【Android-Jetpack进阶】4、LiveData：Activity 监听数据变化，用 LiveData + ViewModel 在 Fragment 间共享数据

文章目录

四、LiveData 监听实时数据变化
- 4.1 LivaData 和 Activity 通信
- 4.2 LiveData 源码
- - 4.2.1 observe() 源码
  - 4.2.2 observeForever() 源码
- 4.3 用 LiveData 使 Fragment 间可通信

四、LiveData 监听实时数据变化

ViewModel 内定义 interface，Activity 或 Fragment 实例化 ViewModel 对象时，实现该 interface。这种方式需要大量的 interface，代码很冗余，所以可以用 LiveData 组件。LiveData 是可被观察的数据容器类，其将数据包装起来，当数据变化时可通知观察者。LiveData 通常放在 ViewModel 中来包装 ViewModel 的数据，使 ViewModel 的数据可被外界来观察，架构如下：

LiveData 有 observe()、postValue()、setValue() 函数，示例如下：

4.1 LivaData 和 Activity 通信

首先，新建项目 Jetpack4LiveDataTest，在项目添加 implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0' 依赖。

然后，新建 TimerWithLiveDataViewModel 类，LiveData是抽象类，通常我们用其直接子类 MutableLiveData 类，代码如下：

package com.bignerdranch.android.jetpack4livedatatest

import android.util.Log
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import java.util.*

class TimerWithLiveDataViewModel : ViewModel() {
    private val TAG = this.javaClass.name
    private var timer: Timer? = null
    private var currentSecond: MutableLiveData<Int>? = null
    fun getCurrentSecond(): LiveData<Int> {
        if (currentSecond == null) {
            currentSecond = MutableLiveData()
        }
        return currentSecond as MutableLiveData<Int>
    }

    // 开始计时
    fun startTiming() {
        if (timer == null) {
            currentSecond!!.value = 0
            timer = Timer()
            val timerTask: TimerTask = object : TimerTask() {
                override fun run() {
                    currentSecond!!.postValue(currentSecond!!.value!! + 1) // 这里要用postValue方法，而不能用setValue方法，否则会报线程异常错误
                }
            }
            timer!!.schedule(timerTask, 1000, 1000) //延迟1秒执行
        }
    }

    // 由于屏幕旋转导致的Activity重建，该方法不会被调用
    // 只有ViewModel已经没有任何Activity与之有关联，系统则会调用该方法，你可以在此清理资源
    override fun onCleared() {
        super.onCleared()
        Log.d(TAG, "onCleared()")
        timer?.cancel()
    }
}

然后，在 activity_main.xml 中添加布局，布局如下：


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

    <TextView
        android:id="@+id/tvTime"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:textSize="40sp"
        android:text="TIME"/>

    <Button
        android:id="@+id/btnResetTime"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:layout_marginTop="20dp"
        android:layout_below="@+id/tvTime"
        android:text="重置时间"/>

RelativeLayout>

MainActivity 可监听 ViewModel 的值并显示在 TextView 上，当按下重置按钮时将 ViewModel 的值置位为0，代码如下：

package com.bignerdranch.android.jetpack4livedatatest

import android.os.Bundle
import android.view.View
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModelProvider


class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        iniComponent()
    }

    private fun iniComponent() {
        val vm = ViewModelProvider(this)[TimerWithLiveDataViewModel::class.java]
        val liveData = vm.getCurrentSecond() as MutableLiveData<Int> // 得到ViewModel中的LiveData
        liveData.observe(this) { second -> (findViewById<View>(R.id.tvTime) as TextView).text = "TIME:$second" } // 对ViewModel中数据变化的观察
        findViewById<View>(R.id.btnResetTime).setOnClickListener { liveData.setValue(0) } // 对ViewModel中数据的更新
        vm.startTiming()
    }
}

运行后，文本不断变化，当按下重置按钮则置位为0，效果如下：

项目代码github详见

4.2 LiveData 源码

4.2.1 observe() 源码

其 observe() 函数的源码如下，第一个参数是 LifecycleOwner 对象（即本例的 Activity），第二个参数是 Observer 对象。最后一行将 owner 和 observer 通过 lifecycle 关联。

