Android toast.makeText 上下文错误

Question

我在位置监听器中调用 toast.Maketext 时遇到问题。上下文不可用，我做错了什么？

private LocationListener ll = new LocationListener() {

    public void onLocationChanged(Location l) {
        // SMSReceiver.l = l;
        String s = "";
        s += "\tTime: " + l.getTime() + "\n";
        s += "\tLatitude:  " + l.getLatitude() + "°\n";
        s += "\tLongitude: " + l.getLongitude() + "°\n";
        s += "\tAccuracy:  " + l.getAccuracy() + " metres\n";
        s += "\tAltitude:  " + l.getAltitude() + " metres\n";
        s += "\tSpeed:  " + l.getSpeed() + " metres\n";

        // TODO Auto-generated method stub
        if (l.hasSpeed()) {
            mySpeed = l.getSpeed();
        }

        Log.i(DEBUG_TAG, "On Location Changed: (" + s + ")");
ERROR HERE-->       Toast.makeText(context, s, Toast.LENGTH_SHORT).show();
    }

    public void onProviderDisabled(String arg0) {
        // TODO Auto-generated method stub

    }

    public void onProviderEnabled(String arg0) {
        // TODO Auto-generated method stub

    }

    public void onStatusChanged(String arg0, int arg1, Bundle arg2) {
        // TODO Auto-generated method stub

    }

};

Answer 1

如果此 LocationListener 声明在 Activity 类中(例如:MyActivity)，您应该将 Toast 创建为:

Toast.makeText(MyActivity.this, s, Toast.LENGTH_SHORT).show();

如果 LocationListener 是在无上下文类中声明的，例如在您的情况下是 BroadcastReceiver，您可以将上下文传递给它的构造函数:

private final class MyReceiver extends BroadcastReceiver
{
    private MyLocationListener listener; 
    public MyReceiver(final Context context)
    {
        this.listener = new MyLocationListener(context);
    }

    private final class MyLocationListener implements LocationListener
    {
        private Context context;
        public MyLocationListener(final Context context)
        {
            this.context = context; 
        }

        @Override
        public void onLocationChanged(Location location)
        {
            // ...
            Toast.makeText(context, "Toast message here", Toast.LENGTH_SHORT).show();
        }

        // implement the rest of the methods
    }

    @Override
    public void onReceive(Context context, Intent intent)
    {
        // Note that you have a context here, which you can use when receiving an broadcast message
    }
}