如何正确处理位置服务切换

Question

在我的应用程序中，我有一个使用 FusedLocationProviderClient 获取用户当前位置的按钮。这可以正常工作，直到位置服务关闭然后再在设备上重新打开。发生这种情况时，FusedLocationProviderClient.getLastLocation() 总是 returns null。我应该如何处理这种情况？我需要以某种方式重新初始化 FusedLocationProviderClient 吗？

我从documentation that when location services are turned off, the last cached location is flushed. This tells me that I need to force a location request so the cache gets refreshed. However, I'm unsure of how to do this. I've also followed this Medium article了解到FusedLocationProviderClient的设置，但它所做的只是检查定位结果是否为空，当它为空时不做任何处理。

我正在用 Kotlin 编写应用程序。以下是我如何初始化 FusedLocationProviderClient 并在我的片段中获取用户的当前位置：

class LocationFragment : Fragment() {

    private lateinit var mFusedLocationProviderClient: FusedLocationProiderClient

    override fun onCreate(savedInstanceState: Bundle?) {
        mFusedLocationProviderClient = LocationServices.getFusedLocationProviderClient(activity)
    }

    override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
        // Initialize button on-click callback.
    }

    private fun getCurrentLocation() {
        // Check that the location service is enabled.
        // Check coarse and fine location permissions.

        mFusedLocationProviderClient.lastLocation.addOnSuccessListener {
            if (it != null) { // Always false if location services are toggled.
                print(it.latitude, it.longitude)
            }
        }
    }
}

Answer 1

我没有对 FusedLocationProviderClient 做很多事情，但据我了解（文档似乎也同意），使用 lastLocation（getLastLocation() for Java 人）不会'不是真的出去 "fetch" 当前设备位置，而只是 return 设备获取的最后一个位置。通常，设备上的任何数量的服务都在请求位置，因此缓存中始终有一个相当准确的值供 lastLocation 使用。但是，如果您关闭并重新打开定位服务，然后立即调用 lastLocation，您很可能会收到 null returned。另请注意 lastLocation 状态的文档：

It is particularly well suited for applications that do not require an accurate location

并且来自 getLocationAvailability() 调用的文档：

Note it's always possible for getLastLocation() to return null even when this method returns true (e.g. location settings were disabled between calls).

这些也证实了该方法实际上并不进行位置查找，而只是 return 从缓存中进行最近的查找，当您切换服务时 off/on，缓存是空的。

如果您需要确保获取设备的当前位置并确保其相当准确，您可能需要使用 requestLocationUpdates() 让位置客户端实际计算出设备的当前位置和 return 它在你提供的回调中，然后在你完成后删除回调。

自从我与位置提供商合作以来已经有一段时间了，但我认为你可以做这样的事情，假设你只需要一个位置：

    private val locationRequest: LocationRequest by lazy {
        LocationRequest.create().apply {
            interval = (LOCATION_UPDATE_INTERVAL_SECONDS * 1000).toLong()
            fastestInterval = (LOCATION_FAST_INTERVAL_SECONDS * 1000).toLong()
            priority = LocationRequest.PRIORITY_HIGH_ACCURACY
            smallestDisplacement =
                MINIMUM_DISPLACEMENT_METERS
        }
    }

    private fun isValidLocation(location: Location): Boolean {
        /* TODO: validate that the location meets all of your
                 requirements for accuracy, etc, and return the
                 appropriate true/false value
         */
        return true  // or false, for example if the location accuracy is not good enough.    
    }    


    private val locationCallback = object: LocationCallback() {
        override fun onLocationResult(locationResult: LocationResult) {

            Log.d(TAG, "Location Received: $locationResult")

            val location = locationResult.lastLocation

            if (isValidLocation(location)) {
                Log.d(TAG, "Valid Location dectected: $location")

                // we have a valid location, so stop receiving further location updates.
                mFusedLocationClient.removeLocationUpdates(this)

                //TODO: we have a valid location! Use it as you wish to update the
                //      view, or emit it via a LiveData, etc.
            } else {
                // nothing to do, wait for more results.
                Log.d(TAG, "Location received was not valid: $location")
            }

        }
    }    

    private fun watchLocation() {

        // Check Location Permissions
        // Check Google Play Services Version
        // Check Location Settings
        // If all three are good, then:

        mFusedLocationClient.requestLocationUpdates(locationRequest, locationCallback, Looper.myLooper())
    }

上面的代码片段设置了一个 LocationCallback，它在收到位置时被调用。它检查位置（通过 isValidLocation 方法），如果它有效，它会从提供者中删除自己，这样您就不会获得额外的位置更新。在它自己删除的那一行 (mFusedLocationClient.removeLocationUpdates(this)) 之后，您可以对 location 对象做任何您需要做的工作。如果收到的位置无效，它会在收到位置时继续收到额外的回调，直到您收到一个有效的回调。要开始这一切，只需调用 watchLocation()

如何正确处理位置服务切换

How to properly handle location service toggling

android

android-location

kotlin

android-fusedlocation