文章目錄

• 1、功能分析
• • 1.1、秒表功能界面
  • 1.2、App結(jié)構(gòu)
• 2、開發(fā)視圖布局
• • 2.1、activity_main.xml
  • 2.2、string.xml
• 3、Activity實現(xiàn)
• • 3.1、MainActivity類
• 4、生命周期的應(yīng)用
• • 4.1、問題分析
  • 4.2、Activity運(yùn)行過程
  • 4.2、屏幕旋轉(zhuǎn)，計時不重置
  • 4.3、App被切換至后臺，秒表可以暫停
• 5、MainActivity完整代碼

1、功能分析

1.1、秒表功能界面

1.2、App結(jié)構(gòu)

• 1個Activity ：MainActivity
• 1個Layout ：activity_main.xml

2、開發(fā)視圖布局

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"><LinearLayoutandroid:layout_width="match_parent"android:layout_height="match_parent"android:gravity="center_horizontal"android:orientation="vertical"><TextViewandroid: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"/><Buttonandroid: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"/><Buttonandroid: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"/><Buttonandroid: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實現(xiàn)

3.1、MainActivity類

public class MainActivity extends AppCompatActivity {//計時的秒數(shù)private int seconds = 0;//計時的狀態(tài)private boolean running = false;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);//在Activity啟動時，onCreate()方法中啟動runTimer()runTimer();}//啟動計時public void onClickStart(View view){running = true;}//停止計時public void onClickStop(View view){running = false;}//重置秒表public void onClickReset(View view){running = false;seconds = 0;}//循環(huán)計時方法private void runTimer(){final TextView timeView = findViewById(R.id.time_view);//創(chuàng)建UI線程的handler,用于消息處理final Handler handler = new Handler();handler.post(new Runnable() {//立即交一個Runnable，任務(wù)在Runnable的run()方法中@Overridepublic 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++;} //每隔1000ms,重復(fù)提交該任務(wù)handler.postDelayed(this,1000);}});} }

4、生命周期的應(yīng)用

4.1、問題分析

• 問題一：旋轉(zhuǎn)屏幕，Android檢測到屏幕方向變化，計時會重置
• 問題二：App被切換至后臺，秒表不能暫停

4.2、Activity運(yùn)行過程

4.2、屏幕旋轉(zhuǎn)，計時不重置

• 設(shè)備配置變化時，如屏幕旋轉(zhuǎn)，需保存狀態(tài)，重啟時恢復(fù)

• 在運(yùn)行后（running），銷毀（onDestroy()）前會調(diào)用 onSaveInstanceState()，保存狀態(tài)到Bundle

  • 保存狀態(tài)需要覆蓋onSaveInstanceState()方法

  • Bundle可存儲鍵值對

    bundle.put*(“name”,value)

  • 在Bundle中存儲running和seconds

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

• 在onCreate()方法中恢復(fù)，Bundle是其參數(shù)，進(jìn)程第一次新建Activity時為null，
  之后為onSaveInstanceState()保存的Bundle

  • 從Bundle取出鍵值對

    bundle.get*(“name”);

  • 在Bundle中取出running和seconds

    @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(); }

• 程序運(yùn)行流程

  • 用戶啟動App，點擊start按鈕，開始計時
    runTimer()方法開始遞增seconds，并顯示到文本框time_view中
  • 旋轉(zhuǎn)手機(jī)，Android檢測到屏幕方向變化，銷毀原Activity前，調(diào)用onSaveInstanceState()保存實例變量
  • Android銷毀Activity，再次新建該Activity再次調(diào)用onCreate()方法，將保存的Bundle作為參數(shù)傳入
  • onCreate()方法中，取出Bundle存儲的值并恢復(fù)到銷毀前的狀態(tài)
  • runTimer()方法從旋轉(zhuǎn)前的seconds繼續(xù)計時

4.3、App被切換至后臺，秒表可以暫停

• 解決方法

  • 覆蓋onStop()，在消失前停止計時

    @Override protected void onStop(){super.onStop();Log.d("life cycle","onStop");wasRunning = running; // 記錄之前是否運(yùn)行，running = false; //將running設(shè)置為false以停止計時 }

  • 覆蓋生命周期方法前，必須先調(diào)用父類的生命周期

    super.onStop();

  • 覆蓋onStart()，在可見前繼續(xù)計時

    @Override //如果之前running==true,則將running設(shè)置為true,繼續(xù)計時 protected void onStart(){super.onStart();Log.d("life cycle","onStop");if(wasRunning){ running = true;} }

  • App被切換至后臺，Activity對象仍存在，可以使用實例變量存儲狀態(tài)

    //計時的秒數(shù) private int seconds = 0; //計時的狀態(tài) private boolean running = false; //新的變量，用于在onStop()中保存消失前running的狀態(tài) private boolean wasRunning = false;@Override protected void onStop(){super.onStop();Log.d("life cycle","onStop"); //日志并暫停wasRunning = running;running = false; }@Override protected void onStart(){super.onStart();Log.d("life cycle","onStop"); //日志并恢復(fù)if(wasRunning){running = true;} }

  • Activity銷毀前在Bundle保存wasRunning,
    Activity重新實例化后從Bundle恢復(fù)wasRunning

    @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); }

• 運(yùn)行流程

  • 用戶啟動App，點擊Start按鈕，runTimer()開始遞增seconds并更新文本框
  • 用戶點擊Home鍵，Activity消失，Android調(diào)用onStop()
  • 用戶返回秒表App，Activity可見，Android調(diào)用onStart()

5、MainActivity完整代碼

public class MainActivity extends AppCompatActivity {//計時的秒數(shù)private int seconds = 0;//計時的狀態(tài)private boolean running = false;//新的變量，用于在onStop()中保存消失前running的狀態(tài)private boolean wasRunning = false;@Overrideprotected 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");}//在Activity啟動時，onCreate()方法中啟動runTimer()runTimer();}@Overrideprotected void onStart(){super.onStart();Log.d("life cycle","onStop");if(wasRunning){running = true;}}@Overrideprotected void onStop(){super.onStop();Log.d("life cycle","onStop");wasRunning = running;running = false;}@Overridepublic 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");}//啟動計時public void onClickStart(View view){running = true;}//停止計時public void onClickStop(View view){running = false;}//重置秒表public void onClickReset(View view){running = false;seconds = 0;wasTiming = false;}//循環(huán)計時方法private void runTimer(){final TextView timeView = findViewById(R.id.time_view);//創(chuàng)建UI線程的handler,用于消息處理final Handler handler = new Handler();handler.post(new Runnable() {//立即交一個Runnable,任務(wù)在Runnable的run()方法中@Overridepublic 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++;}//每隔1000ms,重復(fù)提交該任務(wù)handler.postDelayed(this,1000);}});} }

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