自从Android 3.0中引入fragments 的概念,根据词海的翻译可以译为:碎片、片段。其上的是为了解决不同屏幕分辩率的动态和灵活UI设计。大屏幕如平板小屏幕如手机，平板电脑的设计使得其有更多的空间来放更多的UI组件，而多出来的空间存放UI使其会产生更多的交互，从而诞生了fragments 。fragments 的设计不需要你来亲自管理view hierarchy 的复杂变化，通过将Activity 的布局分散到frament 中，可以在运行时修改activity 的外观，并且由activity 管理的back stack 中保存些变化。
在设计应用时特别是Android 应用 ，有众多的分辨率要去适应，而fragments 可以让你在屏幕不同的屏幕上动态管理UI。例如：通讯应用程序（QQ），用户列表可以在左边，消息窗口在右边的设计。而在手机屏幕用户列表填充屏幕当点击某一用户时，则弹出对话窗口的设计，如下图：
对话框式的Fragments，可以将一个fragments 对话框并到activity 管理的fragments back stack 中，允许用户回到一个前曾摒弃fragments.
package com.xuzhi.fragment; import android.app.Activity;
import android.app.AlertDialog;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.app.ListFragment;
import android.os.Bundle;
import android.util.TypedValue;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.ScrollView;
import android.widget.TextView;
public class FragmentDemoActivity
extends Activity {
public static String[] array = { "text1,", "text2", "text3", "text4",
"text5,", "text6", "text7", "text8" };
/** Called when the activity is first created. */ @Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
public static class TitlesFragment
extends ListFragment {
boolean mDualPane;
int mCurCheckPosition = 0;
@Override
public void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub super.onCreate(savedInstanceState);
System.out.println("Fragment-->onCreate");
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// TODO Auto-generated method stub System.out.println("Fragment-->onCreateView");
return super.onCreateView(inflater, container, savedInstanceState);
}
@Override
public void onPause() {
// TODO Auto-generated method stub super.onPause();
System.out.println("Fragment-->onPause");
}
@Override
public void onStop() {
// TODO Auto-generated method stub super.onStop();
System.out.println("Fragment-->onStop");
}
@Override
public void onAttach(Activity activity) {
// TODO Auto-generated method stub super.onAttach(activity);
System.out.println("Fragment-->onAttach");
}
@Override
public void onStart() {
// TODO Auto-generated method stub super.onStart();
System.out.println("Fragment-->onStart");
}
@Override
public void onResume() {
// TODO Auto-generated method stub super.onResume();
System.out.println("Fragment-->onResume");
}
@Override
public void onDestroy() {
// TODO Auto-generated method stub super.onDestroy();
System.out.println("Fragment-->onDestroy");
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
// TODO Auto-generated method stub super.onActivityCreated(savedInstanceState);
System.out.println("Fragment-->onActivityCreted");
setListAdapter(
new ArrayAdapter<String>(getActivity(),
android.R.layout.simple_list_item_1, array));
View detailsFrame = getActivity().findViewById(R.id.details);
mDualPane = detailsFrame !=
null && detailsFrame.getVisibility() == View.VISIBLE;
if (savedInstanceState !=
null) {
mCurCheckPosition = savedInstanceState.getInt("curChoice", 0);
// 从保存的状态中取出数据 }
if (mDualPane) {
getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
showDetails(mCurCheckPosition);
}
}
@Override
public void onSaveInstanceState(Bundle outState) {
// TODO Auto-generated method stub super.onSaveInstanceState(outState);
outState.putInt("curChoice", mCurCheckPosition);
// 保存当前的下标 }
@Override
public void onListItemClick(ListView l, View v,
int position,
long id) {
// TODO Auto-generated method stub super.onListItemClick(l, v, position, id);
showDetails(position);
}
void showDetails(
int index) {
mCurCheckPosition = index;
if (mDualPane) {
getListView().setItemChecked(index,
true);
DetailsFragment details = (DetailsFragment) getFragmentManager()
.findFragmentById(R.id.details);
if (details ==
null || details.getShownIndex() != index) {
details = DetailsFragment.newInstance(mCurCheckPosition);
// 得到一个fragment 事务（类似sqlite的操作） FragmentTransaction ft = getFragmentManager()
.beginTransaction();
ft.replace(R.id.details, details);
// 将得到的fragment 替换当前的viewGroup内容，add则不替换会依次累加 ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
// 设置动画效果 ft.commit();
// 提交 }
}
else {
new AlertDialog.Builder(getActivity()).setTitle(
android.R.string.dialog_alert_title).setMessage(
array[index]).setPositiveButton(android.R.string.ok,
null).show();
}
}
}
/** * 作为界面的一部分，为fragment 提供一个layout * @author terry * */ public static class DetailsFragment
extends Fragment {
public static DetailsFragment newInstance(
int index) {
DetailsFragment details =
new DetailsFragment();
Bundle args =
new Bundle();
args.putInt("index", index);
details.setArguments(args);
return details;
}
public int getShownIndex() {
return getArguments().getInt("index", 0);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// TODO Auto-generated method stub if (container ==
null)
return null;
ScrollView scroller =
new ScrollView(getActivity());
TextView text =
new TextView(getActivity());
int padding = (
int) TypedValue.applyDimension(
TypedValue.COMPLEX_UNIT_DIP, 4, getActivity()
.getResources().getDisplayMetrics());
text.setPadding(padding, padding, padding, padding);
scroller.addView(text);
text.setText(array[getShownIndex()]);
return scroller;
}
}
}
Activity在加载一个新的Fragment时会重新生成菜单，也就是会再次调用onCreateOptionsMenu(Menu menu)。注意：不是每次加载Fragment都会重新调用onCreateOptionsMenu(Menu menu)。只有在要加载的Fragment上注明过setHasOptionsMenu(true);在加载到Activity是会调用onCreateOptionsMenu(Menu menu)。由此进行加载Fragment判断后就可以对不同的Fragment加载不同的菜单。同时也可以通过此方法恢复初始菜单项。
