RxJava与Retrofit

在出现LiveData之前，Android上实现网络请求最常用的方式是使用Retrofit+Rxjava。通常是RxJavaCallAdapterFactory将请求转成Observable(或者Flowable等)被观察者对象，调用时通过subscribe方式实现最终的请求。为了实现线程切换，需要将订阅时的线程切换成io线程，请求完成通知被观察者时切换成ui线程。代码通常如下：

observable.subscribeOn(Schedulers.io())
          .observeOn(AndroidSchedulers.mainThread())
          .subscribe(subscriber)

为了能够让请求监听到生命周期变化，onDestroy时不至于发生view空指针，要需要使用RxLifecycle或AutoDispose让Observable能够监听到Activity和Fragment的生命周期，在适当的生命周期下取消订阅。

LiveData与Retrofit

LiveData和Rxjava中的Observable类似，是一个被观察者的数据持有类。但是不同的是LiveData具有生命周期感知，相当于RxJava+RxLifecycle。LiveData使用起来相对简单轻便，所以当它加入到项目中后，再使用RxJava便显得重复臃肿了(RxJava包1～2M容量)。为了移除RxJava，我们将Retrofit的Call请求适配成LiveData，因此我们需要自定义CallAdapterFactory。根据接口响应格式不同，对应的适配器工厂会有所区别。本次便以广为人知的wanandroid的api为例子，来完成LiveData网络请求实战。首先根据它的响应格式：

{
	data:[],//或者{}
	errorCode:0,
	errorMsg:""
}

定义一个通用的响应实体ApiResponse

class ApiResponse<T>(
    var data: T?,
    var errorCode: Int,
    var errorMsg: String
)

然后我们定义对应的LiveDataCallAdapterFactory

import androidx.lifecycle.LiveData
import retrofit2.CallAdapter
import retrofit2.Retrofit
import java.lang.reflect.Type
import retrofit2.CallAdapter.Factory
import java.lang.reflect.ParameterizedType

class LiveDataCallAdapterFactory : Factory() {
    override fun get(returnType: Type, annotations: Array<Annotation>, retrofit: Retrofit): CallAdapter<*, *>? {
        if (getRawType(returnType) != LiveData::class.java) return null
        //获取第一个泛型类型
        val observableType = getParameterUpperBound(0, returnType as ParameterizedType)
        val rawType = getRawType(observableType)
        if (rawType != ApiResponse::class.java) {
            throw IllegalArgumentException("type must be ApiResponse")
        }
        if (observableType !is ParameterizedType) {
            throw IllegalArgumentException("resource must be parameterized")
        }
        return LiveDataCallAdapter<Any>(observableType)
    }
}

然后在LiveDataCallAdapter将Retrofit的Call对象适配成LiveData

import androidx.lifecycle.LiveData
import retrofit2.Call
import retrofit2.CallAdapter
import retrofit2.Callback
import retrofit2.Response
import java.lang.reflect.Type
import java.util.concurrent.atomic.AtomicBoolean

class LiveDataCallAdapter<T>(private val responseType: Type) : CallAdapter<T, LiveData<T>> {
    override fun adapt(call: Call<T>): LiveData<T> {
        return object : LiveData<T>() {
            private val started = AtomicBoolean(false)
            override fun onActive() {
                super.onActive()
                if (started.compareAndSet(false, true)) {//确保执行一次
                    call.enqueue(object : Callback<T> {
                        override fun onFailure(call: Call<T>, t: Throwable) {
                            val value = ApiResponse<T>(null, -1, t.message ?: "") as T
                            postValue(value)
                        }

