如何取消正在运行的LiveData协程块 [英] How to cancel a running LiveData Coroutine Block

问题描述

通过使用LiveData的最新版本"androidx.lifecycle:lifecycle-livedata-ktx:2.2.0-alpha03"，我使用LiveData的新构件(LiveData +协程(Coroutine)，可使用Retrofit执行同步网络调用，并相应地更新ViewModel中的不同标志(isLoading，isError).我在查询" LiveData上使用Transforamtions.switchMap，因此无论何时从UI中对查询"进行更改，搜索产品"代码都将使用Transformations.switchMap开始执行.一切正常，除了查询" LiveData中发生更改时我想取消以前的改装呼叫".目前，我看不到任何方法可以做到这一点.任何帮助，将不胜感激.

By using LiveData's latest version "androidx.lifecycle:lifecycle-livedata-ktx:2.2.0-alpha03", I have developed a code for a feature called "Search Products" in the ViewModel using LiveData's new building block (LiveData + Coroutine) that performs a synchronous network call using Retrofit and update different flags (isLoading, isError) in ViewModel accordingly. I am using Transforamtions.switchMap on "query" LiveData so whenever there is a change in "query" from the UI, the "Search Products" code starts its executing using Transformations.switchMap. Every thing is working fine, except that i want to cancel the previous Retrofit Call whenever a change happens in "query" LiveData. Currently i can't see any way to do this. Any help would be appreciated.

class ProductSearchViewModel : ViewModel() {
    val completableJob = Job()
    private val coroutineScope = CoroutineScope(Dispatchers.IO + completableJob)

    // Query Observable Field
    val query: MutableLiveData<String> = MutableLiveData()

    // IsLoading Observable Field
    private val _isLoading = MutableLiveData<Boolean>()
    val isLoading: LiveData<Boolean> = _isLoading


    val products: LiveData<List<ProductModel>> = query.switchMap { q ->
        liveData(context = coroutineScope.coroutineContext) {
            emit(emptyList())
            _isLoading.postValue(true)
            val service = MyApplication.getRetrofitService()
            val response = service?.searchProducts(q)
            if (response != null && response.isSuccessful && response.body() != null) {
                _isLoading.postValue(false)
                val body = response.body()
                if (body != null && body.results != null) {
                    emit(body.results)
                }
            } else {
                _isLoading.postValue(false)
            }
        }
    }
}

推荐答案

您可以通过两种方式解决此问题:

You can solve this problem in two ways:

方法1(简易方法)

就像梅尔(Mel)在其答案中所解释的那样，您可以将引用保持在switchMap外部的作业实例并取消立即在switchMap中返回新的liveData之前立即执行该作业.

Just like Mel has explained in his answer, you can keep a referece to the job instance outside of switchMap and cancel instantance of that job right before returning your new liveData in switchMap.

class ProductSearchViewModel : ViewModel() {

    // Job instance
    private var job = Job()

    val products = Transformations.switchMap(_query) {
        job.cancel() // Cancel this job instance before returning liveData for new query
        job = Job() // Create new one and assign to that same variable

        // Pass that instance to CoroutineScope so that it can be cancelled for next query
        liveData(CoroutineScope(job + Dispatchers.IO).coroutineContext) { 
            // Your code here
        }
    }

    override fun onCleared() {
        super.onCleared()
        job.cancel()
    }
}

方法2(不是很干净，但是可以独立使用并且可重复使用)

由于 liveData {} 构建器块在协程范围内运行，因此您可以结合使用 CompletableDeffered 和协程 launch 构建器来暂停liveData阻止并手动观察 query liveData以启动网络请求作业.

Since liveData {} builder block runs inside a coroutine scope, you can use a combination of CompletableDeffered and coroutine launch builder to suspend that liveData block and observe query liveData manually to launch jobs for network requests.

class ProductSearchViewModel : ViewModel() {

    private val _query = MutableLiveData<String>()

    val products: LiveData<List<String>> = liveData {
        var job: Job? = null // Job instance to keep reference of last job

        // LiveData observer for query
        val queryObserver = Observer<String> {
            job?.cancel() // Cancel job before launching new coroutine
            job = GlobalScope.launch {
                // Your code here
            }
        }

        // Observe query liveData here manually
        _query.observeForever(queryObserver)

        try {
            // Create CompletableDeffered instance and call await.
            // Calling await will suspend this current block 
            // from executing anything further from here
            CompletableDeferred<Unit>().await()
        } finally {
            // Since we have called await on CompletableDeffered above, 
            // this will cause an Exception on this liveData when onDestory
            // event is called on a lifeCycle . By wrapping it in 
            // try/finally we can use this to know when that will happen and 
            // cleanup to avoid any leaks.
            job?.cancel()
            _query.removeObserver(queryObserver)
        }
    }
}

您可以在演示项目

更新了方法1，以在yasir的注释中指出的onCleared方法上添加作业取消.

Updated Method # 1 to add job cancellation on onCleared method as pointed out by yasir in comments.

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