Fragment简介

最新推荐文章于 2021-08-15 00:37:52 发布

carterrao

最新推荐文章于 2021-08-15 00:37:52 发布

阅读量1.1k

点赞数

分类专栏： Android

Android 专栏收录该内容

24 篇文章

订阅专栏

以下所述内容参考自：http://developer.android.com/guide/components/fragments.html。

Android 3.0版本（API Level 11）开始引入Fragment，主要是为了更好的支持像平板这样的大屏幕设备中UI界面的显示与交互。Fragment可以看作是Activity界面的一个逻辑单元，类似于Activity布局中的一个控件，但与普通控件不同的时，它有类似于Activity的生命周期函数，用于在不同生命周期过程中进行相关处理，并且它是一个可重用的组件，可以在一个Activity中使用多个Fragment来构成UI界面，也可以在多个Activity界面中重用同一个Fragment，并且各Fragment之间还可以进行数据交互。例如一个新闻类的应用程序，主要包括两个逻辑界面：一个界面显示所有新闻的标题，另一个界面显示当前选中新闻的内容。可以把这两个逻辑界面包装为两个Fragment，当程序在平板电脑这样的大屏幕设备上运行时，可以在一个Activity中同时显示这两个Fragment，而在手机这样的小屏幕设备中，就使用两个Activity，每个Activity显示一个Fragment，如下图1所示。

图1

由于Fragment有自己的生命周期函数，它还可以不提供UI界面，而只是作为Activity的一个后台工作者。

要创建并显示一个Fragment界面，需要创建一个类，这个类继承Fragment类，或者继承DialogFragment、ListFragment、PreferenceFragment等，并重写其中的onCreateView()函数，在此函数最后返回要显示的内容的view对象，如果返回null值则Fragment就没有UI界面，如以下代码所示：

public static class ExampleFragment extends Fragment {

@Override

public View onCreateView(LayoutInflater inflater, ViewGroup container,

Bundle savedInstanceState) {

return inflater.inflate(R.layout.example_fragment, container, false);

}

上面的例子中，Fragment的具体显示内容定义在布局文件example_fragment.xml文件中，container代表的是当前Activity的布局，用inflater.inflate()函数就可以把Fragment的显示内容填充到当前Activity中，第三个参数用的false，因为这个参数代表是否要将Fragment附加到container中，执行inflate函数后就已经填充过了，没必要再附加一次，所以这里参数用false。如果在上面onCreateView()函数最后用的是return null，则这个Fragment没有UI显示内容，只作为Activity的后台工作者。如果创建的类继承的是ListFragment类，那也可以不重写onCreateView()函数，因为默认的onCreateView()函数实现中返回了一个ListView对象来显示Fragment的内容。

跟普通控件一样，也有两种方式将Fragment添加到Activity中，一种是在Activity的布局文件中用<fragment>标签来添加，如下所示：

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="horizontal"

android:layout_width="match_parent"

android:layout_height="match_parent">

<fragment android:name="com.example.news.ArticleListFragment"

android:id="@+id/list"

android:layout_weight="1"

android:layout_width="0dp"

android:layout_height="match_parent" />

<fragment android:name="com.example.news.ArticleReaderFragment"

android:id="@+id/viewer"

android:layout_weight="2"

android:layout_width="0dp"

android:layout_height="match_parent" />

</LinearLayout>

这里添加了两个fragment，fragment标签中的android:name属性的值是对应的Fragment的实现类，当Activity加载这个布局文件时，会依次初始化这两个Fragment实现类，调用其中重写的onCreateView()函数来显示Fragment的显示内容。另外也可以用代码的方式来添加Fragment到Activity中，如下代码所示：

FragmentManager fragmentManager = getFragmentManager();

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

ExampleFragment fragment = new ExampleFragment();

fragmentTransaction.add(R.id.fragment_container, fragment);

fragmentTransaction.commit();

第一行代码获取一个FragmentManager对象；第二行代码开始执行一个Fragment的创建事务；第三行代码创建要添加的Fragment对象；第四行代码将fragment对象添加到一个布局容器中，这里的fragment_container可以是Activity的布局中包含的任意一个ViewGroup对象，例如在Activity的布局文件中嵌入的一个LinearLayout；第五行代码是提交这次的添加事务，对Fragment操作后，包括添加和移除等，都要记得调用commit()函数来提交修改。除了add()事务操作外，还有remove()、replace()、addToBackStack()等事务操作。当在commit()执行前，如果调用了addToBackStack()，则之前的操作会加入到Activity为Fragment维护的回退堆栈中，当按返回键时会依次恢复执行的事务操作，如下代码所示：

