android - 在Activity/Fragment中，如何获取/等待ViewModel协程操作的返回值？

Question

根据 Google (link) 的 Codelab 演示，我尝试将代码重构为 ViewModel + 协程。我的问题是，我不想只插入数据( original code )，而是想等待插入操作的结果，如果插入成功，则应返回 id，然后根据结果执行某些操作。那么该怎么做呢？

目前，我将一个方法作为回调发送到 ViewModel 插入方法。当然，观察 ViewModel 是另一种选择。但还有更好的解决办法吗？

我当前的代码:

事件 Activity :

viewModel.insert(Event("name"), {
    if (it == -1L) {
        Log.i("insert", "failure")
    } else {
        Log.i("insert", "success: $it")
    }
})

事件 View 模型:

private val mEventDao: EventDao = AppDatabase.getDatabase(application).eventDao()
private val mJob = Job()
private val mScope = CoroutineScope(Dispatchers.Main + mJob)

fun insert(event: Event, callback: (id: Long) -> Unit) {
    mScope.launch(Dispatchers.IO) {
        val result =
            try {
                // just for testing delay situation
                delay(5000)
                val id = mEventDao.insertEvent(event)
                id
            } catch (e: Exception) {
                -1L
            }
        withContext(Dispatchers.Main) {
            callback(result)
        }
    }
}

EventDao:

@Dao
interface EventDao {
    fun insertEvent(event: Event): Long
}

Answer 1

您可以将 LiveData 对象添加到 EventViewModel，插入完成后更新它并在 Activity 中订阅它:

class EventViewModel : ViewModel() {
    //...
    var insertionId = MutableLiveData<Long>()

    fun insert(event: String) {
        mScope.launch(Dispatchers.IO) {
            val result =
                    try {
                        // just for testing delay situation
                        delay(5000)
                        val id = mEventDao.insertEvent(event)
                        id
                    } catch (e: Exception) {
                        -1L
                    }

            insertionId.postValue(result)
        }
    }
}

并在EventActivity中订阅:

class EventActivity : AppCompatActivity() {

    lateinit var viewModel: EventViewModel

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

        viewModel = ViewModelProviders.of(this).get(EventViewModel::class.java)
        viewModel.insertionId.observe(this, android.arch.lifecycle.Observer { id ->
            // Use `id` for example to update UI. 
        })

        // ...

        viewModel.insert(Event("name"))
    }
}