Kotlin中getCurrentLocation函数返回初始值问题求助

问题核心分析 #

你遇到的问题本质是异步操作的执行顺序问题：

• fusedLocationProviderClient.lastLocation.addOnCompleteListener是异步任务，它的回调代码会在后台获取到位置信息后才执行。
• 但你的getCurrentLocation函数是同步执行的——在回调还没来得及更新DBLatitude和DBLongitude之前，函数就已经执行到return语句，返回了初始的0.0值。
• 而UI控件能正常显示，是因为回调执行时确实更新了binding.tvLatitude和binding.tvLongitude，这时候函数的返回值已经无关紧要了。

解决方案1：使用Kotlin协程（推荐） #

将函数改为挂起函数，利用Tasks.await()把异步任务转为同步等待，确保获取到位置后再返回值：

private suspend fun getCurrentLocation(): String {
    var DBLatitude: Double = 0.0
    var DBLongitude: Double = 0.0

    if (checkPermissions()) {
        if (isLocationEnabled()) {
            if (ActivityCompat.checkSelfPermission(
                    this,
                    Manifest.permission.ACCESS_FINE_LOCATION
                ) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(
                    this,
                    Manifest.permission.ACCESS_COARSE_LOCATION
                ) != PackageManager.PERMISSION_GRANTED
            ) {
                checkPermissions()
            }
            try {
                // 等待异步任务完成，获取位置结果
                val location = Tasks.await(fusedLocationProviderClient.lastLocation)
                if (location == null) {
                    Toast.makeText(this, "Null obtido", Toast.LENGTH_SHORT).show()
                } else {
                    binding.tvLatitude.text = "" + location.latitude
                    binding.tvLongitude.text = "" + location.longitude

                    DBLatitude = location.latitude
                    DBLongitude = location.longitude
                }
            } catch (e: Exception) {
                e.printStackTrace()
                Toast.makeText(this, "获取位置失败", Toast.LENGTH_SHORT).show()
            }
        } else {
            Toast.makeText(this, "Ligue a localização", Toast.LENGTH_SHORT).show()
            val intent = Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS)
            startActivity(intent)
        }
    } else {
        requestPermission()
    }
    return "" + DBLatitude + " " + DBLongitude
}

调用方式 #

需要在协程作用域中调用该函数，比如使用页面的生命周期协程：

lifecycleScope.launch {
    val locationStr = getCurrentLocation()
    // 此处拿到的locationStr就是正确的经纬度，可直接发送到数据库
}

解决方案2：使用回调接口 #

如果不想用协程，可以定义回调接口，在位置获取完成后再处理结果：

1. 定义回调接口 #

interface LocationCallback {
    fun onLocationReceived(locationStr: String)
}

2. 修改获取位置的函数 #

private fun getCurrentLocation(callback: LocationCallback) {
    var DBLatitude: Double = 0.0
    var DBLongitude: Double = 0.0

    if (checkPermissions()) {
        if (isLocationEnabled()) {
            if (ActivityCompat.checkSelfPermission(
                    this,
                    Manifest.permission.ACCESS_FINE_LOCATION
                ) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(
                    this,
                    Manifest.permission.ACCESS_COARSE_LOCATION
                ) != PackageManager.PERMISSION_GRANTED
            ) {
                checkPermissions()
            }
            fusedLocationProviderClient.lastLocation.addOnCompleteListener(this) { task ->
                val location = task.result

                if (location == null) {
                    Toast.makeText(this, "Null obtido", Toast.LENGTH_SHORT).show()
                    callback.onLocationReceived("$DBLatitude $DBLongitude")
                } else {
                    binding.tvLatitude.text = "" + location.latitude
                    binding.tvLongitude.text = "" + location.longitude

                    DBLatitude = location.latitude
                    DBLongitude = location.longitude
                    // 获取到位置后调用回调返回结果
                    callback.onLocationReceived("$DBLatitude $DBLongitude")
                }
            }
        } else {
            Toast.makeText(this, "Ligue a localização", Toast.LENGTH_SHORT).show()
            val intent = Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS)
            startActivity(intent)
            callback.onLocationReceived("$DBLatitude $DBLongitude")
        }
    } else {
        requestPermission()
        callback.onLocationReceived("$DBLatitude $DBLongitude")
    }
}

3. 调用方式 #

getCurrentLocation(object : LocationCallback {
    override fun onLocationReceived(locationStr: String) {
        // 此处拿到正确的经纬度字符串，执行发送到数据库的逻辑
    }
})

内容的提问来源于stack exchange，提问作者Bruno