任务目标
制作一个简易计时器（秒表），实现Start, Stop, Restart三个按钮的功能。
在此基础上，实现当屏幕旋转后依旧可以继续计时，且当程序在后台运行时（如返回桌面或打开其它软件）暂停计时，并在返回该程序时继续计时。
界面编辑
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:id="@+id/time_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="00：00：00"
android:textAppearance="@android:style/TextAppearance.Large"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.2" />
<Button
android:id="@+id/button_start"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Start"
android:onClick="onClickStart"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="@+id/button_stop"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Stop"
android:onClick="onClickStop"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.4" />
<Button
android:id="@+id/button_Reset"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Reset"
android:onClick="onClickReset"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.3" />
</androidx.constraintlayout.widget.ConstraintLayout>Java编程逻辑
我们通过Handler.post(Runnable())来更新UI界面，Runnable会调用重写的run()函数，然后通过handler.postDelayed()函数控制每次更新UI的延迟时间。具体原理还挺复杂的哈，希望深入了解handler机制和runnable函数的原理可以自行查阅资料或参考你不知道的Runnable接口，深度解析Runnable接口_程序员七哥的博客-CSDN博客_runnable
public class MainActivity extends AppCompatActivity
{
private int seconds = 0;
private boolean running;
public void onClickStart(View view)
{
running = true;
}
public void onClickStop(View view)
{
running = false;
}
public void onClickReset(View view)
{
running = false;
seconds = 0;
//下面这两行是为了防止点击Reset按钮后有1s的延迟
final TextView timeView = findViewById(R.id.time_view);
timeView.setText("00:00:00");
}
private void runTimer()
{
final TextView timeView = findViewById(R.id.time_view);
final Handler handler = new Handler();
handler.post(new Runnable()
{
@Override
public void run()
{
if (running)
seconds++;
int hours = seconds / 3600;
int minutes = seconds % 3600 / 60;
int secs = seconds % 3600 % 60;
String time = String.format(Locale.getDefault(), "%02d:%02d:%02d", hours, minutes, secs);
timeView.setText(time);
handler.postDelayed(this, 1000);
}
});
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
runTimer();
}
}以上，我们仅仅实现了最基本的计时器功能，如何实现在屏幕旋转后仍然计时呢？我们需要通过Bundle来暂时储存一些变量，在程序在后台运行或程序自动刷新时通过bundle.getInt()即使保存这些变量，然后在适当的时间通过bundle.putInt()释放这些保存的变量。这里我们要重写onCreate(), onStop(), onStart()等函数来完成目标任务，具体逻辑如下不做过多解释，代码如下：
private boolean wasRunning;
@Override
protected void onCreate(Bundle bundle)
{
super.onCreate(bundle);
setContentView(R.layout.activity_main);
if(bundle != null)
{
seconds = bundle.getInt("seconds");
running = bundle.getBoolean("running");
wasRunning = bundle.getBoolean("wasRunning");
}
runTimer();
}
@Override
public void onSaveInstanceState(Bundle bundle)
{
super.onSaveInstanceState(bundle);
bundle.putInt("seconds", seconds);
bundle.putBoolean("running", running);
bundle.putBoolean("wasRunning", wasRunning);
}
@Override
protected void onStop()
{
super.onStop();
wasRunning = running;
running = false;
}
@Override
protected void onStart()
{
super.onStart();
if(wasRunning)
running = true;
}Kotlin编程逻辑
class MainActivity : AppCompatActivity()
{
private var seconds = 0
private var running = false
fun onClickStart(view: View?)
{
running = true
}
fun onClickStop(view: View?)
{
running = false
}
@SuppressLint("SetTextI18n")
fun onClickReset(view: View?)
{
running = false
seconds = 0
val timeView = findViewById<TextView>(R.id.time_view)
timeView.text = "00:00:00"
}
private fun runTimer()
{
val timeView = findViewById<TextView>(R.id.time_view)
val handler = Handler()
handler.post(object : Runnable
{
override fun run()
{
if (running) seconds++
val hours = seconds / 3600
val minutes = seconds % 3600 / 60
val secs = seconds % 3600 % 60
val time =
String.format(Locale.getDefault(), "%02d:%02d:%02d", hours, minutes, secs)
timeView.text = time
handler.postDelayed(this, 1000)
}
})
}
private var wasRunning = false
override fun onCreate(bundle: Bundle?)
{
super.onCreate(bundle)
setContentView(R.layout.activity_main)
if (bundle != null)
{
seconds = bundle.getInt("seconds")
running = bundle.getBoolean("running")
wasRunning = bundle.getBoolean("wasRunning")
}
runTimer()
}
public override fun onSaveInstanceState(bundle: Bundle)
{
super.onSaveInstanceState(bundle)
bundle.putInt("seconds", seconds)
bundle.putBoolean("running", running)
bundle.putBoolean("wasRunning", wasRunning)
}
override fun onStop()
{
super.onStop()
wasRunning = running
running = false
}
override fun onStart()
{
super.onStart()
if (wasRunning) running = true
}
}
