為什么我無法從Android Studio中的MainActivity獲取上下文？

Question

我的MainActivity調用getLoc(MainActivity.this)因此它將上下文傳遞給我的另一個活動CurrentLocation。 在CurrentLocation活動中， getLoc(Context context1)設置了全局上下文， onLocationChanged(Location location)也使用該全局上下文來調用JsonTaskWeather()但上下文似乎始終為null。

package com.tramtime.owner.croydontramtime;

import android.Manifest;
import android.annotation.TargetApi;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Looper;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.util.Log;
import android.widget.TextView;

import com.tramtime.owner.croydontramtime.Controllers.JsonTask.JsonTaskWeather;
import com.tramtime.owner.croydontramtime.Views.MainActivity;

public class CurrentLocation extends Activity implements LocationListener {
    private LocationManager myManager;
    private CurrentLocation loc;
    private double latitude;
    private double longitude;
    private Context context;

@Override
public void onLocationChanged(Location location) {
    String src = "lat="+location.getLatitude()+"&lon="+location.getLongitude();

    JsonTaskWeather jsonTaskWeather = new JsonTaskWeather(context);
    jsonTaskWeather.execute(
            "http://api.openweathermap.org/data/2.5/weather?"+src+"&appid=e58a3347bfbb6123ea384743a3f2e686");

}

public String see(){
    String foo = "lat and lon"+latitude+" "+longitude;
    return foo;
}

@Override
public void onStatusChanged(String provider, int status, Bundle extras) {

}

@Override
public void onProviderEnabled(String provider) {

}

@Override
public void onProviderDisabled(String provider) {

}

@TargetApi(Build.VERSION_CODES.M)
public void getLoc(Context context1) {
    this.context = context1;
    loc = new CurrentLocation();
    myManager = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);
    if (ActivityCompat.checkSelfPermission(context, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED
            && ActivityCompat.checkSelfPermission(context, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
        requestPermissions(new String[]{
                Manifest.permission.ACCESS_FINE_LOCATION, Manifest.permission.ACCESS_COARSE_LOCATION,
                Manifest.permission.INTERNET
        }, 10);
        return;
    }else{
        myManager.requestSingleUpdate(LocationManager.GPS_PROVIDER, loc, Looper.myLooper());
    }

}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    switch (requestCode){
        case 10:
            if (grantResults.length>0 && grantResults[0] == PackageManager.PERMISSION_GRANTED);
    }
}

}

這是注銷的內容：

08-28 08:26:11.607 8752-8752/com.tramtime.owner.croydontramtime E/AndroidRuntime: FATAL EXCEPTION: main
                                                                              Process: com.tramtime.owner.croydontramtime, PID: 8752
                                                                              java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
                                                                                  at com.tramtime.owner.croydontramtime.CurrentLocation.onLocationChanged(CurrentLocation.java:36)
                                                                                  at android.location.LocationManager$ListenerTransport._handleMessage(LocationManager.java:288)
                                                                                  at android.location.LocationManager$ListenerTransport.access$000(LocationManager.java:217)
                                                                                  at android.location.LocationManager$ListenerTransport$2.handleMessage(LocationManager.java:240)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                  at android.os.Looper.loop(Looper.java:152)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:5507)
                                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
08-28 08:26:11.704 8752-8752/com.tramtime.owner.croydontramtime I/Process: Sending signal. PID: 8752 SIG: 9
08-28 08:33:11.592 9367-9367/com.tramtime.owner.croydontramtime I/logging again: 0.0
08-28 08:33:11.624 9367-9447/com.tramtime.owner.croydontramtime I/DpmTcmClient: RegisterTcmMonitor from: com.android.okhttp.TcmIdleTimerMonitor

Answer 1

問題是這是一個有缺陷的設計。 首先，如果它不是靜態的，我將無法理解如何調用getLoc函數？ MainActivity是否擴展CurrentLocation類？

無論如何，這里是關於如何使其工作的建議

1）在MainActivity中直接實現LocationListener

2）使用意圖調用CurrentLocation類，並通過執行以下操作在CurrentLocation的onCreate內更新內容

Content context;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    context = this;
    ....
}

現在，上下文已正確初始化，您可以在類中的任何位置使用它。