来自Transformation的数据绑定LiveData - Android Kotlin

Question

我正在学习kotlin和android架构组件.我在谷歌地图上有一个简单的地图切换按钮用例.

我想使用数据绑定将地图切换按钮标签绑定到我的ViewModel中的MutableLiveData字段.

我在Activity中的onCreate方法中设置MapViewModel中的mapType val.如果我理解正确,这应该触发mapLabel val由于使用Transformations.map而改变.

它不起作用......为什么？

这是我的版本:

• Android studio 3.2 Canary 4
• kotlin_version ='1.2.21'
• support ="27.1.0"
• arch_core ="1.1.0"
• 数据绑定="3.2.0-alpha04"

MapViewModel.kt

class MapViewModel : ViewModel() {

    val mapType: MutableLiveData<MapType> = MutableLiveData()

    val mapLabel: LiveData<String> = Transformations.map(mapType, {
        if (it == MapType.MAP) "SAT" else "MAP"
    })
}

enum class MapType {
    SAT, MAP
}

activity_maps.xml

<?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>

        <variable
            name="vm"
            type="uk.co.oliverdelange.wcr_android_kt.ui.map.MapViewModel" />
    </data>

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <fragment
            android:id="@+id/map"
            android:name="com.google.android.gms.maps.SupportMapFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            tools:context=".MapsActivity">

            <Button
                android:id="@+id/map_toggle"
                style="@style/Wcr_MapToggle"
                android:layout_marginTop="110dp"
                android:layout_marginEnd="12dp"
                android:layout_marginBottom="7dp"
                android:layout_gravity="top|end" 
                android:text="@{vm.mapLabel}" />
        </fragment>    
    </FrameLayout>
</layout>

MapsActivity.kt

class MapsActivity : AppCompatActivity(), OnMapReadyCallback {

    private lateinit var mMap: GoogleMap
    private lateinit var viewModel: MapViewModel
    private lateinit var binding: ActivityMapsBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = DataBindingUtil.setContentView(this, R.layout.activity_maps)
        viewModel = ViewModelProviders.of(this).get(MapViewModel::class.java)
        binding.vm = viewModel

        val mapFragment = supportFragmentManager.findFragmentById(R.id.map) as SupportMapFragment
        mapFragment.getMapAsync(this)

        // I can do it this way, but I don't want to. 
        // viewModel.mapLabel.observe(this, Observer { map_toggle.text = it })

        // Here is where i'm setting the MapType on the ViewModel.
        viewModel.mapType.value = MapType.MAP
    }

    override fun onMapReady(googleMap: GoogleMap) {
        mMap = googleMap
    }
}

我用MutableLiveData对象测试了绑定,我在活动中设置了字符串,它工作正常.问题似乎与Transformations.map - 我刚刚理解错了吗？

此外,在调试时,我看到我的ViewModel中的mapType val没有观察者(不确定这是对还是错,只是有趣)

Answer 1

这里的问题是尽管被绑定到该mapLabel字段,但是当mapLabel字段的值发生变化时,视图绑定没有被更新.

原因是我没有在绑定上设置生命周期所有者.

binding.setLifecycleOwner(this)

我第10次阅读这篇博文后意识到自己的错误.

我的新MapsActivity.kt

class MapsActivity : AppCompatActivity(), OnMapReadyCallback {

    private lateinit var mMap: GoogleMap
    private lateinit var viewModel: MapViewModel
    private lateinit var binding: ActivityMapsBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = DataBindingUtil.setContentView(this, R.layout.activity_maps)
        binding.setLifecycleOwner(this) //<- NEW!
        viewModel = ViewModelProviders.of(this).get(MapViewModel::class.java)
        binding.vm = viewModel

        val mapFragment = supportFragmentManager.findFragmentById(R.id.map) as SupportMapFragment
        mapFragment.getMapAsync(this)

        // Here is where i'm setting the MapType on the ViewModel.
        viewModel.mapType.value = MapType.MAP
    }

    override fun onMapReady(googleMap: GoogleMap) {
        mMap = googleMap
    }
}

从好的方面来说,我学到了很多关于LiveData内部如何工作的知识!