusKT.with("data1", String::class.java).postValue("2222")}thread {Thread.sleep(4000)OkLiveDataBusJava.getInstance().with("data2", String::class.java).postValue("4444")}}
}