Fragment newFragment = new ExampleFragment();

FragmentTransaction transaction = getFragmentManager().beginTransaction();

transaction.replace(R.id.fragment_container, newFragment);

transaction.addToBackStack(null);

transaction.commit();

在上面代码中，执行replace()将Fragment的内容替换为新的Fragment，由于接着将事务操作添加到了回退堆栈中，当上面代码执行完后，按下返回键，Fragment就会恢复到执行替换之前的内容。

当需要在Fragment类中访问其宿主Activity的其他控件时，可以使用以下方式：

View listView = getActivity().findViewById(R.id.list);

其中getActivity()函数返回Fragment的宿主Activity，list是Activity中的一个普通控件。同样，在Activity中也可以用以下方式来访问其包含的Fragment对象：

ExampleFragment fragment=(ExampleFragment) getFragmentManager().findFragmentById(R.id.example_fragment);或者

ExampleFragment fragment=(ExampleFragment) getFragmentManager().findFragmentByTag(fragment_tag);

在<fragment>标签中，可以为Fragment添加id或tag属性，以上两种方式分别是用的id和tag属性值来访问Fragment。

Fragment还可以附加选项菜单到其宿主Activity中，只要在Fragment的实现类中重写Fragment的onCreateOptionsMenu()函数，并在Fragment实现类的onCreate()函数中调用setHasOptionsMenu()，那么在Activity中按下菜单键时，Fragment的菜单项就会附加到Activity的菜单项后面，同时Fragment还可以重写onOptionsItemSelected()函数来响应菜单项选择事件。另外还可以调用registerForContextMenu()函数并重写onCreateContextMenu()函数来添加上下文菜单，用onContextItemSelected()来处理上下文菜单项选择时间。要注意的是，Fragment的宿主Activity优先处理菜单项选择事件，只有当Activity没有处理Fragment的菜单项时，Fragment的菜单项选择处理函数才会被调用。

Fragment的生命周期函数有点类似于Activity，也主要有三种状态：

Resumed：Fragment在当前运行的Activity中可见。

Paused：另一个Activity覆盖在Fragment的宿主Activity上，并且Fragment的宿主Activity依然可见。

Stopped：当Fragment的宿主Activity不可见，或者Fragment在宿主Activity中被移除后，就处于不可见状态。

同时，也像Activity那样，当程序进程被系统中止时，Fragment也用一个Bundle对象在其函数onSaveInstanceState()中保存数据，并在onCreate()、onCreateView()、onActivityCreated()这三个中的任意一个函数中恢复数据。Fragment的生命周期函数及其宿主Activity的生命周期函数对其影响如下图2、图3所示。

图2 图3

以下通过一个程序例子来演示Fragment的简单用法，程序的功能是在横屏时，左边区域显示莎士比亚的一些戏剧的标题，右边区域显示当前选中戏剧的内容概要；而在竖屏时，在一个Activity中显示标题，当选择一个标题项时，启动另一个Activity显示内容概要，以下列出程序的主要文件内容，包括manifest文件、布局文件和源代码文件，共5个文件：

\AndroidManifest.xml:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

package="com.study.fragment"

android:versionCode="1"

android:versionName="1.0" >

<uses-sdk

android:minSdkVersion="11"

android:targetSdkVersion="15" />

<application

android:icon="@drawable/ic_launcher"

android:label="@string/app_name"

android:theme="@style/AppTheme" >

<activity

android:name=".MainActivity"

android:label="@string/title_activity_main" >

<intent-filter>

</intent-filter>

</activity>

</application>

</manifest>

\res\ layout-land\fragment_layout.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="horizontal"

android:layout_width="match_parent"

android:layout_height="match_parent" >

<fragment class="com.study.fragment.MainActivity$TitlesFragment"

android:id="@+id/titles"

android:layout_weight="1"

android:layout_width="0px"

android:layout_height="match_parent" />

<FrameLayout android:id="@+id/details"

android:layout_weight="1"

android:layout_width="0px"

android:layout_height="match_parent"

</LinearLayout>

\res\ layout\fragment_layout.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="match_parent"

android:layout_height="match_parent" >

<fragment class="com.study.fragment.MainActivity$TitlesFragment"

android:id="@+id/titles"

android:layout_width="match_parent"

android:layout_height="match_parent" />

</LinearLayout>

\src\com\study\fragment\MainActivity.java:

package com.study.fragment;

