NativeScript

Question

我正在编写一个使用 nativescript-geolocation API 的简单应用程序。函数 getCurrentLocation 基本上工作正常，但是当我移动到另一个名为 maps-module.js 的文件并从文件 detail.js 的主线程调用它时，它的对象位置 return 为 NULL。打印控制台对象后，我意识到变量 returned_location 在函数完成查找位置之前被 returned。我认为这是多线程问题，但我真的不知道如何解决。这是我的文件。

detail.js

var Frame = require("ui/frame");
var Observable = require("data/observable");

var MapsModel = require("../../view-models/maps-model");


var defaultMapInfo = new MapsModel({
    latitude: "10.7743332",
    longitude: "106.6345204",
    zoom: "0",
    bearing: "0",
    tilt: "0",
    padding: "0"
});

var page;
var mapView;

exports.pageLoaded = function(args) {
    page = args.object;
    var data = page.navigationContext;
    page.bindingContext = defaultMapInfo;
}

exports.onBackTap = function () {
    console.log("Back to home");
    var topmost = Frame.topmost();
    topmost.goBack();
}

function onMapReady(args) {
    mapView = args.object;
    mapView.settings.zoomGesturesEnabled = true;
}

function onMarkerSelect(args) {
    console.log("Clicked on " + args.marker.title);
}

function onCameraChanged(args) {
    console.log("Camera changed: " + JSON.stringify(args.camera)); 
}

function getCurPos(args) {
    var returned_location = defaultMapInfo.getCurrentPosition(); // variable is returned before function finished
    console.dir(returned_location);
}


exports.onMapReady = onMapReady;
exports.onMarkerSelect = onMarkerSelect;
exports.onCameraChanged = onCameraChanged;
exports.getCurPos = getCurPos;

地图-module.js

var Observable = require("data/observable");

var Geolocation = require("nativescript-geolocation");
var Gmap = require("nativescript-google-maps-sdk");

function Map(info) {
    info = info || {};
    var _currentPosition;

    var viewModel = new Observable.fromObject({
        latitude: info.latitude || "",
        longitude: info.longitude || "",
        zoom: info.zoom || "",
        bearing: info.bearing || "",
        tilt: info.bearing || "",
        padding: info.padding || "",
    });

    viewModel.getCurrentPosition = function() {
        if (!Geolocation.isEnabled()) {
            Geolocation.enableLocationRequest();
        }

        if (Geolocation.isEnabled()) {
            var location = Geolocation.getCurrentLocation({
                desiredAccuracy: 3, 
                updateDistance: 10, 
                maximumAge: 20000, 
                timeout: 20000
            })
            .then(function(loc) {
                if (loc) {
                    console.log("Current location is: " + loc["latitude"] + ", " + loc["longitude"]);
                    return Gmap.Position.positionFromLatLng(loc["latitude"], loc["longitude"]);
                }
            }, function(e){
                console.log("Error: " + e.message);
            });

            if (location)
                console.dir(location);
        }
    }

    return viewModel;
}

module.exports = Map;

Answer 1

由于获取位置是一个异步过程，因此您的 viewModel.getCurrentPosition 应该 return 一个承诺，并且看起来像这样，

viewModel.getCurrentPosition() {
    return new Promise((resolve, reject) => {
        geolocation
            .getCurrentLocation({
                desiredAccuracy: enums.Accuracy.high,
                updateDistance: 0.1,
                maximumAge: 5000,
                timeout: 20000
            })
            .then(r => {
                resolve(r);
            })
            .catch(e => {
                reject(e);
            });
    });
}

然后当你使用它时，它看起来像这样

defaultMapInfo.getCurrentPosition()
    .then(latlng => {
       // do something with latlng {latitude: 12.34, longitude: 56.78}
    }.catch(error => {
       // couldn't get location
    }
}

希望对您有所帮助 :)

更新：顺便说一句，geolocation.enableLocationRequest() 也是一种异步方法。

Answer 2

如果湿婆普拉萨德的脚注...