                        override fun onResponse(call: Call<T>, response: Response<T>) {
                            postValue(response.body())
                        }
                    })
                }
            }
        }
    }

    override fun responseType() = responseType
}

第一个请求

以首页banner接口(https://www.wanandroid.com/banner/json)为例，完成第一个请求。新建一个WanApi接口，加入Banner列表api，以及Retrofit初始化方法,为方便查看http请求和响应，加入了okhttp自带的日志拦截器。

interface WanApi {
    companion object {
        fun get(): WanApi {
            val clientBuilder = OkHttpClient.Builder()
                .connectTimeout(60, TimeUnit.SECONDS)
            if (BuildConfig.DEBUG) {
                val loggingInterceptor = HttpLoggingInterceptor()
                loggingInterceptor.level = HttpLoggingInterceptor.Level.BODY
                clientBuilder.addInterceptor(loggingInterceptor)
            }
            return Retrofit.Builder()
                .baseUrl("https://www.wanandroid.com/")
                .client(clientBuilder.build())
                .addCallAdapterFactory(LiveDataCallAdapterFactory())
                .addConverterFactory(GsonConverterFactory.create())
                .build()
                .create(WanApi::class.java)
        }
    }
    /**
     * 首页banner
     */
    @GET("banner/json")
    fun bannerList(): LiveData<ApiResponse<List<BannerVO>>>
}

BannerVO实体

data class BannerVO(
    var id: Int,
    var title: String,
    var desc: String,
    var type: Int,
    var url: String,
    var imagePath:String
)

我们在MainActivity中发起请求

 private fun loadData() {
    val bannerList = WanApi.get().bannerList()
    bannerList.observe(this, Observer {
        Log.e("main", "res:$it")
    })
 }

调试结果如下：

LiveData的map与switchMap操作

LiveData可以通过Transformations的map和switchMap操作，将一个LiveData转成另一种类型的LiveData，效果与RxJava的map/switchMap操作符类似。可以看看两个函数的声明

public static <X, Y> LiveData<Y> map(
            @NonNull LiveData<X> source,
            @NonNull final Function<X, Y> mapFunction)


public static <X, Y> LiveData<Y> switchMap(
            @NonNull LiveData<X> source,
            @NonNull final Function<X, LiveData<Y>> switchMapFunction)

根据以上代码，我们可以知道，对应的变换函数返回的类型是不一样的：map是基于泛型类型的变换，而switchMap则返回一个新的LiveData。

还是以banner请求为例，我们将map和switchMap应用到实际场景中： 1: 为了能够手动控制请求，我们需要一个refreshTrigger触发变量，当这个变量被设置为true时，通过switchMap生成一个新的LiveData用作请求banner

private val refreshTrigger = MutableLiveData<Boolean>()
private val api = WanApi.get()
private val bannerLis:LiveData<ApiResponse<List<BannerVO>>> = Transformations.switchMap(refreshTrigger) {
    //当refreshTrigger的值被设置时，bannerList
    api.bannerList()
}

2: 为了展示banner，我们通过map将ApiResponse转换成最终关心的数据是List<BannerVO>

val banners: LiveData<List<BannerVO>> = Transformations.map(bannerList) {
	it.data ?: ArrayList()
}

LiveData与ViewModel结合

为了将LiveData与Activity解耦，我们通过ViewModel来管理这些LiveData。

class HomeVM : ViewModel() {
    private val refreshTrigger = MutableLiveData<Boolean>()
    private val api = WanApi.get()
    private val bannerList: LiveData<ApiResponse<List<BannerVO>>> = Transformations.switchMap(refreshTrigger) {
        //当refreshTrigger的值被设置时，bannerList
        api.bannerList()
    }

    val banners: LiveData<List<BannerVO>> = Transformations.map(bannerList) {
        it.data ?: ArrayList()
    }

    fun loadData() {
        refreshTrigger.value = true
    }
}

在activity_main.xml中加入banner布局，这里使用BGABanner-Android来显示图片

<?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="io.github.iamyours.wandroid.ui.home.HomeVM"/>
    </data>
    <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

        <cn.bingoogolapple.bgabanner.BGABanner
                android:id="@+id/banner"
                android:layout_width="match_parent"
                android:layout_height="120dp"
                android:paddingLeft="16dp"
                android:paddingRight="16dp"
                app:banner_indicatorGravity="bottom|right"
                app:banner_isNumberIndicator="true"
                app:banner_pointContainerBackground="#0000"
                app:banner_transitionEffect="zoom"/>

