Android 中为什么要用Fragment.setArguments(Bundle bundle)来传递参数

fragment在android3.0开始提供，并且在兼容包中也提供了fragment特性的支持。fragment的推出让我们编写和管理用户界面更快捷更方便了。

但当我们实例化自定义fragment时，为什么官方推荐fragment.setarguments(bundle bundle)这种方式来传递参数，而不推荐通过构造方法直接来传递参数呢？为了弄清这个问题，我们可以做一个测试，分别测试下这两种方式的不同

首先，我们来测试下通过构造方法传递参数的情况

public class framenttestactivity extends actionbaractivity { 
  @override 
  protected void oncreate(bundle savedinstancestate) { 
    super.oncreate(savedinstancestate); 
    setcontentview(r.layout.activity_main); 
    if (savedinstancestate == null) { 
      getsupportfragmentmanager().begintransaction() 
          .add(r.id.container, new testfragment("param")).commit(); 
    } 
  } 
  public static class testfragment extends fragment { 
    private string marg = "non-param"; 
    public testfragment() { 
      log.i("info", "testfragment non-parameter constructor"); 
    } 
    public testfragment(string arg){ 
      marg = arg; 
      log.i("info", "testfragment construct with parameter"); 
    } 
    @override 
    public view oncreateview(layoutinflater inflater, viewgroup container, 
        bundle savedinstancestate) { 
      view rootview = inflater.inflate(r.layout.fragment_main, container, 
          false); 
      textview tv = (textview) rootview.findviewbyid(r.id.tv); 
      tv.settext(marg); 
      return rootview; 
    } 
  } 
} 

可以看到我们传递过来的数据正确的显示了，现在来考虑一个问题，如果设备配置参数发生变化，这里以横竖屏切换来说明问题，显示如下

发生了什么问题呢？我们传递的参数哪去了？为什么会显示默认值？不急着讨论这个问题，接下来我们来看看fragment.setarguments(bundle bundle)这种方式的运行情况

public class framenttest2activity extends actionbaractivity { 
    @override 
    protected void oncreate(bundle savedinstancestate) { 
       super.oncreate(savedinstancestate); 
       setcontentview(r.layout. activity_main); 
       if (savedinstancestate == null) { 
          getsupportfragmentmanager().begintransaction() 
                 .add(r.id. container, testfragment.newinstance("param")).commit(); 
       } 
    } 
    public static class testfragment extends fragment { 
       private static final string arg = "arg"; 
       public testfragment() { 
          log. i("info", "testfragment non-parameter constructor" ); 
       } 
       public static fragment newinstance(string arg){ 
          testfragment fragment = new testfragment(); 
          bundle bundle = new bundle(); 
          bundle.putstring( arg, arg); 
          fragment.setarguments(bundle); 
           return fragment; 
       } 
       @override 
       public view oncreateview(layoutinflater inflater, viewgroup container, 
              bundle savedinstancestate) { 
          view rootview = inflater.inflate(r.layout. fragment_main, container, 
                 false); 
          textview tv = (textview) rootview.findviewbyid(r.id. tv); 
          tv.settext(getarguments().getstring( arg)); 
           return rootview; 
       } 
    } 
} 

我们再来看看横竖屏切换后的运行情况

看到了吧，我们传递的参数在横竖屏切换的情况下完好保存了下来，正确的显示给用户
那么这到底是怎么回事呢，我们知道设备横竖屏切换的话，当前展示给用户的activity默认情况下会重新创建并展现给用户，那依附于activity的fragment会进行如何处理呢，我们可以通过源码来查看

先来看看activity的oncreate(bundle saveinstance)方法

 protected void oncreate(bundle savedinstancestate) { 
  if (debug_lifecycle ) slog.v( tag, "oncreate " + this + ": " + savedinstancestate); 
  if (mlastnonconfigurationinstances != null) { 
    mallloadermanagers = mlastnonconfigurationinstances .loaders ; 
  } 
  if (mactivityinfo .parentactivityname != null) { 
    if (mactionbar == null) { 
      menabledefaultactionbarup = true ; 
    } else { 
      mactionbar .setdefaultdisplayhomeasupenabled( true); 
    } 
  } 
  if (savedinstancestate != null) { 
    parcelable p = savedinstancestate.getparcelable( fragments_tag ); 
    mfragments .restoreallstate(p, mlastnonconfigurationinstances != null 
        ? mlastnonconfigurationinstances .fragments : null); 
  } 
  mfragments .dispatchcreate(); 
  getapplication().dispatchactivitycreated( this , savedinstancestate); 
  mcalled = true ; 
} 

