使用Fuel发送请求时，ProgressBar为何在inline协程函数后才显示？

在平板上使用Fuel发送HTTP请求时，希望在请求相关函数开始执行前显示ProgressBar，但它总是在函数执行完成后才出现。单独测试ProgressBar的显示/隐藏功能完全正常。

ProgressBar初始化代码 #

protected fun initProgressBar(constraintLayoutID: Int): ProgressBar {
    val originalConstraintLayout = findViewById<ConstraintLayout>(constraintLayoutID)
    progressBar = ProgressBar(this).apply {
        id = getInteger(R.integer.INT_PROGRESS_BAR_ID)
        elevation = 999F
        isIndeterminate = true
        layoutParams = ViewGroup.LayoutParams(300, 300)
        visibility = View.GONE

        originalConstraintLayout.addView(this)
    }

    ConstraintSet().run {
        clone(originalConstraintLayout)

        for(position in listOf(ConstraintSet.TOP, ConstraintSet.START, ConstraintSet.END, ConstraintSet.BOTTOM))
            connect(progressBar!!.id, position, constraintLayoutID, position,0)
        applyTo(originalConstraintLayout)
    }
    return progressBar!!
}

原请求函数（inline函数） #

原本期望在发送请求前显示ProgressBar的inline函数：

inline fun <reified T : DefaultResponse> sendRequest(
    routeName: String = "",
    routeArgs: String = ""
): T = runBlocking {
    owner.runOnUiThread {
        owner.showProgressBar() // 应该在这里显示，而不是请求完成后
    }

    val adaptedResponse = ResponseFactory.getInstance(owner).getResponseObjectFor(T::class.java.name, routeName)
    val (request, response, result) = Fuel.get(
        "%s%s?%s%s".format(
            networkManager.getURLConnection(),
            (adaptedResponse as DefaultResponse).getRouteName(),
            routeArgs,
            getIndustrialTrypticArgs()
        )
    ).awaitObjectResponseResult(adaptedResponse)

    val exception =
        FailedRequestException(owner.getString(R.string.STR_EXCEPTION_RM_FAILED_REQUEST)).apply {
            setHTTPMessage(response.responseMessage)
            setStatusCode(response.statusCode)
            setUsedRequest(request.url)
        }

    return@runBlocking result.fold<T>(
        { responseObject ->
            if (response.statusCode != 200)
                throw exception
            return@fold responseObject as T
        },
        { error ->
            throw exception.setOriginalError(error.exception)
        }
    )
}

已尝试的解决方式 #

• 使用runOnUiThread调用ProgressBar显示逻辑
• 将显示逻辑封装在launch开启的不同协程线程中

以上方法均无效，ProgressBar仍在inline函数执行结束后才出现。目前疑问：是因为inline函数的特性导致，还是存在并发线程调度问题？

更新1：改为suspend函数 #

按照建议将sendRequest改为suspend函数：

suspend inline fun <reified T : DefaultResponse> sendRequest(
    routeName: String = "",
    routeArgs: String = "",
    isBlocking: Boolean = true
): T? {
    val adaptedResponse = ResponseFactory.getInstance(owner).getResponseObjectFor(T::class.java.name, routeName) // 1
    val fuelRequest = Fuel.get( // 2
        "%s%s?%s%s".format(
            networkManager.getURLConnection(),
            (adaptedResponse as DefaultResponse).getRouteName(),
            routeArgs,
            getIndustrialTrypticArgs()
        )
    )
    var fuelResponse: T? = null // 3

    CoroutineScope(Dispatchers.Main).launch {
        owner.showProgressBar() // 4
    }
    try {
        fuelResponse =
            if(isBlocking) {
                val answer = fuelRequest.awaitObjectResponseResult(adaptedResponse) // 5

                treatsResponse(answer) as T // 6
            }
            else {
                fuelRequest.responseObject(adaptedResponse) { httpRequest, httpResponse, httpResult ->
                    try {
                        treatsResponse(httpRequest, httpResponse, httpResult) as T
                    }
                    catch (e: FailedRequestException) {
                        println(e.getOriginalError())
                        println(e.getUsedRequest())
                    }
                    finally {
                        owner.hideProgressBar()
                    }
                }
                null
            }
    }
    catch (e: FailedRequestException) {
        println(e.getOriginalError())
        println(e.getUsedRequest())
    }
    finally {
        owner.hideProgressBar()
    }
    return fuelResponse
}

sendRequest的调用被封装在CoroutineScope(Dispatchers.Main).launch中。情况有所改善：ProgressBar开始在函数内部出现而非调用后，但仍希望它在fuelRequest.awaitObjectResponseResult(adaptedResponse)（第5行）之前显示。awaitObjectResponseResult本身也是suspend函数。

更新2：执行顺序问题 #

逐行调试发现：当owner.showProgressBar()被封装在CoroutineScope(Dispatchers.Main).launch（第4行）中时，它明显在第5行指令之后执行，实际执行顺序始终是1->2->3->5->4->6。当将Dispatchers.Main作为awaitObjectResponseResult的第二个参数传入时，执行顺序变为1->2->3->5->6->4，该网络请求函数似乎始终拥有执行优先级。

内容的提问来源于stack exchange，提问作者Omiganox