Activity作为Android的四大组件之一，在实际开发中扮演着很重要的角色，了解它的声明周期对我们以后的开发会很有帮助，下面我们来看看在不同场景下Activity的生命周期

准备两个Activity，IndexActivity和TwoActivity

public class IndexActivity extends AppCompatActivity {

public static final String TAG = "IndexActivity";

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

findViewById(R.id.text_request_one).setOnClickListener(new View.OnClickListener() {

@Override

public void onClick(View v) {

startActivity(new Intent(IndexActivity.this, TwoActivity.class));

}

});

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");

}

@Override

protected void onNewIntent(Intent intent) {

super.onNewIntent(intent);

Log.i(TAG, "onNewIntent");

}

}

public class TwoActivity extends AppCompatActivity {

public static final String TAG = "TwoActivity";

@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");

}

}

启动App IndexActivity的生命周期

01-22 15:44:02.131 23541-23541/fe.dotwoven I/IndexActivity: onCreate

01-22 15:44:02.131 23541-23541/fe.dotwoven I/IndexActivity: onStart

01-22 15:44:02.133 23541-23541/fe.dotwoven I/IndexActivity: onResume

启动IndexActivity分别调用了onCreate，onStart， onResume方法。

按Home键回到桌面 Activity生命周期

01-22 15:46:52.583 23541-23541/fe.dotwoven I/IndexActivity: onPause

01-22 15:46:53.110 23541-23541/fe.dotwoven I/IndexActivity: onStop

回到桌面会调用IndexActivity的onPause，onStop方法

从桌面再次进入IndexActivity 生命周期

01-22 15:47:51.388 23541-23541/fe.dotwoven I/IndexActivity: onRestart

01-22 15:47:51.388 23541-23541/fe.dotwoven I/IndexActivity: onStart

01-22 15:47:51.388 23541-23541/fe.dotwoven I/IndexActivity: onResume

再次进入的时候调用了IndexActivity的onRestart，onStart，onResume方法

IndexActivity上按返回键时生命周期

01-22 15:50:03.004 23541-23541/fe.dotwoven I/IndexActivity: onPause

01-22 15:50:03.558 23541-23541/fe.dotwoven I/IndexActivity: onStop

01-22 15:50:03.558 23541-23541/fe.dotwoven I/IndexActivity: onDestroy

分别执行了IndexActivity的onPause，onStop，onDestroy方法

从IndexActivity跳转到TwoActivity生命周期

01-22 15:51:54.183 23541-23541/fe.dotwoven I/IndexActivity: onPause

01-22 15:51:54.270 23541-23541/fe.dotwoven I/TwoActivity: onCreate

01-22 15:51:54.271 23541-23541/fe.dotwoven I/TwoActivity: onStart

01-22 15:51:54.273 23541-23541/fe.dotwoven I/TwoActivity: onResume

01-22 15:51:54.688 23541-23541/fe.dotwoven I/IndexActivity: onStop

首先会执行IndexActivity的onPause方法，接下来执行TwoActivity的onCreate，onStart，onResume方法；最后才会执行IndexActivity的onStop方法，最后这点大家要记住

TwoActivity按返回键返回IndexActivity生命周期

01-22 15:53:49.444 23541-23541/fe.dotwoven I/TwoActivity: onPause

01-22 15:53:49.484 23541-23541/fe.dotwoven I/IndexActivity: onRestart

01-22 15:53:49.485 23541-23541/fe.dotwoven I/IndexActivity: onStart

01-22 15:53:49.494 23541-23541/fe.dotwoven I/IndexActivity: onResume

01-22 15:53:49.870 23541-23541/fe.dotwoven I/TwoActivity: onStop

01-22 15:53:49.870 23541-23541/fe.dotwoven I/TwoActivity: onDestroy

首先会执行TwoActivity的onPause方法，接下来调用IndexActivity的onRestart，onStart，onResume方法，最后再调用TwoActivity的onStop和onDestroy方法。

启动模式不同 Activity的生命周期

如果IndexActivity的启动模式是singleTask或是singleInstance，然后从IndexActivity跳转到TwoActivity，再从TwoActivity跳转到IndexActivity，这个时候生命周期会有什么不同呢？我们来看看

01-22 16:07:53.739 18403-18403/fe.dotwoven I/TwoActivity: onPause

01-22 16:07:53.762 18403-18403/fe.dotwoven I/IndexActivity: onNewIntent

01-22 16:07:53.762 18403-18403/fe.dotwoven I/IndexActivity: onRestart

01-22 16:07:53.765 18403-18403/fe.dotwoven I/IndexActivity: onStart

01-22 16:07:53.767 18403-18403/fe.dotwoven I/IndexActivity: onResume

01-22 16:07:54.103 18403-18403/fe.dotwoven I/TwoActivity: onStop

01-22 16:07:54.103 18403-18403/fe.dotwoven I/TwoActivity: onDestroy

IndexActivity多调用了一个onNewIntent方法。

如果IndexActivity的启动模式是singleTop，然后从IndexActivity启动到IndexActivity，它其实显示的还是IndexActivity，那IndexActivity的生命周期会有什么不同呢？

01-22 16:13:03.392 4830-4830/fe.dotwoven I/IndexActivity: onPause

01-22 16:13:03.392 4830-4830/fe.dotwoven I/IndexActivity: onNewIntent

01-22 16:13:03.393 4830-4830/fe.dotwoven I/IndexActivity: onResume

可以看到它先调用了自己的onPause方法，然后调用了onNewIntent，onResume方法。

总结

到此Activity各个场景的生命周期差不多都分析完了，理解好Activity的生命周期有利于我们的开发工作