3. 使用 FusedLocationProviderClient 获取最后已知位置

为什么 getLastLocation 是“先问缓存，再决定要不要动”

FusedLocationProviderClient 的 getLastLocation() 返回的是系统位置缓存中最近一次被任意应用写入的位置，它不触发新的定位硬件工作，因此通常毫秒级返回，但可能很旧、精度很差，甚至为 null。正确用法不是“拿到就用”，而是先判断时效性与精度，再决定是否发起新的定位请求。

时效性判断的三个维度

• 是否为空：返回 null 表示缓存中没有任何可用位置，必须发起新请求。
• 时间戳 age：System.currentTimeMillis() - location.getTime()，超过阈值（如 2 分钟）视为过期。
• 精度 accuracy：location.getAccuracy() 单位为米，数值越大越差，超过阈值（如 100 m）视为不可用。

完整实现：缓存优先 + 过期回退

class LastKnownLocationActivity : AppCompatActivity() {

    private lateinit var fusedClient: FusedLocationProviderClient
    private val maxAgeMs = 2 * 60 * 1000L   // 缓存最大可接受年龄：2 分钟
    private val maxAccuracyM = 100f          // 缓存最大可接受精度：100 米

    private val locationPermissionRequest =
        registerForActivityResult(ActivityResultContracts.RequestMultiplePermissions()) { grants ->
            if (grants[Manifest.permission.ACCESS_FINE_LOCATION] == true ||
                grants[Manifest.permission.ACCESS_COARSE_LOCATION] == true) {
                readLastKnownLocation()
            } else {
                showMessage("未授予定位权限，无法读取缓存位置")
            }
        }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        fusedClient = LocationServices.getFusedLocationProviderClient(this)
        requestPermissionThenRead()
    }

    private fun requestPermissionThenRead() {
        val fine = ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION)
        val coarse = ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION)
        if (fine == PackageManager.PERMISSION_GRANTED || coarse == PackageManager.PERMISSION_GRANTED) {
            readLastKnownLocation()
        } else {
            locationPermissionRequest.launch(arrayOf(
                Manifest.permission.ACCESS_FINE_LOCATION,
                Manifest.permission.ACCESS_COARSE_LOCATION))
        }
    }

    @SuppressLint("MissingPermission")
    private fun readLastKnownLocation() {
        fusedClient.lastLocation
            .addOnSuccessListener { location -> handleLastLocation(location) }
            .addOnFailureListener { e -> showMessage("读取缓存位置失败：${e.message}") }
    }

    private fun handleLastLocation(location: Location?) {
        if (location == null) {
            showMessage("缓存为空，需要发起新定位请求")
            requestFreshLocation()
            return
        }
        val ageMs = System.currentTimeMillis() - location.time
        val accuracy = location.accuracy
        val fresh = ageMs <= maxAgeMs && accuracy <= maxAccuracyM

        renderLocation(location, ageMs, accuracy, fresh)

        if (!fresh) {
            showMessage("缓存已过期或精度不足，后台刷新中…")
            requestFreshLocation()
        }
    }

    @SuppressLint("MissingPermission")
    private fun requestFreshLocation() {
        val request = LocationRequest.Builder(Priority.PRIORITY_HIGH_ACCURACY, 1000L)
            .setMaxUpdates(1)
            .build()
        fusedClient.requestLocationUpdates(request, object : LocationCallback() {
            override fun onLocationResult(result: LocationResult) {
                result.lastLocation?.let { fresh ->
                    renderLocation(fresh, 0L, fresh.accuracy, true)
                }
                fusedClient.removeLocationUpdates(this)
            }
        }, Looper.getMainLooper())
    }

    private fun renderLocation(loc: Location, ageMs: Long, accuracy: Float, fresh: Boolean) {
        val tag = if (fresh) "可用" else "过期"
        showMessage("[$tag] lat=${loc.latitude}, lon=${loc.longitude}, " +
                "age=${ageMs / 1000}s, acc=${accuracy}m")
    }

    private fun showMessage(msg: String) {
        Toast.makeText(this, msg, Toast.LENGTH_SHORT).show()
    }
}

判断逻辑可视化

关键参数与判读

验证方法

1. 首次安装后立即调用：预期返回 null，触发新请求路径。
2. 打开地图应用定位一次后回到本页：预期返回非 null，age 较小，直接使用缓存。
3. 关闭定位服务后调用：预期返回 null 或旧位置，观察是否走新请求分支并收到失败回调。
4. 用 Toast 输出 age 与 accuracy，确认阈值判断与预期一致。