我认为通过一个例子来解释在Android中的MVVM会更有帮助。完整的文章以及GitHub存储库信息可以在这里找到。
假设我们使用本系列第一部分介绍的相同基准电影应用程序示例。用户输入电影的搜索词并按下“查找”按钮,然后应用程序根据此搜索包含该搜索词的电影列表并显示它们。单击列表上的每个电影将显示其详细信息。
![enter image description here](https://istack.dev59.com/FElE9.webp)
我现在将解释这个应用程序是如何使用MVVM实现的,然后是完整的Android应用程序,可在我的GitHub页面上找到。
当用户在视图上点击“查找”按钮时,将从ViewModel调用一个方法,并将搜索词作为其参数:
main_activity_button.setOnClickListener({
showProgressBar()
mMainViewModel.findAddress(main_activity_editText.text.toString())
})
ViewModel 然后调用 Model 中的 findAddress 方法来搜索电影名称:
fun findAddress(address: String) {
val disposable: Disposable = mainModel.fetchAddress(address)!!.subscribeOn(schedulersWrapper.io()).observeOn(schedulersWrapper.main()).subscribeWith(object : DisposableSingleObserver<List<MainModel.ResultEntity>?>() {
override fun onSuccess(t: List<MainModel.ResultEntity>) {
entityList = t
resultListObservable.onNext(fetchItemTextFrom(t))
}
override fun onError(e: Throwable) {
resultListErrorObservable.onNext(e as HttpException)
}
})
compositeDisposable.add(disposable)
}
当来自Model的响应到达时,RxJava观察者的onSuccess方法携带了成功的结果,但是由于ViewModel是与View无关的,它没有或不使用任何View实例来传递结果以供显示。相反,它通过调用resultListObservable.onNext(fetchItemTextFrom(t))在resultListObservable中触发一个事件,该事件被View观察到:
mMainViewModel.resultListObservable.subscribe({
hideProgressBar()
updateMovieList(it)
})
因此,可观察对象在视图和视图模型之间扮演中介者的角色:
- 视图模型在其可观察对象中触发事件
- 视图通过订阅视图模型的可观察对象来更新UI
这是视图的完整代码。在此示例中,视图是Activity类,但Fragment也可以同样使用:
class MainActivity : AppCompatActivity() {
private lateinit var mMainViewModel: MainViewModel
private lateinit var addressAdapter: AddressAdapter
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
mMainViewModel = MainViewModel(MainModel())
loadView()
respondToClicks()
listenToObservables()
}
private fun listenToObservables() {
mMainViewModel.itemObservable.subscribe(Consumer { goToDetailActivity(it) })
mMainViewModel.resultListObservable.subscribe(Consumer {
hideProgressBar()
updateMovieList(it)
})
mMainViewModel.resultListErrorObservable.subscribe(Consumer {
hideProgressBar()
showErrorMessage(it.message())
})
}
private fun loadView() {
setContentView(R.layout.activity_main)
addressAdapter = AddressAdapter()
main_activity_recyclerView.adapter = addressAdapter
}
private fun respondToClicks() {
main_activity_button.setOnClickListener({
showProgressBar()
mMainViewModel.findAddress(main_activity_editText.text.toString())
})
addressAdapter setItemClickMethod {
mMainViewModel.doOnItemClick(it)
}
}
fun showProgressBar() {
main_activity_progress_bar.visibility = View.VISIBLE
}
fun hideProgressBar() {
main_activity_progress_bar.visibility = View.GONE
}
fun showErrorMessage(errorMsg: String) {
Toast.makeText(this, "Error retrieving data: $errorMsg", Toast.LENGTH_SHORT).show()
}
override fun onStop() {
super.onStop()
mMainViewModel.cancelNetworkConnections()
}
fun updateMovieList(t: List<String>) {
addressAdapter.updateList(t)
addressAdapter.notifyDataSetChanged()
}
fun goToDetailActivity(item: MainModel.ResultEntity) {
var bundle = Bundle()
bundle.putString(DetailActivity.Constants.RATING, item.rating)
bundle.putString(DetailActivity.Constants.TITLE, item.title)
bundle.putString(DetailActivity.Constants.YEAR, item.year)
bundle.putString(DetailActivity.Constants.DATE, item.date)
var intent = Intent(this, DetailActivity::class.java)
intent.putExtras(bundle)
startActivity(intent)
