为什么从最近的任务中清除应用程序后调用StopTimer（）方法会导致应用程序崩溃？

Question

我正在尝试创建一个可监听wifi状态更改并在更改之间的服务中运行倒数计时器的应用。如果我在活动处于前台时启用wifi，然后在清除最近的任务后禁用wifi，则会导致应用崩溃。崩溃后，它会向负责停止倒数计时器的方法抛出空指针异常。但是，如果我在清除最近的任务后同时启用和禁用，则该应用程序不会崩溃。

这是来自BroadcastReciever的OnReceive方法的代码，该方法负责处理WIFI_STATE_DISABLED状态：

case WifiManager.WIFI_STATE_DISABLED:
                    count++;
                    sharedPreferences.edit().putInt("Count", count).apply();
                    sharedPreferences.edit().putString(Integer.toString(count), "Disabled at -" + simpleDateFormat.format(new Date())).apply();

                    if (activityRunning) {
                        arrayList.add("Disabled at " + simpleDateFormat.format(new Date()));
                        arrayAdapter.notifyDataSetChanged();
                        textView.setText("Disabled");
                        aSwitch.setChecked(false);
                    }

                    if (context.getSharedPreferences("com.example.wifilimiter",MODE_PRIVATE).getBoolean("timerRunning", false)) {
                        stopTimer(context);    
                        sharedPreferences.edit().putLong("millisLeft", millisleft).apply();
                    }
                    break;

上述上述StopTimer方法：

    public static void stopTimer(Context context) {
        if (context.getSharedPreferences("com.example.wifilimiter",MODE_PRIVATE).getBoolean("timerRunning", false)) {
            countDownTimer.cancel(); // The app crushes here
            context.getSharedPreferences("com.example.wifilimiter",MODE_PRIVATE).edit().putBoolean("timerRunning",false).apply();
            Log.d("Timer Running : ","false");
        }

这是CountDownTimer：

public static void startTimer(final Context context) {
        context.getSharedPreferences("com.example.wifilimiter",MODE_PRIVATE).edit().putBoolean("timerRunning", true).apply();
        countDownTimer = new CountDownTimer(millisleft, 1000) {
            @Override
            public void onTick(long millisUntilFinished) {

                millisleft = millisUntilFinished;
                String hms = String.format(Locale.getDefault(), "%02d:%02d:%02d", MILLISECONDS.toHours(millisleft),
                        MILLISECONDS.toMinutes(millisleft) - HOURS.toMinutes(MILLISECONDS.toHours(millisleft)),
                        MILLISECONDS.toSeconds(millisleft) - MINUTES.toSeconds(MILLISECONDS.toMinutes(millisleft)));

                Toast.makeText(context, hms, Toast.LENGTH_SHORT).show();
                if (context.getSharedPreferences("com.example.wifilimiter",MODE_PRIVATE).getBoolean("activityStatus",false)) {
                    Log.d(TAG, "onTick: " + hms);
                    timer.setText(hms);
                }
            }

            @Override
            public void onFinish() {
            }
        }.start();

    }

这是下面的日志：

    Process: com.example.wifilimiter, PID: 5306
    java.lang.RuntimeException: Unable to start receiver com.example.wifilimiter.BroadcastReceiver: java.lang.NullPointerException
        at android.app.ActivityThread.handleReceiver(ActivityThread.java:2424)
        at android.app.ActivityThread.access$1700(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1272)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5019)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
        at com.example.wifilimiter.TimerService.stopTimer(TimerService.java:67) //This refers to the countDownTimer.cancel() method
        at com.example.wifilimiter.BroadcastReceiver.onReceive(BroadcastReceiver.java:85)//This refers to the StopTimer() method
        at android.app.ActivityThread.handleReceiver(ActivityThread.java:2417)
        at android.app.ActivityThread.access$1700(ActivityThread.java:135) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1272) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:136) 
        at android.app.ActivityThread.main(ActivityThread.java:5019) 
        at java.lang.reflect.Method.invokeNative(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:515) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
        at dalvik.system.NativeStart.main(Native Method) 

Answer 1

当您从最近任务列表中删除该应用程序时，托管该应用程序的OS进程将被杀死。您的计时器已死。

您需要通过检查onReceive()是否为countDownTimer来检查计时器是否在null中运行。如果是这样，则您的计时器没有在运行，您不再需要停止它。