只有当页面为激活状态（Lifecycle.State.ON_STARTED 和 Lifecycle.State.ON_RESUME）时，页面才能收到来自 LiveData 的通知。
若页面被销毁（Lifecycle.State.ON_DESTROY）时 LiveData 会自动与页面清除关联，避免内存泄漏。

源码如下：

    /**
     * Adds the given observer to the observers list within the lifespan of the given
     * owner. The events are dispatched on the main thread. If LiveData already has data
     * set, it will be delivered to the observer.
     * 
     * The observer will only receive events if the owner is in {@link Lifecycle.State#STARTED}
     * or {@link Lifecycle.State#RESUMED} state (active).
     * 

     * If the owner moves to the {@link Lifecycle.State#DESTROYED} state, the observer will
     * automatically be removed.
     * 

     * When data changes while the {@code owner} is not active, it will not receive any updates.
     * If it becomes active again, it will receive the last available data automatically.
     * 

     * LiveData keeps a strong reference to the observer and the owner as long as the
     * given LifecycleOwner is not destroyed. When it is destroyed, LiveData removes references to
     * the observer & the owner.
     * 

     * If the given owner is already in {@link Lifecycle.State#DESTROYED} state, LiveData
     * ignores the call.
     * 
     * If the given owner, observer tuple is already in the list, the call is ignored.
     * If the observer is already in the list with another owner, LiveData throws an
     * {@link IllegalArgumentException}.
     *
     * @param owner    The LifecycleOwner which controls the observer
     * @param observer The observer that will receive the events
     */
    @MainThread
    public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
        assertMainThread("observe");
        if (owner.getLifecycle().getCurrentState() == DESTROYED) {
            // ignore
            return;
        }
        LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
        ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
        if (existing != null && !existing.isAttachedTo(owner)) {
            throw new IllegalArgumentException("Cannot add the same observer"
                    + " with different lifecycles");
        }
        if (existing != null) {
            return;
        }
        owner.getLifecycle().addObserver(wrapper);
    }

4.2.2 observeForever() 源码

无论页面是什么状态，LiveData 都会通知，因此使用完之后，要用 removeObserver() 来停止对 LiveData 的观察，防止内存泄露，源码如下：

    /**
     * Adds the given observer to the observers list. This call is similar to
     * {@link LiveData#observe(LifecycleOwner, Observer)} with a LifecycleOwner, which
     * is always active. This means that the given observer will receive all events and will never
     * be automatically removed. You should manually call {@link #removeObserver(Observer)} to stop
     * observing this LiveData.
     * While LiveData has one of such observers, it will be considered
     * as active.
     * 
     * If the observer was already added with an owner to this LiveData, LiveData throws an
     * {@link IllegalArgumentException}.
     *
     * @param observer The observer that will receive the events
     */
    @MainThread
    public void observeForever(@NonNull Observer<? super T> observer) {
        assertMainThread("observeForever");
        AlwaysActiveObserver wrapper = new AlwaysActiveObserver(observer);
        ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
        if (existing instanceof LiveData.LifecycleBoundObserver) {
            throw new IllegalArgumentException("Cannot add the same observer"
                    + " with different lifecycles");
        }
        if (existing != null) {
            return;
        }
        wrapper.activeStateChanged(true);
    }

4.3 用 LiveData 使 Fragment 间可通信

因为 Fragment 是 Activity 的子页面，我们可用 LiveData，使 Fragment 间可通信，架构如下：

首先，新建 Jetpack4FragmentlShardLiveDataTest 项目，引入 implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0' 依赖。

然后，新建 SharedDateViewModel 类，内有一个 process 变量，代码如下：

package com.bignerdranch.android.jetpack4fragmentlshardlivedatatest

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel


class ShareDataViewModel : ViewModel() {
    private var progress: MutableLiveData<Int>? = null
    fun getProgress(): LiveData<Int> {
        if (progress == null) {
            progress = MutableLiveData()
        }
        return progress as MutableLiveData<Int>
    }

    // 由于屏幕旋转导致的Activity重建，该方法不会被调用, 只有ViewModel已经没有任何Activity与之有关联，系统则会调用该方法，你可以在此清理资源
    override fun onCleared() {
        super.onCleared()
        progress = null
    }
}