}
class AddressAdapter : RecyclerView.Adapter<AddressAdapter.Holder>() {
var mList: List<String> = arrayListOf()
private lateinit var mOnClick: (position: Int) -> Unit
override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): Holder {
val view = LayoutInflater.from(parent!!.context).inflate(R.layout.item, parent, false)
return Holder(view)
}
override fun onBindViewHolder(holder: Holder, position: Int) {
holder.itemView.item_textView.text = mList[position]
holder.itemView.setOnClickListener { mOnClick(position) }
}
override fun getItemCount(): Int {
return mList.size
}
infix fun setItemClickMethod(onClick: (position: Int) -> Unit) {
this.mOnClick = onClick
}
fun updateList(list: List<String>) {
mList = list
}
class Holder(itemView: View?) : RecyclerView.ViewHolder(itemView)
}
}
这是ViewModel:
class MainViewModel() {
lateinit var resultListObservable: PublishSubject<List<String>>
lateinit var resultListErrorObservable: PublishSubject<HttpException>
lateinit var itemObservable: PublishSubject<MainModel.ResultEntity>
private lateinit var entityList: List<MainModel.ResultEntity>
private val compositeDisposable: CompositeDisposable = CompositeDisposable()
private lateinit var mainModel: MainModel
private val schedulersWrapper = SchedulersWrapper()
constructor(mMainModel: MainModel) : this() {
mainModel = mMainModel
resultListObservable = PublishSubject.create()
resultListErrorObservable = PublishSubject.create()
itemObservable = PublishSubject.create()
}
fun findAddress(address: String) {
val disposable: Disposable = mainModel.fetchAddress(address)!!.subscribeOn(schedulersWrapper.io()).observeOn(schedulersWrapper.main()).subscribeWith(object : DisposableSingleObserver<List<MainModel.ResultEntity>?>() {
override fun onSuccess(t: List<MainModel.ResultEntity>) {
entityList = t
resultListObservable.onNext(fetchItemTextFrom(t))
}
override fun onError(e: Throwable) {
resultListErrorObservable.onNext(e as HttpException)
}
})
compositeDisposable.add(disposable)
}
fun cancelNetworkConnections() {
compositeDisposable.clear()
}
private fun fetchItemTextFrom(it: List<MainModel.ResultEntity>): ArrayList<String> {
val li = arrayListOf<String>()
for (resultEntity in it) {
li.add("${resultEntity.year}: ${resultEntity.title}")
}
return li
}
fun doOnItemClick(position: Int) {
itemObservable.onNext(entityList[position])
}
}
最后是模型:
class MainModel {
private var mRetrofit: Retrofit? = null
fun fetchAddress(address: String): Single<List<MainModel.ResultEntity>>? {
return getRetrofit()?.create(MainModel.AddressService::class.java)?.fetchLocationFromServer(address)
}
private fun getRetrofit(): Retrofit? {
if (mRetrofit == null) {
val loggingInterceptor = HttpLoggingInterceptor()
loggingInterceptor.level = HttpLoggingInterceptor.Level.BODY
val client = OkHttpClient.Builder().addInterceptor(loggingInterceptor).build()
mRetrofit = Retrofit.Builder().baseUrl("http://bechdeltest.com/api/v1/").addConverterFactory(GsonConverterFactory.create()).addCallAdapterFactory(RxJava2CallAdapterFactory.create()).client(client).build()
}
return mRetrofit
}
class ResultEntity(val title: String, val rating: String, val date: String, val year: String)
interface AddressService {
@GET("getMoviesByTitle")
fun fetchLocationFromServer(@Query("title") title: String): Single<List<ResultEntity>>
}
}
完整文章在这里