LiveData+Retrofit 网络请求实战
作者:星星y
星星y
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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( var data: T?, var errorCode: Int, var errorMsg: String)
然后我们定义对应的LiveDataCallAdapterFactory
class LiveDataCallAdapterFactory : Factory() { override fun get(returnType: Type, annotations: Array, 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(observableType) }}
然后在LiveDataCallAdapter将Retrofit的Call对象适配成LiveData
class LiveDataCallAdapter(private val responseType: Type) : CallAdapter> { override fun adapt(call: Call): LiveData { return object : LiveData() { 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 { override fun onFailure(call: Call, t: Throwable) { val value = ApiResponse(null, -1, t.message ?: "") as T postValue(value) } override fun onResponse(call: Call, response: Response) { 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>>}
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:
banner请求结果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 LiveData map( @NonNull LiveData source, @NonNull final Function mapFunction)public static LiveData switchMap( @NonNull LiveData source, @NonNull final Function> 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:
private val refreshTrigger = MutableLiveData()private val api = WanApi.get()private val bannerLis:LiveData>> = 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
val banners: LiveData = Transformations.map(bannerList) {
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() private val api = WanApi.get() private val bannerList: LiveData>> = Transformations.switchMap(refreshTrigger) { //当refreshTrigger的值被设置时,bannerList api.bannerList() } val banners: LiveData> = 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"?> vm.loadData()}" android:text="加载Banner"/>
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 { _, image, model, _ -> image.displayWithUrl(model?.imagePath) } banner.setAdapter(bannerAdapter) vm?.banners?.observe([email protected], Observer { banner.setData(it, null) }) } }}
最终效果如下:
banner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()val banners: LiveData> = 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
vm.loadData()}" app:refreshing="@{vm.loading}"> ...
然后我们再看下效果:
SwipeRefreshLayout进度控制加载对话框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 { 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:
加载对话框显示我们还可以将LoadingObserver注册到BaseActivity
class BaseActivity : AppCompatActivity() { val loadingState = MutableLiveData() 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) { 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() //分页数据val refreshing = MutableLiveData()//下拉刷新状态val moreLoading = MutableLiveData()//上拉加载更多状态val hasMore = MutableLiveData()//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。
@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
vm.refresh()}" app:refreshing="@{vm.refreshing}" app:moreLoading="@{vm.moreLoading}" app:hasMore="@{vm.hasMore}" app:onLoadMoreListener="@{()->vm.loadMore()}" android:layout_height="match_parent">
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:
wanandroid首页数据项目地址https://github.com/iamyours/Wandroid
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