angular - Ionic 3 值绑定(bind)地理定位

Question

ERROR TypeError: Cannot set property 'items' of null

这是结果:

</ion-card>
<button ion-button round outline 
(click)="logEvent($startTrackingButton)">Start tracking me</button>
<ion-card>
<ion-item>
<h2>Locations:</h2>
<ion-list no-lines>
<h3 ion-item *ngFor="let item of items" (click)="itemSelected(item)">
{{ item }}
</h3>
</ion-list>
</ion-item>
</ion-card>
</ion-content>

(在 Component.html 中)，现在是它的 .ts 文件:

logEvent1(TrackNowButton) {

    /*Initializing geolocation*/

    // onSuccess Callback
    // This method accepts a Position object, 
    // which contains the
    // current GPS coordinates
    var onSuccess = function(position) {
        this.items = ([{
            key: "1", value: {
                Latitude: position.coords.latitude,
                Longitude: position.coords.longitude
            }
        }, {
            key: "2", value: {
                Altitude: position.coords.altitude,
                Accuracy: position.coords.accuracy
            }
        }, {
            key: "3", value: {
                Altitude_Accuracy: position.coords.altitudeAccuracy,
                Heading: position.coords.heading
            }
        }, {
            key: "4", value: {
                Speed: position.coords.speed,
                Timestamp: position.timestamp
            }
        }]);
    };

    // onError Callback receives a PositionError 
    // object
    function onError(error) {
        console.log('code: ' + error.code + '\n' + 'message: ' + error.message + '\n');
    }
    navigator.geolocation.getCurrentPosition(onSuccess, onError);
}

itemSelected(item: string) {
    console.log("Selected Item", item);
}

我希望这是可读的...我不知道我在数据绑定(bind)方面做错了什么，因为那是我认为错误的地方...我还认为 .ts 文件中的部分有问题，我在其中写道:this.items= (...) 因为我认为该值不能绑定(bind)到 html 或其他东西。我正在开发一个小程序，它经常跟踪运动并使用给定参数记录它们。稍后我想添加 Mapbox 支持，但我需要先解决一些错误。

Answer 1

您收到的错误表明 this 为空。用箭头函数替换分配给 onSuccess 的函数。它将为 this 使用正确的值。

var onSuccess = (position: any) => {
    this.items = ([{
        key: "1", value: {
            Latitude: position.coords.latitude,
            Longitude: position.coords.longitude
        }
    }, {
        key: "2", value: {
            Altitude: position.coords.altitude,
            Accuracy: position.coords.accuracy
        }
    }, {
        key: "3", value: {
            Altitude_Accuracy: position.coords.altitudeAccuracy,
            Heading: position.coords.heading
        }
    }, {
        key: "4", value: {
            Speed: position.coords.speed,
            Timestamp: position.timestamp
        }
    }]);
};

您可以使用 JSON.stringify 将所选项目转换为字符串:

itemSelected(item: any) {
    console.dir(item);
    let str = JSON.stringify(item);
    ...
}

---

另一方面，如果每一项都需要定义为字符串，则可以为数组中的每一项调用JSON.stringify。

var onSuccess = (position: any) => {
    this.items = ([JSON.stringify({
        key: "1", value: {
            Latitude: position.coords.latitude,
            Longitude: position.coords.longitude
        }
    }), JSON.stringify({
        key: "2", value: {
            Altitude: position.coords.altitude,
            Accuracy: position.coords.accuracy
        }
    }), JSON.stringify({
        key: "3", value: {
            Altitude_Accuracy: position.coords.altitudeAccuracy,
            Heading: position.coords.heading
        }
    }), JSON.stringify({
        key: "4", value: {
            Speed: position.coords.speed,
            Timestamp: position.timestamp
        }
    })]);
}