Android入门——四大组件之Activity生命周期

 

转载请注明出处：https://blog.csdn.net/eternally__/article/details/82463016

Activity是Android四大组件之一，代表用户可以操作界面，可以和用户进行交互。比如打电话，发微信等等。并且在上面可以显示一些其他的控件，来实现不同的界面。

1.Activity生命周期方法

onCreate() 第一次创建的时候调用
onStart() 由不可见变为可见时调用
onResume() 在准备好和用户交互的时候调用，此时activity位于返回栈的栈顶
onPause() 准备去启动或者另一个activity的时候调用，通常在这个方法中将一些消耗CUP资源释放掉
onStop() 完全不可见的时候调用，如果启动的新activity是一个对话框的形式，那么就不会执行onStop，只会执行onPause
onDestroy() activity被销毁之前调用
onRestart() activity由停止状态变为运行状态之前调用

2.单个Activity的生命周期

XML

MainActivity

public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Log.i(TAG, "onCreate()");
}

@Override
protected void onStart() {
    super.onStart();
    Log.i(TAG, "onStart()");
}

@Override
protected void onRestart() {
    super.onRestart();
    Log.i(TAG, "onRestart()");
}

@Override
protected void onResume() {
    super.onResume();
    Log.i(TAG, "onResume()");
}

@Override
protected void onPause() {
    super.onPause();
    Log.i(TAG, "onPause()");
}

@Override
protected void onStop() {
    super.onStop();
    Log.i(TAG, "onStop()");
}

@Override
protected void onDestroy() {
    super.onDestroy();
    Log.i(TAG, "onDestroy()");
}
}
public void startActivity(View view) {
    Intent intent = new Intent(MainActivity.this, SecondActivity.class);
    startActivity(intent);
}

首次启动Activity

I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onResume()

点击Home键，将程序退到后台(锁屏)

I/MainActivity: onPause()
I/MainActivity: onStop()

将程序从后台切到前台（解锁屏幕）

I/MainActivity: onRestart()
I/MainActivity: onStart()
I/MainActivity: onResume()

退出程序

I/MainActivity: onPause()
I/MainActivity: onStop()
I/MainActivity: onDestroy()

3.两个Activity的生命周期

public class SecondActivity extends AppCompatActivity {
private static final String TAG = "SecondActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_second);
    Log.i(TAG, "onCreate()");
}

@Override
protected void onStart() {
    super.onStart();
    Log.i(TAG, "onStart()");
}

@Override
protected void onRestart() {
    super.onRestart();
    Log.i(TAG, "onRestart()");
}

@Override
protected void onResume() {
    super.onResume();
    Log.i(TAG, "onResume()");
}

@Override
protected void onPause() {
    super.onPause();
    Log.i(TAG, "onPause()");
}

@Override
protected void onStop() {
    super.onStop();
    Log.i(TAG, "onStop()");
}

@Override
protected void onDestroy() {
    super.onDestroy();
    Log.i(TAG, "onDestroy()");
}
}

首次启动MainActivity

I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onResume()

在MainActivity上启动SecondActivity

I/MainActivity: onPause()
I/SecondActivity: onCreate()
I/SecondActivity: onStart()
I/SecondActivity:onResume()
I/MainActivity: onStop

返回到MainActivity

I/SecondActivity: onPause()
I/MainActivity: onRestart()
I/MainActivity: onStart()
I/MainActivity: onResume()
I/SecondActivity: onStop()
I/SecondActivity: onDestroy()

退出MainActivity

I/MainActivity: onPause()
I/MainActivity: onStop()
I/MainActivity: onDestroy()

4.启动的第二个activity的主题为Dialog

在AndroidManifest.xml中修改Secondactivity主题

在MainActivity上启动SecondActivity

I/MainActivity: onPause()
I/SecondActivity: onCreate()
I/SecondActivity: onStart()
I/SecondActivity:onResume()

返回到MainActivity

I/SecondActivity: onPause()
I/MainActivity: onResume()
I/SecondActivity: onStop()
I/SecondActivity: onDestroy()

其他的生命周期和非Dialog的Activity一样

5.onSaveInstanceState

通常activity因为内存不够，被系统回收掉，一些重要的临时数据没有保存下来。通过横竖屏切换来实现这种场景

首先在MainActivity中重写onSaveInstanceState

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    String data = mEditText.getText().toString();
    outState.putString("data",data);
    Log.i(TAG, "onSaveInstanceState()");
}

然后在MainActivity的onCreate方法中

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mEditText=findViewById(R.id.edit);
    Log.i(TAG, "onCreate()");
    if(savedInstanceState!=null){
        //将保存的数据再拿出来
        String name = (String) savedInstanceState.get("name");
        mEditText.setText(name);
    }
}

6.横竖屏切换时的生命周期

每切换一次横竖屏activity都会重新创建一次。那么大家就要问了，如何才能不让activity每次都重新创建呢？ 接下来在AndroidManifest.xml文件中

通过上面在activity中配置android:configChanges后，横竖屏切换不会再重新创建activity，只会执行onConfigurationChanged方法

@Override
public void onConfigurationChanged(Configuration newConfig) {
    super.onConfigurationChanged(newConfig);
    //这里面添加横竖屏切换后的操作
}

7.MiniSdkVersion大于等于 13的 时候（在android7.0上运行）

生命周期会执行两次，不设置configChanges

//切换之前
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onResume()
//切换之后
I/MainActivity: onPause()
I/MainActivity: onSaveInstanceState()
I/MainActivity: onStop()
I/MainActivity: onDestroy()
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onRestoreInstanceState()
I/MainActivity: onResume()
//切换回到竖屏
I/MainActivity: onPause()
I/MainActivity: onSaveInstanceState()
I/MainActivity: onStop()
I/MainActivity: onDestroy()
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onRestoreInstanceState()
I/MainActivity: onResume()

生命周期会执行一次，设置android:configChanges="orientation"或android:configChanges="orientation|keyboardHidden"

//切换之前（竖屏）
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onResume()
//切换到横屏
I/MainActivity: onConfigurationChanged
I/MainActivity: onPause()
I/MainActivity: onSaveInstanceState()
I/MainActivity: onStop()
I/MainActivity: onDestroy()
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onRestoreInstanceState()
I/MainActivity: onResume()
//切换回到横屏
I/MainActivity: onConfigurationChanged

设置android:configChanges="orientation|keyboardHidden|screenSize"

//切换之前（竖屏）
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onResume()
//之后切换都会执行
I/MainActivity: onConfigurationChanged

8.MiniSdkVersion小于 13 的时候（在android2.3上运行）

设置android:configChanges="orientation"或android:configChanges="orientation|keyboardHidden"或android:configChanges="orientation|keyboardHidden|screenSize"结果都是一样的

//切换之前（竖屏）
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onResume()
//之后切换都会执行
I/MainActivity: onConfigurationChanged
I/MainActivity: onConfigurationChanged

不管是横屏启动还是竖屏启动，生命周期会执行两次，不设置configChanges

//切换之前
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onResume()
//切换之后  
I/MainActivity: onSaveInstanceState()
I/MainActivity: onPause()
I/MainActivity: onStop()
I/MainActivity: onDestroy()
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onRestoreInstanceState()
I/MainActivity: onResume()
//切换回到竖屏
I/MainActivity: onSaveInstanceState()
I/MainActivity: onPause()
I/MainActivity: onStop()
I/MainActivity: onDestroy()
I/MainActivity: onCreate()
I/MainActivity: onStart()
I/MainActivity: onRestoreInstanceState()
I/MainActivity: onResume()

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