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;

import android.app.Activity;

import android.app.Fragment;

import android.app.FragmentTransaction;

import android.app.ListFragment;

import android.content.Intent;

import android.content.res.Configuration;

public class MainActivity extends Activity {

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.fragment_layout);

}

public static class TitlesFragment extends ListFragment {

boolean mDualPane;

int mCurCheckPosition = 0;

@Override

public void onActivityCreated(Bundle savedInstanceState) {

super.onActivityCreated(savedInstanceState);

//Populate list with our static array of titles.

setListAdapter(new ArrayAdapter<String>(getActivity(),

android.R.layout.simple_list_item_activated_1, Shakespeare.TITLES));

/*Check to see if we have a frame in which to embed the details

fragment directly in the containing UI.

View detailsFrame = getActivity().findViewById(R.id.details);

mDualPane = detailsFrame != null && detailsFrame.getVisibility() == View.VISIBLE;

if (savedInstanceState != null) {

mCurCheckPosition = savedInstanceState.getInt("curChoice", 0);

}

if (mDualPane) {

//In dual-pane mode, the list view highlights the selected item.

getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);

//Make sure out UI is in the correct state.

showDetails(mCurCheckPosition);

}

@Override

public void onSaveInstanceState(Bundle outState) {

super.onSaveInstanceState(outState);

outState.putInt("curChoice", mCurCheckPosition);

}

@Override

public void onListItemClick(ListView l, View v, int position, long id) {

showDetails(position);

}

/**

* Helper function to show the details of a selected item, either by

* displaying a fragment in-place in the current UI, or starting a

* whole new activity in which it is displayed.

void showDetails(int index) {

mCurCheckPosition = index;

if (mDualPane) {

*We can display everything in-place with fragment, so update

*the list to highlight the selected item and show the data.

getListView().setItemChecked(index, true);

//Check what fragment is currently shown, replace if needed.

DetailsFragment details = (DetailsFragment)

getFragmentManager().findFragmentById(R.id.details);

if (details == null || details.getShownIndex() != index) {

//Make new fragment to show this selection.

details = DetailsFragment.newInstance(index);

/*Execute a transaction, replacing any existing fragment

* with this one inside the frame.

FragmentTransaction ft = getFragmentManager().beginTransaction();

ft.replace(R.id.details, details);

ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);

ft.commit();

}

} else {

/*Otherwise we need to launch a new activity to display the

* dialog fragment with selected text.

Intent intent = new Intent();

intent.setClass(getActivity(), DetailsActivity.class);

intent.putExtra("index", index);

startActivity(intent);

}

public static class DetailsFragment extends Fragment {

/**

* Create a new instance of DetailsFragment, initialized to

* show the text at 'index'.

public static DetailsFragment newInstance(int index) {

DetailsFragment f = new DetailsFragment();

// Supply index input as an argument.

Bundle args = new Bundle();

args.putInt("index", index);

f.setArguments(args);

return f;

}

public int getShownIndex() {

return getArguments().getInt("index", 0);

}

@Override

public View onCreateView(LayoutInflater inflater, ViewGroup container,

Bundle savedInstanceState) {

if (container == null) {

// We have different layouts, and in one of them this

// fragment's containing frame doesn't exist. The fragment

// may still be created from its saved state, but there is

// no reason to try to create its view hierarchy because it

// won't be displayed. Note this is not needed -- we could

// just run the code below, where we would create and return

// the view hierarchy; it would just never be used.

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(Shakespeare.DIALOGUE[getShownIndex()]);

return scroller;

}

public static class DetailsActivity extends Activity {

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

if (getResources().getConfiguration().orientation

== Configuration.ORIENTATION_LANDSCAPE) {

// If the screen is now in landscape mode, we can show the

// dialog in-line with the list so we don't need this activity.

finish();

return;

}

if (savedInstanceState == null) {

// During initial setup, plug in the details fragment.