"geolocation.enableLocationRequest() is also an asynchorous method"

... 是正确的，那么由 geolocation.enableLocationRequest() 编辑的 Promise return 必须适当地处理并且代码将发生相当大的变化。

试试这个：

viewModel.getCurrentPosition = function(options) {
    var settings = Object.assign({
        'desiredAccuracy': 3,
        'updateDistance': 10,
        'maximumAge': 20000,
        'timeout': 20000
    }, options || {});

    var p = Promise.resolve() // Start promise chain with a resolved native Promise.
    .then(function() {
        if (!Geolocation.isEnabled()) {
            return Geolocation.enableLocationRequest(); // return a Promise
        } else {
            // No need to return anything here.
            // `undefined` will suffice at next step in the chain.
        }
    })
    .then(function() {
        if (Geolocation.isEnabled()) {
            return Geolocation.getCurrentLocation(settings); // return a Promise
        } else { // <<< necessary to handle case where Geolocation didn't enable.
            throw new Error('Geolocation could not be enabled');
        }
    })
    .then(function(loc) {
        if (loc) {
            console.log("Current location is: " + loc.latitude + ", " + loc.longitude);
            return Gmap.Position.positionFromLatLng(loc.latitude, loc.longitude);
        } else { // <<< necessary to handle case where loc was not derived.
            throw new Error('Geolocation enabled, but failed to derive current location');
        }
    })
    .catch(function(e) {
        console.error(e);
        throw e; // Rethrow the error otherwise it is considered caught and the promise chain will continue down its success path.
        // Alternatively, return a manually-coded default `loc` object.
    });

    // Now race `p` against a timeout in case enableLocationRequest() hangs.
    return Promise.race(p, new Promise(function(resolve, reject) {
        setTimeout(function() {
            reject(new Error('viewModel.getCurrentPosition() timed out'));
        }, settings.timeout);
    }));
}
return viewModel;

备注：

使用已解析的本机 Promise 启动链与包装在 new Promise(...) 中的效果大致相同，但更清晰，主要是因为链中的意外抛出保证会传递一个 Error 对象链的错误路径而不需要 try/catch/reject()。此外，在标记为 "return a Promise" 的两行中，我们不需要关心 return 是一个 Promise 还是一个值；两者都会被原生的 Promise 链同化。
包含两个 else 子句以应对不会自动抛出的失败情况。
Promise.race() 不是必需的，但它是针对报告的问题 here 的保障措施。内置的 'timeout' 机制可能就足够了。这种额外的超时机制是一项 "belt-and-braces" 措施。
包含一种机制，通过传递 options 对象来覆盖 viewModel.getCurrentPosition 中的硬编码默认值。要使用默认值运行，只需调用 viewModel.getCurrentPosition()。引入此功能主要是为了允许 settings.timeout 在 Promise.race().

编辑：

感谢@grantwparks 提供 Geolocation.isEnabled() 也 return 承诺的信息。

所以现在我们可以用 p = Geolocation.isEnabled().... 启动 Promise 链并测试异步传递的布尔值。如果 false 则尝试启用。

从那时起，如果地理定位最初已启用或已启用，则将遵循 Promise 链的成功路径。启用地理定位的进一步测试消失了。

这应该有效：

viewModel.getCurrentPosition = function(options) {
    var settings = Object.assign({
        'desiredAccuracy': 3,
        'updateDistance': 10,
        'maximumAge': 20000,
        'timeout': 20000
    }, options || {});

    var p = Geolocation.isEnabled() // returned Promise resolves to true|false.
    .then(function(isEnabled) {
        if (isEnabled) {
            // No need to return anything here.
            // `undefined` will suffice at next step in the chain.
        } else {
            return Geolocation.enableLocationRequest(); // returned Promise will cause main chain to follow success path if Geolocation.enableLocationRequest() was successful, or error path if it failed;
        }
    })
    .then(function() {
        return Geolocation.getCurrentLocation(settings); // return Promise
    })
    .then(function(loc) {
        if (loc) {
            console.log("Current location is: " + loc.latitude + ", " + loc.longitude);
            return Gmap.Position.positionFromLatLng(loc.latitude, loc.longitude);
        } else { // <<< necessary to handle case where loc was not derived.
            throw new Error('Geolocation enabled, but failed to derive current location');
        }
    })
    .catch(function(e) {
        console.error(e);
        throw e; // Rethrow the error otherwise it is considered caught and the promise chain will continue down its success path.
        // Alternatively, return a manually-coded default `loc` object.
    });

    // Now race `p` against a timeout in case Geolocation.isEnabled() or Geolocation.enableLocationRequest() hangs.
    return Promise.race(p, new Promise(function(resolve, reject) {
        setTimeout(function() {
            reject(new Error('viewModel.getCurrentPosition() timed out'));
        }, settings.timeout);
    }));
}
return viewModel;

NativeScript - 地理定位：使用 getCurrentLocation promise 函数的正确方法

NativeScript - Geolocation: right way to use getCurrentLocation promise function

javascript

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promise