由于我们的fragment是由fragmentmanager来管理，所以可以跟进fragmentmanager.restoreallstate()方法，通过对当前活动的fragmnet找到下面的代码块

 for (int i=0; i<fms.mactive.length; i++) { 
      fragmentstate fs = fms.mactive[i]; 
      if (fs != null) { 
       fragment f = fs.instantiate(mactivity, mparent); 
        if (debug) log.v(tag, "restoreallstate: active #" + i + ": " + f); 
        mactive.add(f); 
        // now that the fragment is instantiated (or came from being 
        // retained above), clear minstance in case we end up re-restoring 
        // from this fragmentstate again. 
        fs.minstance = null; 
      } else { 
        mactive.add(null); 
        if (mavailindices == null) { 
          mavailindices = new arraylist<integer>(); 
        } 
        if (debug) log.v(tag, "restoreallstate: avail #" + i); 
        mavailindices.add(i); 
      } 
} 

接下来我们可以看看fragmentstate.instantitate()方法的实现

public fragment instantiate(activity activity, fragment parent) { 
    if (minstance != null) { 
      return minstance ; 
    } 
    if (marguments != null) { 
      marguments .setclassloader(activity.getclassloader()); 
    } 
    minstance = fragment.instantiate(activity, mclassname , marguments ); 
    if (msavedfragmentstate != null) { 
      msavedfragmentstate .setclassloader(activity.getclassloader()); 
      minstance .msavedfragmentstate = msavedfragmentstate ; 
    } 
    minstance .setindex(mindex , parent); 
    minstance .mfromlayout = mfromlayout ; 
    minstance .mrestored = true; 
    minstance .mfragmentid = mfragmentid ; 
    minstance .mcontainerid = mcontainerid ; 
    minstance .mtag = mtag ; 
    minstance .mretaininstance = mretaininstance ; 
    minstance .mdetached = mdetached ; 
    minstance .mfragmentmanager = activity.mfragments; 
    if (fragmentmanagerimpl.debug) log.v(fragmentmanagerimpl.tag, 
        "instantiated fragment " + minstance ); 
    return minstance ; 
  } 

可以看到最终转入到fragment.instantitate()方法

public static fragment instantiate(context context, string fname, bundle args) { 
  try { 
    class<?> clazz = sclassmap .get(fname); 
    if (clazz == null) { 
      // class not found in the cache, see if it's real, and try to add it 
      clazz = context.getclassloader().loadclass(fname); 
      sclassmap .put(fname, clazz); 
    } 
    fragment f = (fragment)clazz.newinstance(); 
    if (args != null) { 
      args.setclassloader(f.getclass().getclassloader()); 
      f. marguments = args; 
    } 
    return f; 
  } catch (classnotfoundexception e) { 
    throw new instantiationexception( "unable to instantiate fragment " + fname 
        + ": make sure class name exists, is public, and has an" 
        + " empty constructor that is public" , e); 
  } catch (java.lang.instantiationexception e) { 
    throw new instantiationexception( "unable to instantiate fragment " + fname 
        + ": make sure class name exists, is public, and has an" 
        + " empty constructor that is public" , e); 
  } catch (illegalaccessexception e) { 
    throw new instantiationexception( "unable to instantiate fragment " + fname 
        + ": make sure class name exists, is public, and has an" 
        + " empty constructor that is public" , e); 
  } 

通过此方法可以看到，最终会通过反射无参构造实例化一个新的fragment，并且给margments初始化为原先的值，而原来的fragment实例的数据都丢失了，并重新进行了初始化

通过上面的分析，我们可以知道activity重新创建时，会重新构建它所管理的fragment，原先的fragment的字段值将会全部丢失，但是通过fragment.setarguments(bundle bundle)方法设置的bundle会保留下来。所以尽量使用fragment.setarguments(bundle bundle)方式来传递参数

以上所述是小编给大家介绍的android 中为什么要用fragment.setarguments(bundle bundle)来传递参数，希望对大家有所帮助