livedata

引用

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

MainActivity

class MainActivity : AppCompatActivity() {
    var mViewModel: MyViewModel? = null
    private lateinit var activityMainBinding: ActivityMainBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        activityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        // ViewModel和控制器绑定
        mViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
        // 给LiveData设置一个观察者，观察数据的变化，数据发生变化，就直接更新到View之上
        mViewModel!!.getLikedNumber().observe(this, Observer {integer ->
            integer?.let {
                activityMainBinding.textView.setText(integer.toString())
            }
        })
        activityMainBinding.button.setOnClickListener {
            mViewModel!!.setLikedNumber(1)
        }
        activityMainBinding.button2.setOnClickListener {
            mViewModel!!.setLikedNumber(2)
        }
    }
}

ViewModel

class MyViewModel : ViewModel() {
    // MutableLiveData 是 LiveData 的一个最简单实现，它可以接收数据更新并通知观察者
    private var mLikedNumber: MutableLiveData<Int>? = null

    fun getLikedNumber(): MutableLiveData<Int> {
        if (mLikedNumber == null) {
            mLikedNumber = MutableLiveData()
            mLikedNumber!!.value = 0
        }
        return mLikedNumber!!
    }

    fun setLikedNumber(value: Int) {
        mLikedNumber!!.value = mLikedNumber!!.value!! + value
    }
}

activity_main

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <data>

    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <TextView
            android:id="@+id/textView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello World!"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <Button
            android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintTop_toBottomOf="@+id/textView"
            app:layout_constraintLeft_toLeftOf="@+id/textView"
            app:layout_constraintRight_toRightOf="@+id/textView"
            android:layout_marginTop="30dp"
            android:text="+1"/>

        <Button
            android:id="@+id/button2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintTop_toBottomOf="@+id/button"
            app:layout_constraintLeft_toLeftOf="@+id/textView"
            app:layout_constraintRight_toRightOf="@+id/textView"
            android:layout_marginTop="30dp"
            android:text="+1"/>

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>