        <TextView
                android:layout_width="match_parent"
                android:layout_height="44dp"
                android:background="#ccc"
                android:gravity="center"
                android:onClick="@{()->vm.loadData()}"
                android:text="加载Banner"/>
    </LinearLayout>
</layout>

然后在MainActivity完成Banner初始化,通过监听ViewModel中的banners实现轮播图片的展示。

class MainActivity : AppCompatActivity() {
    lateinit var binding: ActivityMainBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        val vm = ViewModelProviders.of(this).get(HomeVM::class.java)
        binding.lifecycleOwner = this
        binding.vm = vm
        initBanner()
    }

    private fun initBanner() {
        binding.run {
            val bannerAdapter = BGABanner.Adapter<ImageView, BannerVO> { _, image, model, _ ->
                image.displayWithUrl(model?.imagePath)
            }
            banner.setAdapter(bannerAdapter)
            vm?.banners?.observe(this@MainActivity, Observer {
                banner.setData(it, null)
            })
        }
    }
}

最终效果如下：

加载进度显示

SwipeRefreshLayout

请求网络过程中，必不可少的是加载进度的展示。这里我们列举两种常用的的加载方式，一种在布局中的进度条(如SwipeRefreshLayout)，另一种是加载对话框。为了控制加载进度条显示隐藏，我们在HomeVM中添加loading变量，在调用loadData时通过loading.value=true控制进度条的显示，在map中的转换函数中控制进度的隐藏

val loading = MutableLiveData<Boolean>()
val banners: LiveData<List<BannerVO>> = Transformations.map(bannerList) {
    loading.value = false
    it.data ?: ArrayList()
}
fun loadData() {
    refreshTrigger.value = true
    loading.value = true
}

我们在activity_main.xml的外层嵌套一个SwipeRefreshLayout，通过databinding设置加载状态，添加刷新事件

<androidx.swiperefreshlayout.widget.SwipeRefreshLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:onRefreshListener="@{() -> vm.loadData()}"
        app:refreshing="@{vm.loading}">
        ...
</androidx.swiperefreshlayout.widget.SwipeRefreshLayout>

然后我们再看下效果：

加载对话框KProgressHUD

为了能和ViewModel解藕，我们将加载对话框封装到一个Observer中。

class LoadingObserver(context: Context) : Observer<Boolean> {
    private val dialog = KProgressHUD(context)
        .setStyle(KProgressHUD.Style.SPIN_INDETERMINATE)
        .setCancellable(false)
        .setAnimationSpeed(2)
        .setDimAmount(0.5f)

    override fun onChanged(show: Boolean?) {
        if (show == null) return
        if (show) {
            dialog.show()
        } else {
            dialog.dismiss()
        }
    }
}

然后在MainActivity添加这个Observer

vm.loading.observe(this, LoadingObserver(this))

效果：

我们还可以将LoadingObserver注册到BaseActivity

class BaseActivity : AppCompatActivity() {
    val loadingState = MutableLiveData<Boolean>()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        loadingState.observe(this, LoadingObserver(this))
    }
}

然后在HomeVM中添加一个attachLoading方法

class HomeVM:ViewModel{
     fun attachLoading(otherLoadingState: MutableLiveData<Boolean>) {
        loading.observeForever {
            otherLoadingState.value = it
        }
    }
}

最终如果想要显示进度对话框，在BaseActivity到子类中，只需调用vm.attachLoading(loadingState)即可。

分页请求

分页请求是另个一常用请求，它的请求状态就比刷新数据多了几种。以wanandroid首页文章列表api为例，我们在HomeVM中加入page,refreshing,moreLoading，hasMore变量控制分页请求

private val page = MutableLiveData<Int>() //分页数据
val refreshing = MutableLiveData<Boolean>()//下拉刷新状态
val moreLoading = MutableLiveData<Boolean>()//上拉加载更多状态
val hasMore = MutableLiveData<Boolean>()//是否还有更多数据
private val articleList = Transformations.switchMap(page) {
    api.articleList(it)
}

val articlePage = Transformations.map(articleList) {
    refreshing.value = false
    moreLoading.value = false
    hasMore.value = !(it?.data?.over ?: false)
    it.data
}

fun loadMore() {
    page.value = (page.value ?: 0) + 1
    moreLoading.value = true
}

fun refresh() {
    loadBanner()
    page.value = 0
    refreshing.value = true
}

