爲何console.log(mainSvr.getLastKnownLocation(lm.GPS_PROVIDER)) 是null?

爲何console.log(mainSvr.getLastKnownLocation(lm.GPS_PROVIDER)) 是null?

ar context = plus.android.importClass("android.content.Context");
var lm=plus.android.importClass("android.location.LocationManager");
var main=plus.android.runtimeMainActivity();
var mainSvr=main.getSystemService(context.LOCATION_SERVICE);

console.log(mainSvr.getLastKnownLocation(lm.GPS_PROVIDER));

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null at index.html:65