View Model中的JSON值仅显示为[object Object]

这将是一个菜鸟问题 . 我正在开发我的第一个NativeScript应用程序，一个简单的GPS跟踪器 . 这只是一种观点 . 我得到了 Location 插件，它可以很好地将位置数据输出到控制台 . 它似乎也更新了View Model . 但是当我尝试将它绑定到 Label 元素的文本时，它只显示[object Object]，无论我做什么 .

The view model:

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

function viewModel() {

    return new observableModule.fromObject({
        "locations": [],
        "lastLocation": {
            "latitude": "0",
            "longitude": "0",
            "altitude": 0,
            "horizontalAccuracy": 0,
            "verticalAccuracy": 0,
            "speed": "0",
            "direction": "0",
            "timestamp": new Date().toISOString()
        },
        "lastUpload": "Unknown"
    });

}


module.exports = viewModel;

The view XML: （只是重要部分）

<Label text="Current location:"/>
<Label text="{{ lastLocation?.latitude }} ; {{ lastLocation?.longitude }}" />

<Label text="Last location update:"/>
<Label text="{{ lastLocation?.timestamp }}" />

<Label text="Last successful upload:"/>
<Label text="{{ lastUpload }}" />

The code:

var frameModule = require("ui/frame");
var viewModel = require("./home-view-model");
var geolocation = require("nativescript-geolocation");
var dialog = require("ui/dialogs");

var viewModel = new viewModel();

function onLoaded(args) {

    page = args.object;
    page.bindingContext = viewModel;

    var enableLocation = new Promise((resolve, reject) => {
        geolocation.enableLocationRequest(true)
            .then(
            success => resolve(),
            error => reject(error)
            )
    })

        .then(
        () => {
            watchId = geolocation.watchLocation(
                function (loc) {
                    if (loc) {
                        viewModel.lastLocation = loc;
                        console.log(viewModel.lastLocation.latitude + ' ; ' + viewModel.lastLocation.longitude);
                    }
                },
                function (e) {
                    dialogsModule.alert('Location error: ' + e.message);
                },
                {
                    desiredAccuracy: 3,
                    updateDistance: 0,
                    minimumUpdateTime: 10000
                })
        },

        error => {
            dialogsModule.alert('Location error: ' + e.message);
        }
        )

}

exports.onLoaded = onLoaded;

console.log 行正确显示纬度和经度值 . 视图显示[object Object]，但它显示 lastUpload 值 . 我在视图中做错了什么？