用SmartRefreshLayout作为分页组件，来实现WanAndroid首页文章列表数据的展示。

绑定SmartRefreshLayout属性和事件

通过@BindingAdapter注解，将绑定SmartRefreshLayout属性和事件封装一样，便于我们在布局文件通过databinding控制它。新建一个CommonBinding.kt文件，注意在gradle中引入kotlin-kapt

@BindingAdapter(value = ["refreshing", "moreLoading", "hasMore"], requireAll = false)
fun bindSmartRefreshLayout(
    smartLayout: SmartRefreshLayout,
    refreshing: Boolean,
    moreLoading: Boolean,
    hasMore: Boolean

) {
    if (!refreshing) smartLayout.finishRefresh()
    if (!moreLoading) smartLayout.finishLoadMore()
    smartLayout.setEnableLoadMore(hasMore)
}

@BindingAdapter(value = ["onRefreshListener", "onLoadMoreListener"], requireAll = false)
fun bindListener(
    smartLayout: SmartRefreshLayout,
    refreshListener: OnRefreshListener?,
    loadMoreListener: OnLoadMoreListener?
) {
    smartLayout.setOnRefreshListener(refreshListener)
    smartLayout.setOnLoadMoreListener(loadMoreListener)
}

然后在布局中使用

<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="io.github.iamyours.wandroid.ui.home.HomeVM"/>
    </data>

    <com.scwang.smartrefresh.layout.SmartRefreshLayout
            android:id="@+id/refreshLayout"
            android:layout_width="match_parent"
            app:onRefreshListener="@{()->vm.refresh()}"
            app:refreshing="@{vm.refreshing}"
            app:moreLoading="@{vm.moreLoading}"
            app:hasMore="@{vm.hasMore}"
            app:onLoadMoreListener="@{()->vm.loadMore()}"
            android:layout_height="match_parent">

        <androidx.core.widget.NestedScrollView
                android:layout_width="match_parent"
                android:layout_height="match_parent">

            <LinearLayout
                    android:layout_width="match_parent"
                    android:orientation="vertical"
                    android:layout_height="wrap_content">

                <cn.bingoogolapple.bgabanner.BGABanner
                        android:id="@+id/banner"
                        android:layout_width="match_parent"
                        android:layout_height="140dp"
                        app:banner_indicatorGravity="bottom|right"
                        app:banner_isNumberIndicator="true"
                        app:banner_pointContainerBackground="#0000"
                        app:banner_transitionEffect="zoom"/>

                <androidx.recyclerview.widget.RecyclerView
                        android:id="@+id/recyclerView"
                        android:layout_width="match_parent"
                        android:layout_marginTop="5dp"
                        tools:listitem="@layout/item_article"
                        android:layout_height="wrap_content"/>
            </LinearLayout>
        </androidx.core.widget.NestedScrollView>
    </com.scwang.smartrefresh.layout.SmartRefreshLayout>

</layout>

分页实现

然后在MainActivity中完成RecyclerView的逻辑

class MainActivity : AppCompatActivity() {
    lateinit var binding: ActivityMainBinding
    private val adapter = ArticleAdapter()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        val vm = ViewModelProviders.of(this).get(HomeVM::class.java)
        binding.lifecycleOwner = this
        binding.vm = vm
        binding.executePendingBindings()
        initBanner()
        initRecyclerView()
        binding.refreshLayout.autoRefresh()
    }

    private fun initRecyclerView() {
        binding.recyclerView.let {
            it.adapter = adapter
            it.layoutManager = LinearLayoutManager(this)
        }
        binding.vm?.articlePage?.observe(this, Observer {
            it?.run {
                if (curPage == 1) {
                    adapter.clearAddAll(datas)
                } else {
                    adapter.addAll(datas)
                }
            }
        })
    }

    private fun initBanner() {
       ...
    }
}

最终效果：

项目地址

https://github.com/iamyours/Wandroid