我已经阅读了大量教程并看到了一堆示例，但我很难理解活动的生命周期。我明白会发生什么，我的问题是我似乎无法确切地弄清楚它应该如何编码。例如，假设我有一个简单的应用程序，一旦按下按钮就会添加两个数字。我知道要为 onCreate 方法编写什么代码，但我不知道要添加什么 onPause、onRestart 等。我只是删除变量的内容吗？任何帮助将不胜感激。
【问题讨论】：
标签： android android-activity lifecycle oncreate ondestroy
我创建了一个示例来帮助您了解 Activity 的生命周期。第一次启动屏幕时，会调用以下三个生命周期方法：
onCreate()
onStart()
onResume()
每当我们在应用程序运行时按下返回按钮时，都会调用以下生命周期方法：
onPause()
onStop()
onDestroy()
当我们在应用程序运行时按下主页按钮时，会调用以下生命周期方法：
onPause()
onStop()
当我们在按下主页按钮后恢复应用程序时，会调用以下生命周期方法：
onRestart()
onStart()
onResume()
我想这会对你有所帮助。
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toast.makeText(this, "Activity is created", Toast.LENGTH_SHORT).show();
Log.i("onCreate():","Activity is created");
}
@Override
protected void onStart() {
// TODO Auto-generated method stub
super.onStart();
Toast.makeText(this, "Activity is started", Toast.LENGTH_SHORT).show();
Log.i("onStart():","Activity started");
}
@Override
protected void onRestart() {
// TODO Auto-generated method stub
super.onRestart();
Toast.makeText(this, "Activity is Restarted", Toast.LENGTH_SHORT).show();
Log.i("onRestart():","Activity Restarted");
}
@Override
protected void onResume() {
// TODO Auto-generated method stub
super.onResume();
Toast.makeText(this, "Activity is Resumed", Toast.LENGTH_SHORT).show();
Log.i("onResume():","Activity Resumed");
}
@Override
protected void onPause() {
// TODO Auto-generated method stub
super.onPause();
Toast.makeText(this, "Activity is Paused", Toast.LENGTH_SHORT).show();
Log.i("onPause():","Activity paused");
}
@Override
protected void onStop() {
// TODO Auto-generated method stub
super.onStop();
Toast.makeText(this, "Activity is Stopped", Toast.LENGTH_SHORT).show();
Log.i("onStop():","Activity stopped");
}
@Override
protected void onDestroy() {
// TODO Auto-generated method stub
super.onDestroy();
Toast.makeText(this, "Activity is Destroyed", Toast.LENGTH_SHORT).show();
Log.i("onDestroy():","Activity destroyed");
}
}
【讨论】：
在像你这样的单个应用程序上，建议您不需要覆盖任何这些方法......除非您想在任何活动转换之前/之后或应用程序关闭/打开/进入后台时执行重要操作，例如停止服务，启动会话，甚至在应用运行时记录活动状态。
了解所有活动生命周期回调以及如何使用它来优化应用非常重要。
例如：您正在使用智能手机摄像头的应用程序导航，突然接到朋友的电话。应用程序会自动调用onPause() 方法，并且（假设）在你内部有类似myCamera.release() 的东西，它会强制相机停止在后台工作。
为什么需要这样做？以优化电池和内存使用。您不知道该应用程序是否会被杀死并强制其关闭......所以实现它是一个好习惯。
你在Android Developers上有这个例子和解释
希望对您有所帮助。
【讨论】：
这个来自 coursera 实验室的代码检查一下
package course.labs.activitylab;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
public class ActivityOne extends Activity {
// Use these as keys when you're saving state between reconfigurations
private static final String RESTART_KEY = "restart";
private static final String RESUME_KEY = "resume";
private static final String START_KEY = "start";
private static final String CREATE_KEY = "create";
// String for LogCat documentation
private final static String TAG = "Lab-ActivityOne";
// Lifecycle counters
// TODO:
// Create variables named
// mCreate, mRestart, mStart and mResume
// to count calls to onCreate(), onRestart(), onStart() and
// onResume(). These variables should not be defined as static.
int mCreate = 0 , mRestart=0 , mStart = 0 , mResume = 0;
// You will need to increment these variables' values when their
// corresponding lifecycle methods get called.
