实例讲解Android应用开发中Fragment生命周期的控制
一、Fragment的生命周期初探
因为Fragment必须嵌入在Acitivity中使用,所以Fragment的生命周期和它所在的Activity是密切相关的。
如果Activity是暂停状态,其中所有的Fragment都是暂停状态;如果Activity是stopped状态,这个Activity中所有的Fragment都不能被启动;如果Activity被销毁,那么它其中的所有Fragment都会被销毁。
但是,当Activity在活动状态,可以独立控制Fragment的状态,比如加上或者移除Fragment。
当这样进行fragment transaction(转换)的时候,可以把fragment放入Activity的back stack中,这样用户就可以进行返回操作。
使用Fragment时,需要继承Fragment或者Fragment的子类(DialogFragment, ListFragment, PreferenceFragment, WebViewFragment),所以Fragment的代码看起来和Activity的类似。
每当创建一个Fragment时,首先添加以下三个回调方法:
onCreate():系统在创建Fragment的时候调用这个方法,这里应该初始化相关的组件,一些即便是被暂停或者被停止时依然需要保留的东西。
onCreateView():当第一次绘制Fragment的UI时系统调用这个方法,该方法将返回一个View,如果Fragment不提供 UI也可以返回null。注意,如果继承自ListFragment,onCreateView()默认的实现会返回一个ListView,所以不用自己 实现。
onPause():当用户离开Fragment时第一个调用这个方法,需要提交一些变化,因为用户很可能不再返回来。
将Fragment加载到Activity当中有两种方式:
方式一:添加Fragment到Activity的布局文件当中
方式二:在Activity的代码中动态添加Fragment(荐)
第一种方式虽然简单但灵活性不够。添加Fragment到Activity的布局文件当中,就等同于将Fragment及其视图与activity的视图绑定在一起,且在activity的生命周期过程中,无法切换fragment视图。
第二种方式比较复杂,但也是唯一一种可以在运行时控制fragment的方式(加载、移除、替换)。
二、生命周期控制实例
package com.goso.testapp; import android.app.Activity; import android.app.ListFragment; import android.os.Bundle; import android.util.Log; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.ArrayAdapter; import android.widget.ListView; /** * Demonstration of using ListFragment to show a list of items * from a canned array. */ public class FragmentListArray extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.e("HJJ", "Activity &&&& onCreate..."); // Create the list fragment and add it as our sole content. if (getFragmentManager().findFragmentById(android.R.id.content) == null) { ArrayListFragment list = new ArrayListFragment(); getFragmentManager().beginTransaction().add(android.R.id.content, list).commit(); } } @Override protected void onStart() { // TODO Auto-generated method stub super.onStart(); Log.e("HJJ", "Activity &&&& onStart..."); } @Override protected void onResume() { // TODO Auto-generated method stub super.onResume(); Log.e("HJJ", "Activity &&&& onResume..."); } @Override protected void onStop() { // TODO Auto-generated method stub super.onStop(); Log.e("HJJ", "Activity &&&& onStop..."); } @Override protected void onPause() { // TODO Auto-generated method stub super.onPause(); Log.e("HJJ", "Activity &&&& onPause..."); } @Override protected void onDestroy() { // TODO Auto-generated method stub super.onDestroy(); Log.e("HJJ", "Activity &&&& onDestroy..."); } public static class ArrayListFragment extends ListFragment { @Override public void onAttach(Activity activity) { // TODO Auto-generated method stub Log.e("HJJ", "ArrayListFragment **** onAttach..."); super.onAttach(activity); } @Override public void onCreate(Bundle savedInstanceState) { // TODO Auto-generated method stub Log.e("HJJ", "ArrayListFragment **** onCreate..."); super.onCreate(savedInstanceState); } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { // TODO Auto-generated method stub Log.e("HJJ", "ArrayListFragment **** onCreateView..."); return super.onCreateView(inflater, container, savedInstanceState); } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); Log.e("HJJ", "ArrayListFragment **** onActivityCreated..."); String[] array = new String[]{"C++", "JAVA", "PYTHON"}; setListAdapter(new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, array)); } @Override public void onStart() { // TODO Auto-generated method stub Log.e("HJJ", "ArrayListFragment **** onStart..."); super.onStart(); } @Override public void onResume() { Log.e("HJJ", "ArrayListFragment **** onResume..."); // TODO Auto-generated method stub super.onResume(); } @Override public void onPause() { Log.e("HJJ", "ArrayListFragment **** onPause..."); // TODO Auto-generated method stub super.onPause(); } @Override public void onStop() { Log.e("HJJ", "ArrayListFragment **** onStop..."); // TODO Auto-generated method stub super.onStop(); } @Override public void onDestroyView() { Log.e("HJJ", "ArrayListFragment **** onDestroyView..."); // TODO Auto-generated method stub super.onDestroyView(); } @Override public void onDestroy() { // TODO Auto-generated method stub Log.e("HJJ", "ArrayListFragment **** onDestroy..."); super.onDestroy(); } @Override public void onDetach() { Log.e("HJJ", "ArrayListFragment **** onDetach..."); // TODO Auto-generated method stub super.onDetach(); } @Override public void onListItemClick(ListView l, View v, int position, long id) { Log.i("FragmentList", "Item clicked: " + id); } } }
结果:
onCreate过程
01-22 15:30:28.091: E/HJJ(10315): Activity &&&& onCreate... 01-22 15:30:28.091: E/HJJ(10315): ArrayListFragment **** onAttach... 01-22 15:30:28.091: E/HJJ(10315): ArrayListFragment **** onCreate... 01-22 15:30:28.115: E/HJJ(10315): ArrayListFragment **** onCreateView... 01-22 15:30:28.123: E/HJJ(10315): ArrayListFragment **** onActivityCreated...
onStart过程
01-22 15:30:28.123: E/HJJ(10315): Activity &&&& onStart... 01-22 15:30:28.123: E/HJJ(10315): ArrayListFragment **** onStart...
onResume过程
01-22 15:30:28.123: E/HJJ(10315): Activity &&&& onResume... 01-22 15:30:28.123: E/HJJ(10315): ArrayListFragment **** onResume...
onPause过程
01-22 15:31:26.748: E/HJJ(10315): ArrayListFragment **** onPause... 01-22 15:31:26.748: E/HJJ(10315): Activity &&&& onPause...
onStop过程
01-22 15:31:27.638: E/HJJ(10315): ArrayListFragment **** onStop... 01-22 15:31:27.638: E/HJJ(10315): Activity &&&& onStop...
onStart过程
01-22 15:31:57.537: E/HJJ(10315): Activity &&&& onStart... 01-22 15:31:57.537: E/HJJ(10315): ArrayListFragment **** onStart...
onResume过程
01-22 15:31:57.537: E/HJJ(10315): Activity &&&& onResume... 01-22 15:31:57.537: E/HJJ(10315): ArrayListFragment **** onResume...
onPause过程
01-22 15:32:47.412: E/HJJ(10315): ArrayListFragment **** onPause... 01-22 15:32:47.412: E/HJJ(10315): Activity &&&& onPause...
onStop过程
01-22 15:32:47.865: E/HJJ(10315): ArrayListFragment **** onStop... 01-22 15:32:47.865: E/HJJ(10315): Activity &&&& onStop...
onDestroy过程
01-22 15:32:47.865: E/HJJ(10315): ArrayListFragment **** onDestroyView... 01-22 15:32:47.865: E/HJJ(10315): ArrayListFragment **** onDestroy... 01-22 15:32:47.865: E/HJJ(10315): ArrayListFragment **** onDetach... 01-22 15:32:47.865: E/HJJ(10315): Activity &&&& onDestroy...