Android application life cycle to achieve a simple stopwatch App

1. Functional analysis

1.1 stopwatch function interface

1.2 App structure

1 Activity: MainActivity
1 Layout: activity_main.xml

2. Development view layout

2.1,activity_main.xml

<?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">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center_horizontal"
        android:orientation="vertical">

        <TextView
            android:id="@+id/time_view"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center_horizontal"
            android:text="0:00:00"
            android:textAppearance="@style/TextAppearance.AppCompat.Large"
            android:textSize="80sp"/>

        <Button
            android:id="@+id/button_start"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="@string/start"
            android:onClick="onClickStart"/>

        <Button
            android:id="@+id/button_stop"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="@string/stop"
            android:onClick="onClickStop"/>

        <Button
            android:id="@+id/button_reset"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="@string/reset"
            android:onClick="onClickReset"/>
    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

2.2,string.xml

<resources>
    <string name="app_name">Stopwatch</string>
    <string name="start">start</string>
    <string name="stop"> Stop</string>
    <string name="reset">Reset</string>
    <string name="time">Time</string>
</resources>

3. Activity implementation

3.1. MainActivity class

public class MainActivity extends AppCompatActivity {

    //Seconds timed
    private  int seconds = 0;
    //Status of timing
    private  boolean running = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //When the Activity starts, runTimer() is started in the onCreate() method
        runTimer();
    }

    //Start timing
    public void onClickStart(View view){
        running = true;
    }
    //Stop timing
    public void onClickStop(View view){
        running = false;
    }
    //Reset stopwatch
    public void onClickReset(View view){
        running = false;
        seconds = 0;
    }
  	//Cycle timing method
    private void runTimer(){
        final TextView timeView = findViewById(R.id.time_view);
        //Create handler of UI thread for message processing
        final Handler handler = new Handler();
        handler.post(new Runnable() {//Hand in a Runnable immediately, and the task is in the run() method of Runnable
            @Override
            public void run() {
                int hours = seconds/3600;
                int minutes = (seconds%3600)/60;
                int secs = seconds%60;
                String time = String.format("%d:%02d:%02d", hours, minutes, secs);
                timeView.setText(time);
                if(running){
                    seconds++;
                }            
                //Submit the task repeatedly every 1000ms
                handler.postDelayed(this,1000);
            }
        });
    }
}

4. Application of life cycle

4.1 problem analysis

Question 1: when the screen is rotated, Android detects a change in the screen direction, and the timing will be reset
Problem 2: the App is switched to the background, and the stopwatch cannot pause

4.2. Activity running process

4.2. The screen rotates and the timing does not reset

When the device configuration changes, such as the screen rotates, the state needs to be saved and restored when restarting
After running and before destroying (onDestroy()), onSaveInstanceState() will be called to save the state to the Bundle
- The onSaveInstanceState() method needs to be overridden to save the state
- Bundle stores key value pairs
  
  bundle.put*("name",value)
- Store running and seconds in the Bundle
```
@Override
public void onSaveInstanceState(Bundle savedInstanceState){
    super.onSaveInstanceState(savedInstanceState);
    
    savedInstanceState.putInt("seconds",seconds);
    savedInstanceState.putBoolean("running",running);
}
```

Recover in onCreate() method, with Bundle as its parameter, and null when the process creates an Activity for the first time,
The Bundle saved later for onSaveInstanceState()

Take the key value pair from the Bundle

bundle.get*("name");

Take out running and seconds from the Bundle

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.d("life cycle","onCreate");
    setContentView(R.layout.activity_main);
    
    if(savedInstanceState!=null){
        seconds = savedInstanceState.getInt("seconds");
        running = savedInstanceState.getBoolean("running");
    }
    runTimer();
}

Program running process
- The user starts the App and clicks the start button to start timing
  The runTimer() method starts incrementing seconds and displays it in the text box time_ In view
- Rotate the mobile phone, Android detects the change of the screen direction, and destroys the original Activity, then calls onSaveInstanceState() to save the instance variables.
- Android destroys the Activity, creates the Activity again, calls onCreate() method again, and passes in the saved Bundle as a parameter
- In the onCreate() method, retrieve the value stored in the Bundle and restore it to the state before destruction
- The runTimer() method continues timing from the seconds before rotation

4.3. When the App is switched to the background, the stopwatch can pause

resolvent

Override onStop() and stop timing before disappearing

@Override
protected void onStop(){
    super.onStop();
    Log.d("life cycle","onStop");
    wasRunning = running; // Whether it is running before recording,
    running = false; //Set running to false to stop timing
}

