在没有服务的情况下始终保持计时器运行

Question

我正在使用计时器，它需要一直运行直到停止。

我在下面使用的代码运行良好，计时器保持运行ning，但是，当设备重新启动时，我得到负值。

SharedPreferences sharedPreferences = getSharedPreferences("chronometer", MODE_PRIVATE);

    //Check if the start time was saved before
    long elapsedTime = sharedPreferences.getLong("elapsed_time", 0);
    
    if (elapsedTime != 0) {
        //It means the chronometer was running before, so we set the time from SharedPreferences
        chronometer.setBase(elapsedTime);
        chronometer.start();
    }

    //Code below starts the chronometer, after the user manually starts it

    Button startButton = findViewById(R.id.btnStart);
    startButton.setOnClickListener(v -> {
        //Save the starting time
        long elapsedRealtime = SystemClock.elapsedRealtime();
        sharedPreferences.edit().putLong("elapsed_time", elapsedRealtime).apply();

        chronometer.setBase(SystemClock.elapsedRealtime());
        chronometer.start();
    });

为什么天文钟在重启后显示负值，我该如何解决？

Answer 1

您应该将 elapsedMillis 保存到共享首选项中，当设备重新启动并设置计时表的基础时，您应该执行以下操作


chronometer.setBase(SystemClock.elapsedRealtime() - millis)

其中 millis 是您在首选项中的值

Answer 2

来自 Android Documentation

Chronometer

Class that implements a simple timer.

You can give it a start time in the SystemClock#elapsedRealtime timebase, and it counts up from that.

这个class的时基基于SystemClock.elapsedRealtime()。

elapsedRealtime

public static long elapsedRealtime()

Returns milliseconds since boot, including time spent in sleep.

回到你的问题：

Why does the chronometer display negative values after a re-boot?

下面是一个演示此场景的示例。

假设你的 phone 是运行距离上次启动大约 10 分钟，此时用户打开 activity，elapsedRealtime() 方法将 return 600000L（10 分钟 = 10 * 60 * 1000 毫秒）。当用户单击“开始”按钮时，您将 Chronometer 的时间设置为基准时间，并使用 elapsed_time 键将 600000L 保存到 SharePreferences。

现在：elapsedRealTime() = 10 分钟 |计时器 = 00:00
1 分钟后：elapsedRealtime() = 11 分钟 |计时器 = 01:00
10 分钟后：elapsedRealTime() = 20 分钟 |计时器 = 10:00

此时用户按返回键退出应用并重启phone。

phone 有点慢，所以从启动到花了 2 分钟。用户再次打开 activity。此时。

elapsedRealtime() returns 120000L（2 分钟 = 2 * 60 * 1000 毫秒）
您在 SharePreferences (elapsed_time) 中保存的值为 600000L

因为您设置的 Chronometer basetime 的值早于 elapsedRealtime()，所以 Chronometer 将显示 -08:00 并从该值开始向上计数。

How can I sort it out?

您可以保存 Chronometer 的运行时间以及从用户上次离开 activity 到用户下次打开 activity 的运行时间。然后使用这两个值来计算 Chronometer 的基准时间。

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical">

    <Chronometer
        android:id="@+id/chronometer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="20dp"
        android:textColor="#F00"
        android:textSize="24sp" />

    <Button
        android:id="@+id/btnStart"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start" />
</LinearLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private static final String KEY_CHRONOMETER_ELAPSED_TIME = "chronometerElapsedTime";
    private static final String KEY_CHRONOMETER_STOPPED_TIME = "chronometerStoppedTime";

    private Chronometer chronometer;
    private SharedPreferences prefs;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        chronometer = findViewById(R.id.chronometer);

        // Code below starts the chronometer, after the user manually starts it
        Button startButton = findViewById(R.id.btnStart);
        startButton.setOnClickListener(v -> {
            setElapsedTime(-1);
            setStoppedTime(-1);
            chronometer.setBase(SystemClock.elapsedRealtime());
            chronometer.start();
        });
    }

    @Override
    protected void onStart() {
        super.onStart();
        prefs = getSharedPreferences("chronometer", MODE_PRIVATE);
        if (prefs.contains(KEY_CHRONOMETER_ELAPSED_TIME)
                && prefs.contains(KEY_CHRONOMETER_STOPPED_TIME)) {
            long chronometerElapsedTime = prefs.getLong(KEY_CHRONOMETER_ELAPSED_TIME, -1);
            long chronometerStoppedTime = prefs.getLong(KEY_CHRONOMETER_STOPPED_TIME, -1);
            if (chronometerElapsedTime != -1 && chronometerStoppedTime != -1) {
                long now = System.currentTimeMillis();
                long elapsedTimeFromLastStop = now - chronometerStoppedTime; // Including restart time

                long elapsedRealTime = SystemClock.elapsedRealtime();
                long base = elapsedRealTime - (chronometerElapsedTime + elapsedTimeFromLastStop);

                chronometer.setBase(base);
                chronometer.start();
            }
        }
    }

    @Override
    protected void onStop() {
        setElapsedTime(getChronometerTimeMs());
        setStoppedTime(System.currentTimeMillis());
        super.onStop();
    }

    private void setElapsedTime(long elapsedTimeMs) {
        prefs.edit().putLong(KEY_CHRONOMETER_ELAPSED_TIME, elapsedTimeMs).apply();
    }

    private void setStoppedTime(long stoppedTimeMs) {
        prefs.edit().putLong(KEY_CHRONOMETER_STOPPED_TIME, stoppedTimeMs).apply();
    }

    private long getChronometerTimeMs() {
        long chronometerTimeMs = 0;

        // Regex for HH:MM:SS or MM:SS
        String regex = "([0-1]?\d|2[0-3])(?::([0-5]?\d))?(?::([0-5]?\d))?";

        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(chronometer.getText());
        if (matcher.find()) {
            boolean isHHMMSSFormat = matcher.groupCount() == 4;
            if (isHHMMSSFormat) {
                int hour = Integer.valueOf(matcher.group(1));
                int minute = Integer.valueOf(matcher.group(2));
                int second = Integer.valueOf(matcher.group(3));
                chronometerTimeMs = (hour * DateUtils.HOUR_IN_MILLIS)
                        + (minute * DateUtils.MINUTE_IN_MILLIS)
                        + (second * DateUtils.SECOND_IN_MILLIS);
            } else {
                int minute = Integer.valueOf(matcher.group(1));
                int second = Integer.valueOf(matcher.group(2));
                chronometerTimeMs = (minute * DateUtils.MINUTE_IN_MILLIS)
                        + (second * DateUtils.SECOND_IN_MILLIS);
            }
        }

        return chronometerTimeMs;
    }
}

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在没有服务的情况下始终保持计时器运行