然后，在 app/java 菜单栏，右键新建 OneFragment 和 TwoFragment 两个类，设置 activity_main.xml 中包含2个 Fragment，布局如下：


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

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragmentOne"
        android:name="com.bignerdranch.android.jetpack4fragmentlshardlivedatatest.OneFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" />

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragmentTwo"
        android:name="com.bignerdranch.android.jetpack4fragmentlshardlivedatatest.TwoFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" />

LinearLayout>

activity_main 的布局效果如下：

然后，设置 fragment_one.xml 的布局如下：


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

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:layout_above="@+id/seekBar"
        android:text="Fragment_One"/>

    <SeekBar
        android:id="@+id/seekBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:max="100"
        android:layout_centerInParent="true"/>

RelativeLayout>

同样，设置 fragment.xml 布局如下：


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

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:layout_above="@+id/seekBar"
        android:text="Fragment_Two"/>

    <SeekBar
        android:id="@+id/seekBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:max="100"
        android:layout_centerInParent="true"/>

RelativeLayout>

在 OneFragment 中设置逻辑，代码如下：

package com.bignerdranch.android.jetpack4fragmentlshardlivedatatest

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.SeekBar
import android.widget.SeekBar.OnSeekBarChangeListener
import androidx.fragment.app.Fragment
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModelProvider


class OneFragment : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
        val parentView: View = inflater.inflate(R.layout.fragment_one, container, false)
        val seekBar = parentView.findViewById<SeekBar>(R.id.seekBar)

        // 注意：这里ViewModelProvider(requireActivity())这里的参数需要是Activity，而不能是Fragment，否则收不到监听
        val shareDataViewModel = ViewModelProvider(requireActivity())[ShareDataViewModel::class.java]
        val liveData = shareDataViewModel.getProgress() as MutableLiveData<Int>

        // 通过observe方法观察ViewModel中字段数据的变化，并在变化时，得到通知
        liveData.observe(viewLifecycleOwner) { progress -> seekBar.progress = progress!! }
        seekBar.setOnSeekBarChangeListener(object : OnSeekBarChangeListener {
            override fun onProgressChanged(seekBar: SeekBar, progress: Int, fromUser: Boolean) {
                liveData.value = progress // 用户操作SeekBar时，更新ViewModel中的数据
            }

            override fun onStartTrackingTouch(seekBar: SeekBar) {}
            override fun onStopTrackingTouch(seekBar: SeekBar) {}
        })
        return parentView
    }
}

TwoFragment 代码和 OneFragment 代码相同，代码如下：

package com.bignerdranch.android.jetpack4fragmentlshardlivedatatest

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.SeekBar
import android.widget.SeekBar.OnSeekBarChangeListener
import androidx.fragment.app.Fragment
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModelProvider


class TwoFragment : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
        val parentView: View = inflater.inflate(R.layout.fragment_one, container, false)
        val seekBar = parentView.findViewById<SeekBar>(R.id.seekBar)

        // 注意：这里ViewModelProvider(requireActivity())这里的参数需要是Activity，而不能是Fragment，否则收不到监听
        val shareDataViewModel = ViewModelProvider(requireActivity())[ShareDataViewModel::class.java]
        val liveData = shareDataViewModel.getProgress() as MutableLiveData<Int>

        // 通过observe方法观察ViewModel中字段数据的变化，并在变化时，得到通知
        liveData.observe(viewLifecycleOwner) { progress -> seekBar.progress = progress!! }
        seekBar.setOnSeekBarChangeListener(object : OnSeekBarChangeListener {
            override fun onProgressChanged(seekBar: SeekBar, progress: Int, fromUser: Boolean) {
                liveData.value = progress // 用户操作SeekBar时，更新ViewModel中的数据
            }

            override fun onStartTrackingTouch(seekBar: SeekBar) {}
            override fun onStopTrackingTouch(seekBar: SeekBar) {}
        })
        return parentView
    }
}

运行后，无论滑动哪个 Fragment 的进度条，另一个 Fragment 的进度条也都会跟着滑动，且因为使用 ViewModel，所以无论旋转屏幕、按 Home 键、查看概览屏，VIewModel 的 process 变量都会保存，其实就是通过 Fragment 间共享 VIewModel 实现的，效果如下：