The lifecycle of the parent class must be called before overriding the lifecycle method

super.onStop();

Override onStart() to continue timing until it is visible

@Override
//If running==true before, set running to true to continue timing
protected void onStart(){
    super.onStart();
    Log.d("life cycle","onStop");
    if(wasRunning){ 
        running = true;
    }
}

The App is switched to the background, and the Activity object still exists. You can use the instance variable to store the state

//Seconds timed
private  int seconds = 0;
//Status of timing
private  boolean running = false;
//A new variable used to save the running state before disappearing in onStop()
private boolean wasRunning = false;

@Override
protected void onStop(){
    super.onStop();
    Log.d("life cycle","onStop"); //Log and pause
    wasRunning = running;
    running = false;
}

@Override
protected void onStart(){
    super.onStart();
    Log.d("life cycle","onStop"); //Log and recover
    if(wasRunning){
        running = true;
    }
}

Save wasRunning in the Bundle before destroying the Activity,
Recover wasRunning from Bundle after re instantiation of Activity

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.d("life cycle","onCreate");
    setContentView(R.layout.activity_main);
    if(savedInstanceState!=null){
        seconds = savedInstanceState.getInt("seconds");
        running = savedInstanceState.getBoolean("running");
        wasRunning = savedInstanceState.getBoolean("wasRunning");
    }
    runTimer();
}

@Override
public void onSaveInstanceState(Bundle savedInstanceState){
    super.onSaveInstanceState(savedInstanceState);
    savedInstanceState.putInt("seconds",seconds);
    savedInstanceState.putBoolean("running",running);
    savedInstanceState.putBoolean("wasRunning",wasRunning);
}

Operation process
- The user starts the App, clicks the Start button, and runTimer() starts to increment seconds and update the text box
- The user clicks the Home key, the Activity disappears, and Android calls onStop()
- The user returns the stopwatch App, the Activity is visible, and Android calls onStart()

5. MainActivity complete code

public class MainActivity extends AppCompatActivity {

    //Seconds timed
    private  int seconds = 0;
    //Status of timing
    private  boolean running = false;
    //A new variable used to save the running state before disappearing in onStop()
    private boolean wasRunning = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d("life cycle","onCreate");
        setContentView(R.layout.activity_main);
        if(savedInstanceState!=null){
            seconds = savedInstanceState.getInt("seconds");
            running = savedInstanceState.getBoolean("running");
            wasRunning = savedInstanceState.getBoolean("wasRunning");
        }
        //When the Activity starts, runTimer() is started in the onCreate() method
        runTimer();
    }

    @Override
    protected void onStart(){
        super.onStart();
        Log.d("life cycle","onStop");
        if(wasRunning){
            running = true;
        }
    }

    @Override
    protected void onStop(){
        super.onStop();
        Log.d("life cycle","onStop");
        wasRunning = running;
        running = false;
    }


    @Override
    public void onSaveInstanceState(Bundle savedInstanceState){
        super.onSaveInstanceState(savedInstanceState);
        savedInstanceState.putInt("seconds",seconds);
        savedInstanceState.putBoolean("running",running);
        savedInstanceState.putBoolean("wasRunning",wasRunning);
    }

    protected void onDestroy(){
        super.onDestroy();
        Log.d("life cycle","onDestroy");
    }

    //Start timing
    public void onClickStart(View view){
        running = true;
    }
    //Stop timing
    public void onClickStop(View view){
        running = false;
    }
    //Reset stopwatch
    public void onClickReset(View view){
        running = false;
        seconds = 0;
        wasTiming = false;
    }
    //Cycle timing method
    private void runTimer(){
        final TextView timeView = findViewById(R.id.time_view);
        //Create handler of UI thread for message processing
        final Handler handler = new Handler();
        handler.post(new Runnable() {//Hand in a Runnable immediately, and the task is in the run() method of Runnable
            @Override
            public void run() {
                int hours = seconds/3600;
                int minutes = (seconds%3600)/60;
                int secs = seconds%60;
                String time = String.format("%d:%02d:%02d", hours, minutes, secs);
                timeView.setText(time);
                if(running){
                    seconds++;
                }
                //Submit the task repeatedly every 1000ms
                handler.postDelayed(this,1000);
            }
        });
    }
}

Keywords: Java Android UI

Added by zed420 on Thu, 11 Nov 2021 09:11:23 +0200

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