無法從函式內部回傳變數

所以我試圖從函式回傳一個值以在 mainActivity 類中使??用，但是當我嘗試回傳變數時出現錯誤，指出“必須初始化變數”，即使我已經給它一個值。對此有什么想法嗎？

private fun getCLocation() : LocationClass {

        var loc: LocationClass

        if(checkPermission()){

            if(isLocationEnabled()){

                locationVariable.lastLocation.addOnCompleteListener(this) { task->

                    val location:Location? = task.result

                    if(location == null){
                        Toast.makeText(this,"NULL",Toast.LENGTH_LONG).show()
                    }
                    else{
                        loc = LocationClass(location.latitude.toString(),location.longitude.toString())
                    }
                }

            } else {
                // location not enabled,open settings
                val intent = Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS)
                startActivity(intent)
            }

        } else {
            //Permission Not Enabled
            requestPermission()
        }

        return loc
    }

錯誤是：必須初始化變數“loc”

uj5u.com熱心網友回復：

您的代碼的兩個問題：

1. 用戶可能還沒有授予權限，所以它會進入else塊，該函式請求權限，但不做任何事情來設定初始位置值。在這種情況下，該函式不可能回傳有效位置。
2. 獲取位置更新是異步的。即使已經授予權限，您的 OnCompleteListener 也不會被呼叫，直到該函式已經回傳之后的某個時間。您可以在此處閱讀有關什么是異步 API 的更多說明。

這是做什么的基本策略。獲取位置的函式采用回呼引數，而不是直接回傳 LocationClass，因為它是異步的。您需要一個可以在位置或權限可用后重試的更高級別的邏輯。

private var triedPromptingLocationSetting = false
private var locationWorkflowInitiated = false // SET THIS BACK TO FALSE IN onViewDestroyed() IF IN FRAGMENT

private fun fetchCLocation(onReceived: (LocationClass?)->Unit) {
    if (!checkPermission() || isLocationEnabled()) {
        Log.e("Do not call getCLocation() before permission is granted and location is turned on! Ignoring.")
        return
    }
    locationVariable.lastLocation.addOnCompleteListener(this) { task->
        val location:Location? = task.result
        if (location == null) {
            onReceived(null)
        }
        else {
            var loc = LocationClass(location.latitude.toString(),location.longitude.toString())
            onReceived(loc)
        }
    }
}

// This is where the logic is for whatever you wanted to do with this location.
// In this example, it's assumed it would be called in onResume()
private fun doMyLocationWorkflow() {
    if (!checkPermission()) {
        requestPermission()
        return
    }
    if (!isLocationEnabled()) {
        if (triedPromptingLocationSetting) {
            // can't infinitely loop back to settings. 
            showSomeUiRequestingUserToManuallyEnableLocation()
        } else {
            triedPromptingLocationSetting = true
            val intent = Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS)
            startActivity(intent)
        }
        return
    }

    locationWorkflowInitiated = true

    fetchCLocation { location ->
        if (location == null) {
            Toast.makeText(this,"NULL",Toast.LENGTH_LONG).show()
            //...
            return@fetchCLocation 
        }
        // Do something with the LocationClass
    }
}

override fun onResume() {
    super.onResume()
    if (!locationWorkflowInitiated) {
        doMyLocationWorkflow()
    }
}

private val locationPermissionRequest = registerForActivityResult(
        ActivityResultContracts.RequestMultiplePermissions()
    ) { permissions ->
        when {
            permissions.getOrDefault(Manifest.permission.ACCESS_FINE_LOCATION, false) -> {
                doMyLocationWorkflow() // go back to original workflow
            }
            permissions.getOrDefault(Manifest.permission.ACCESS_COARSE_LOCATION, false) -> {
                doMyLocationWorkflow() // go back to original workflow
            } else -> {
                showSomeUiTellingUserPermissionMustBeGranted()
            }
        }
    }

private fun requestPermission() {
    locationPermissionRequest.launch(arrayOf(
    Manifest.permission.ACCESS_FINE_LOCATION,
    Manifest.permission.ACCESS_COARSE_LOCATION))
}

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