DetailsFragment details = new DetailsFragment();

details.setArguments(getIntent().getExtras());

getFragmentManager().beginTransaction().add(android.R.id.content, details).commit();

}

\src\com\study\fragment\Shakespeare.java:

package com.study.fragment;

public final class Shakespeare {

/**

* Our data, part 1.

public static final String[] TITLES =

{

"Henry IV (1)",

"Henry V",

"Henry VIII",

"Richard II",

"Richard III",

"Merchant of Venice",

"Othello",

"King Lear"

};

/**

* Our data, part 2.

public static final String[] DIALOGUE =

{

"So shaken as we are, so wan with care," +

"Find we a time for frighted peace to pant," +

"And breathe short-winded accents of new broils" +

"To be commenced in strands afar remote." +

"No more the thirsty entrance of this soil" +

"Shall daub her lips with her own children's blood;" +

"Nor more shall trenching war channel her fields," +

"Nor bruise her flowerets with the armed hoofs" +

"Of hostile paces: those opposed eyes," +

"Which, like the meteors of a troubled heaven," +

"All of one nature, of one substance bred," +

"Did lately meet in the intestine shock" +

"And furious close of civil butchery" +

"Shall now, in mutual well-beseeming ranks," +

"March all one way and be no more opposed" +

"Against acquaintance, kindred and allies:" +

"The edge of war, like an ill-sheathed knife," +

"No more shall cut his master. Therefore, friends," +

"As far as to the sepulchre of Christ," +

"Whose soldier now, under whose blessed cross" +

"We are impressed and engaged to fight," +

"Forthwith a power of English shall we levy;" +

"Whose arms were moulded in their mothers' womb" +

"To chase these pagans in those holy fields" +

"Over whose acres walk'd those blessed feet" +

"Which fourteen hundred years ago were nail'd" +

"For our advantage on the bitter cross." +

"But this our purpose now is twelve month old," +

"And bootless 'tis to tell you we will go:" +

"Therefore we meet not now. Then let me hear" +

"Of you, my gentle cousin Westmoreland," +

"What yesternight our council did decree" +

"In forwarding this dear expedience.",

"Hear him but reason in divinity," +

"And all-admiring with an inward wish" +

"You would desire the king were made a prelate:" +

"Hear him debate of commonwealth affairs," +

"You would say it hath been all in all his study:" +

"List his discourse of war, and you shall hear" +

"A fearful battle render'd you in music:" +

"Turn him to any cause of policy," +

"The Gordian knot of it he will unloose," +

"Familiar as his garter: that, when he speaks," +

"The air, a charter'd libertine, is still," +

"And the mute wonder lurketh in men's ears," +

"To steal his sweet and honey'd sentences;" +

"So that the art and practic part of life" +

"Must be the mistress to this theoric:" +

"Which is a wonder how his grace should glean it," +

"Since his addiction was to courses vain," +

"His companies unletter'd, rude and shallow," +

"His hours fill'd up with riots, banquets, sports," +

"And never noted in him any study," +

"Any retirement, any sequestration" +

"From open haunts and popularity.",

"I come no more to make you laugh: things now," +

"That bear a weighty and a serious brow," +

"Sad, high, and working, full of state and woe," +

"Such noble scenes as draw the eye to flow," +

"We now present. Those that can pity, here" +

"May, if they think it well, let fall a tear;" +

"The subject will deserve it. Such as give" +

"Their money out of hope they may believe," +

"May here find truth too. Those that come to see" +

"Only a show or two, and so agree" +

"The play may pass, if they be still and willing," +

"I'll undertake may see away their shilling" +

"Richly in two short hours. Only they" +

"That come to hear a merry bawdy play," +

"A noise of targets, or to see a fellow" +

"In a long motley coat guarded with yellow," +

"Will be deceived; for, gentle hearers, know," +

"To rank our chosen truth with such a show" +

"As fool and fight is, beside forfeiting" +

"Our own brains, and the opinion that we bring," +

"To make that only true we now intend," +

"Will leave us never an understanding friend." +

"Therefore, for goodness' sake, and as you are known" +

"The first and happiest hearers of the town," +

"Be sad, as we would make ye: think ye see" +

"The very persons of our noble story" +

"As they were living; think you see them great," +

"And follow'd with the general throng and sweat" +

"Of thousand friends; then in a moment, see" +

"How soon this mightiness meets misery:" +

"And, if you can be merry then, I'll say" +

"A man may weep upon his wedding-day.",

"First, heaven be the record to my speech!" +

"In the devotion of a subject's love," +

"Tendering the precious safety of my prince," +

"And free from other misbegotten hate," +

"Come I appellant to this princely presence." +

