提问人：Anurag Hale 提问时间：8/28/2022 更新时间：8/29/2022 访问量：52
无法从函数内部返回变量
Not able to return variable from inside a function
问：
因此，我尝试从函数返回一个值以在 mainActivity 类中使用，但是当我尝试返回变量时出现错误，指出“变量必须初始化”，即使我已经为它指定了一个值。对此有什么想法吗？
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
}
错误是：变量“loc”必须初始化
答：
0赞
Tenfour04
8/29/2022
#1
代码的两个问题：
- 用户可能尚未授予权限，因此它将进入块，函数在其中请求权限，但不执行任何操作来设置初始位置值。在这种情况下，该函数不可能返回有效位置。
else - 获取位置更新是异步的。即使已授予权限，也要等到将来某个时间，在此函数已返回之后，才会调用 OnCompleteListener。您可以在此处阅读有关异步 API 的更多说明。
这是该做什么的基本策略。获取位置的函数采用回调参数，而不是直接返回 LocationClass，因为它是异步的。您需要一个更高级别的逻辑，可以在位置或权限可用后重试该逻辑。
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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