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

2019-11-09 17:51:39

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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[java] view plain copy

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[java] view plain copy

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[java] view plain copy

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[html] view%20plain copy

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[html] view%20plain copy

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[java] view%20plain copy

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 (Illegal accessException 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

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