"Now, Thomas Mowbray, do I turn to thee," +

"And mark my greeting well; for what I speak" +

"My body shall make good upon this earth," +

"Or my divine soul answer it in heaven." +

"Thou art a traitor and a miscreant," +

"Too good to be so and too bad to live," +

"Since the more fair and crystal is the sky," +

"The uglier seem the clouds that in it fly." +

"Once more, the more to aggravate the note," +

"With a foul traitor's name stuff I thy throat;" +

"And wish, so please my sovereign, ere I move," +

"What my tongue speaks my right drawn sword may prove.",

"Now is the winter of our discontent" +

"Made glorious summer by this sun of York;" +

"And all the clouds that lour'd upon our house" +

"In the deep bosom of the ocean buried." +

"Now are our brows bound with victorious wreaths;" +

"Our bruised arms hung up for monuments;" +

"Our stern alarums changed to merry meetings," +

"Our dreadful marches to delightful measures." +

"Grim-visaged war hath smooth'd his wrinkled front;" +

"And now, instead of mounting barded steeds" +

"To fright the souls of fearful adversaries," +

"He capers nimbly in a lady's chamber" +

"To the lascivious pleasing of a lute." +

"But I, that am not shaped for sportive tricks," +

"Nor made to court an amorous looking-glass;" +

"I, that am rudely stamp'd, and want love's majesty" +

"To strut before a wanton ambling nymph;" +

"I, that am curtail'd of this fair proportion," +

"Cheated of feature by dissembling nature," +

"Deformed, unfinish'd, sent before my time" +

"Into this breathing world, scarce half made up," +

"And that so lamely and unfashionable" +

"That dogs bark at me as I halt by them;" +

"Why, I, in this weak piping time of peace," +

"Have no delight to pass away the time," +

"Unless to spy my shadow in the sun" +

"And descant on mine own deformity:" +

"And therefore, since I cannot prove a lover," +

"To entertain these fair well-spoken days," +

"I am determined to prove a villain" +

"And hate the idle pleasures of these days." +

"Plots have I laid, inductions dangerous," +

"By drunken prophecies, libels and dreams," +

"To set my brother Clarence and the king" +

"In deadly hate the one against the other:" +

"And if King Edward be as true and just" +

"As I am subtle, false and treacherous," +

"This day should Clarence closely be mew'd up," +

"About a prophecy, which says that 'G'" +

"Of Edward's heirs the murderer shall be." +

"Dive, thoughts, down to my soul: here" +

"Clarence comes.",

"To bait fish withal: if it will feed nothing else," +

"it will feed my revenge. He hath disgraced me, and" +

"hindered me half a million; laughed at my losses," +

"mocked at my gains, scorned my nation, thwarted my" +

"bargains, cooled my friends, heated mine" +

"enemies; and what's his reason? I am a Jew. Hath" +

"not a Jew eyes? hath not a Jew hands, organs," +

"dimensions, senses, affections, passions? fed with" +

"the same food, hurt with the same weapons, subject" +

"to the same diseases, healed by the same means," +

"warmed and cooled by the same winter and summer, as" +

"a Christian is? If you prick us, do we not bleed?" +

"if you tickle us, do we not laugh? if you poison" +

"us, do we not die? and if you wrong us, shall we not" +

"revenge? If we are like you in the rest, we will" +

"resemble you in that. If a Jew wrong a Christian," +

"what is his humility? Revenge. If a Christian" +

"wrong a Jew, what should his sufferance be by" +

"Christian example? Why, revenge. The villany you" +

"teach me, I will execute, and it shall go hard but I" +

"will better the instruction.",

"Virtue! a fig! 'tis in ourselves that we are thus" +

"or thus. Our bodies are our gardens, to the which" +

"our wills are gardeners: so that if we will plant" +

"nettles, or sow lettuce, set hyssop and weed up" +

"thyme, supply it with one gender of herbs, or" +

"distract it with many, either to have it sterile" +

"with idleness, or manured with industry, why, the" +

"power and corrigible authority of this lies in our" +

"wills. If the balance of our lives had not one" +

"scale of reason to poise another of sensuality, the" +

"blood and baseness of our natures would conduct us" +

"to most preposterous conclusions: but we have" +

"reason to cool our raging motions, our carnal" +

"stings, our unbitted lusts, whereof I take this that" +

"you call love to be a sect or scion.",

"Blow, winds, and crack your cheeks! rage! blow!" +

"You cataracts and hurricanoes, spout" +

"Till you have drench'd our steeples, drown'd the cocks!" +

"You sulphurous and thought-executing fires," +

"Vaunt-couriers to oak-cleaving thunderbolts," +

"Singe my white head! And thou, all-shaking thunder," +

"Smite flat the thick rotundity o' the world!" +

"Crack nature's moulds, an germens spill at once," +

"That make ingrateful man!"

};

}