Android-Activity的生命周期
Activity的生命周期主要有如下几个
- onCreate
- onRestart
- onStart
- onResume
- onPause
- onStop
- onDestroy
onCreate主要在Activity创建的时候执行
onRestart在返回Activity前台时候执行，例如使用home键挂在后台，然后再次打开前台，就会执行onRestart，或者在一个Activity中打开了另一个Activity，再次返回这个Activity时，也会执行onRestart
onStart执行在onCreate之后或者onRestart之后，此时前台可见但是还不可以与用户进行交互
onResume执行在onStart之后，前台可见也可以与用户进行交互
onPause在退出Activity或者关闭Activity时执行，此时Activity可见但是不可以进行交互，应该在此处保存一些信息，因为此时进程的优先级可能较低，会随时被系统回收
onStop执行时Activity已经不可见，被下一个Activity覆盖了或者退回到桌面了
onDestroy执行时Activity已经被彻底的销毁了
关于生命周期的例子
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| public class MainActivity extends Activity {
private final static String TAG = MainActivity.class.getSimpleName();
@Override
protected void onCreate(Bundle savedInstanceState) {
Log.e(TAG, "onCreate");
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
protected void onResume() {
Log.e(TAG, "onResume");
super.onResume();
}
@Override
public void onWindowFocusChanged(boolean hasFocus) {
Log.e(TAG, "window hasFocus: " + hasFocus);
super.onWindowFocusChanged(hasFocus);
}
@Override
protected void onPause() {
Log.e(TAG, "onPause");
super.onPause();
}
@Override
protected void onStop() {
Log.e(TAG, "onStop");
super.onStop();
}
@Override
protected void onDestroy() {
Log.e(TAG, "onDestroy");
super.onDestroy();
}
}
|
如上单个Activity，当我们打开app时候，执行的顺序为
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| 01-25 17:45:12.330 4998 4998 E MainActivity: onCreate
01-25 17:45:12.366 4998 4998 E MainActivity: onStart
01-25 17:45:12.367 4998 4998 E MainActivity: onResume
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当按返回键时执行
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| 01-25 17:45:54.847 4998 4998 E MainActivity: onPause
01-25 17:45:55.445 4998 4998 E MainActivity: onStop
01-25 17:45:55.448 4998 4998 E MainActivity: onDestroy
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当按home键时
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| 01-25 17:46:46.097 4998 4998 E MainActivity: onPause
01-25 17:46:46.119 4998 4998 E MainActivity: onStop
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按home键，并且再次返回时
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| 01-25 17:47:20.141 4998 4998 E MainActivity: onRestart
01-25 17:47:20.144 4998 4998 E MainActivity: onStart
01-25 17:47:20.145 4998 4998 E MainActivity: onResume
|
我们用两个Activity跳转来观察一下生命周期（MainActivity -> MainActivity1）
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| public class MainActivity extends Activity {
private final static String TAG = MainActivity.class.getSimpleName();
@Override
protected void onCreate(Bundle savedInstanceState) {
Log.e(TAG, "onCreate");
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button toBtn = findViewById(R.id.to_btn);
toBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent();
intent.setAction("com.lightingsui.TEST");
intent.addCategory("android.intent.category.DEFAULT");
}
}
}
@Override
protected void onResume() {
Log.e(TAG, "onResume");
super.onResume();
}
@Override
public void onWindowFocusChanged(boolean hasFocus) {
Log.e(TAG, "window hasFocus: " + hasFocus);
super.onWindowFocusChanged(hasFocus);
}
@Override
protected void onPause() {
Log.e(TAG, "onPause");
super.onPause();
}
@Override
protected void onStop() {
Log.e(TAG, "onStop");
super.onStop();
}
@Override
protected void onDestroy() {
Log.e(TAG, "onDestroy");
super.onDestroy();
}
}
|
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| public class MainActivity1 extends Activity {
private final static String TAG = MainActivity1.class.getSimpleName();
@Override
protected void onCreate(Bundle savedInstanceState) {
Log.e(TAG, "onCreate1");
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main1);
}
@Override
protected void onStart() {
Log.e(TAG, "onStart1");
super.onStart();
}
@Override
protected void onRestart() {
Log.e(TAG, "onRestart1");
super.onRestart();
}
@Override
protected void onResume() {
Log.e(TAG, "onResume1");
super.onResume();
}
@Override
public void onWindowFocusChanged(boolean hasFocus) {
Log.e(TAG, "window hasFocus1: " + hasFocus);
super.onWindowFocusChanged(hasFocus);
}
@Override
protected void onPause() {
Log.e(TAG, "onPause1");
super.onPause();
}
@Override
protected void onStop() {
Log.e(TAG, "onStop1");
super.onStop();
}
@Override
protected void onDestroy() {
Log.e(TAG, "onDestroy1");
super.onDestroy();
}
}
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从MainActivity跳转到MainActivity1
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| 01-25 17:53:56.161 5972 5972 E MainActivity: onPause
01-25 17:53:56.174 5972 5972 E MainActivity1: onCreate1
01-25 17:53:56.203 5972 5972 E MainActivity1: onStart1
01-25 17:53:56.204 5972 5972 E MainActivity1: onResume1
01-25 17:53:56.761 5972 5972 E MainActivity: onStop
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再从MainActivity1返回到MainActivity
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| 01-25 17:54:55.766 5972 5972 E MainActivity1: onPause1
01-25 17:54:55.773 5972 5972 E MainActivity: onRestart
01-25 17:54:55.773 5972 5972 E MainActivity: onStart
01-25 17:54:55.774 5972 5972 E MainActivity: onResume
01-25 17:54:56.302 5972 5972 E MainActivity1: onStop1
01-25 17:54:56.303 5972 5972 E MainActivity1: onDestroy1
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onWindowFocusChanged
在Android的Activity生命周期中，我们上面见到的都不是真正的visible点，而onWindowFocusChanged才是真正的visible点，这个函数当Activity获取到或者失去焦点的时候执行。
当执行这个函数的时候，Activity已经对用户可见并且可以交互了，在这之前执行的是onResume，之前对用户的交互是会受到一点限制的。
用处
- 检测Acticity是否加载完毕
- 获取手机屏幕的宽高
- 获取View组件的宽高
我们对上面MainActivity代码稍作修改，分别在onCreate、onResume、onWindowFocusChanged中获取TextView的宽高并且通过日志打印
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| public class MainActivity extends Activity {
private final static String TAG = MainActivity.class.getSimpleName();
TextView textView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textView = findViewById(R.id.tv_hello_world);
}
@Override
protected void onResume() {
super.onResume();
}
@Override
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
int width = textView.getWidth();
int height = textView.getHeight();
Log.e(TAG, "TextView width: " + width);
Log.e(TAG, "TextView height: " + height);
}
@Override
protected void onPause() {
super.onPause();
}
@Override
protected void onStop() {
super.onStop();
}
@Override
protected void onDestroy() {
super.onDestroy();
}
}
|
通过上述的例子发现，当TextView的宽高为wrap_content时，只有在onWindowFocusChanged中才能获取到其宽高，其他获取到的都为0。
失去焦点的情况
- 软键盘弹出
- 状态栏下拉
- 关闭Activity
- ……
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| @Override
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
}
|
参考链接