// TODO: Create variables for each of the TextViews
// named mTvCreate, mTvRestart, mTvStart, mTvResume.
// for displaying the current count of each counter variable
TextView mTvCreate , mTvRestart , mTvStart , mTvResume ;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_one);
// TODO: Assign the appropriate TextViews to the TextView variables
// Hint: Access the TextView by calling Activity's findViewById()
// textView1 = (TextView) findViewById(R.id.textView1);
mTvCreate = (TextView) findViewById(R.id.create);
mTvRestart = (TextView) findViewById(R.id.restart);
mTvStart = (TextView) findViewById(R.id.start);
mTvResume = (TextView) findViewById(R.id.resume);
Button launchActivityTwoButton = (Button) findViewById(R.id.bLaunchActivityTwo);
launchActivityTwoButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO:
// Launch Activity Two
// Hint: use Context's startActivity() method
// Create an intent stating which Activity you would like to
// start
startActivity(new Intent (ActivityOne.this , ActivityTwo.class));
// Launch the Activity using the intent
}
});
// Has previous state been saved?
if (savedInstanceState != null) {
// TODO:
// Restore value of counters from saved state
// Only need 4 lines of code, one for every count variable
mCreate = savedInstanceState.getInt(CREATE_KEY);
mStart = savedInstanceState.getInt(START_KEY);
mRestart = savedInstanceState.getInt(RESTART_KEY);
mResume = savedInstanceState.getInt(RESUME_KEY);
}
// Emit LogCat message
Log.i(TAG, "Entered the onCreate() method");
// TODO:
// Update the appropriate count variable
// Update the user interface via the displayCounts() method
mCreate ++;
displayCounts();
}
// Lifecycle callback overrides
@Override
public void onStart() {
super.onStart();
// Emit LogCat message
Log.i(TAG, "Entered the onStart() method");
// TODO:
// Update the appropriate count variable
// Update the user interface
mStart++;
displayCounts();
}
@Override
public void onResume() {
super.onResume();
// Emit LogCat message
Log.i(TAG, "Entered the onResume() method");
// TODO:
// Update the appropriate count variable
// Update the user interface
mResume ++;
displayCounts();
}
@Override
public void onPause() {
super.onPause();
// Emit LogCat message
Log.i(TAG, "Entered the onPause() method");
}
@Override
public void onStop() {
super.onStop();
// Emit LogCat message
Log.i(TAG, "Entered the onStop() method");
}
@Override
public void onRestart() {
super.onRestart();
// Emit LogCat message
Log.i(TAG, "Entered the onRestart() method");
// TODO:
// Update the appropriate count variable
// Update the user interface
mRestart ++;
displayCounts();
}
@Override
public void onDestroy() {
super.onDestroy();
// Emit LogCat message
Log.i(TAG, "Entered the onDestroy() method");
}
@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
// TODO:
// Save state information with a collection of key-value pairs
// 4 lines of code, one for every count variable
savedInstanceState.putInt(CREATE_KEY, mCreate);
savedInstanceState.putInt(START_KEY, mStart);
savedInstanceState.putInt(RESTART_KEY, mRestart);
savedInstanceState.putInt(RESUME_KEY, mResume);
}
// Updates the displayed counters
// This method expects that the counters and TextView variables use the
// names
// specified above
public void displayCounts() {
// TODO - uncomment these lines
mTvCreate.setText("onCreate() calls: " + mCreate);
mTvStart.setText("onStart() calls: " + mStart);
mTvResume.setText("onResume() calls: " + mResume);
mTvRestart.setText("onRestart() calls: " + mRestart);
}
}
【讨论】：