Android 从源码的角度分析——为什么要用newInstance来实例化Fragment
最近在看Google技术文档的时候发现了一种新的方式来实例化Fragment，就是采用静态工厂的方式创建Fragment。我们在使用Android studio创建一个类的时候，选择New ->Fragment->Fragment(Blank)可以很直观的看到这种方式的写法：
public class BlankFragment extends Fragment {
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";
private String mParam1;
private String mParam2;
public BlankFragment() {
// Required empty public constructor
}
/** * Use this factory method to create a new instance of * this fragment using the provided parameters. * * @param param1 Parameter 1. * @param param2 Parameter 2. * @return A new instance of fragment BlankFragment. */
// TODO: Rename and change types and number of parameters
public static BlankFragment newInstance(String param1, String param2) {
BlankFragment fragment = new BlankFragment();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
}
}上述代码其实就是在一个Fragment的newInstance方法中传递两个参数，并且通过fragment.setArgument保存在它自己身上，而后通过onCreate()调用的时候将这些参数取出来。
可能有人乍一看，这样写没什么特殊的啊，不就是用静态工厂方法传个参数么，用构造器传参数不一样处理么？No，No，No，如果仅仅是个静态工厂而已，又怎么能成为谷歌推荐呢。
我们先来看一个小例子：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" >
<FrameLayout android:id="@+id/layout_top" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1"/>
<FrameLayout android:id="@+id/layout_bottom" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1"/>
</LinearLayout>在xml中定义两个FrameLayout，平分整个屏幕高度。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if(savedInstanceState == null){
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.add(R.id.layout_top,new TopFragment("顶部的Fragment"));
transaction.add(R.id.layout_bottom,BottomFragment.newInstance("底部的Fragment"));
transaction.commit();
}
}在activity中采用两种不同的方式来实例化Fragment，顶部的Fragment通过构造方法将参数传递给它，而底部的Fragment通过newInstance的方式实例化并传参。两个Fragment的代码如下所示：
public class TopFragment extends Fragment {
private String mTop = "啥也没有";
public TopFragment(){
}
public TopFragment(String top) {
this.mTop = top;
}
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
TextView tv = new TextView(getActivity());
tv.setText(mTop);
tv.setGravity(Gravity.CENTER);
tv.setTextColor(Color.RED);
tv.setTextSize(25);
return tv;
}
}public class BottomFragment extends Fragment {
private String mBottom = "啥也没有";
public static BottomFragment newInstance(String bottom) {
BottomFragment fragment = new BottomFragment();
Bundle bundle = new Bundle();
bundle.putString("bottom",bottom);
fragment.setArguments(bundle);
return fragment;
}
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(getArguments() != null){
mBottom = getArguments().getString("bottom");
}
}
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
TextView tv = new TextView(getActivity());
tv.setText(mBottom);
tv.setGravity(Gravity.CENTER);
tv.setTextColor(Color.RED);
tv.setTextSize(25);
return tv;
}
}在两个Fragment第一行都写一个相同的默认参数：“啥也没有”，ok，运行工程：
嗯，没毛病，两个Fragment都顺利的接收到来自activity的数据。然后我们把屏幕横过来，看看会出现怎样的状况：
咦。。。顶部的Fragment的数据呢？为什么只显示默认的数据？activity给它传过去的数据哪去了呢？
我们来分析一下产生上述情况的原因：当我们横竖屏切换的时候，activity会重建，相应的，依附于它上面的Fragment也会重新创建。好，顺着这个思路，进activity的onCreate方法中看看：
protected void onCreate(@Nullable Bundle savedInstanceState) {
if (DEBUG_LIFECYCLE) Slog.v(TAG, "onCreate " + this + ": " + savedInstanceState);
if (mLastNonConfigurationInstances != null) {
mFragments.restoreLoaderNonConfig(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);//这里，会恢复所有Fragment的状态
}
mFragments.dispatchCreate();
getApplication().dispatchActivityCreated(this, savedInstanceState);
if (mVoiceInteractor != null) {
mVoiceInteractor.attachActivity(this);
}
mCalled = true;
}显而易见，所有Fragment的状态恢复应该是在mFragments.restoreAllState()这个方法，跟进去看看：
public void restoreAllState(Parcelable state, List<Fragment> nonConfigList) {
mHost.mFragmentManager.restoreAllState(state, nonConfigList);
}找到FragmentManager这个类，查看它的restoreAllState方法：
void restoreAllState(Parcelable state, FragmentManagerNonConfig nonConfig) {
...
for (int i=0; i<fms.mActive.length; i++) {
FragmentState fs = fms.mActive[i];
if (fs != null) {
FragmentManagerNonConfig childNonConfig = null;
if (childNonConfigs != null && i < childNonConfigs.size()) {
childNonConfig = childNonConfigs.get(i);
}
Fragment f = fs.instantiate(mHost, mParent, childNonConfig);//实例化
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);
}
}
...
}寻找关键代码，我们发现了Fragment f = fs.instantiate(mHost, mParent, childNonConfig);这句，这里应该是Fragment重新实例化的地方了吧，赶紧点进去瞧瞧：
public Fragment instantiate(FragmentHostCallback host, Fragment parent,
FragmentManagerNonConfig childNonConfig) {
if (mInstance == null) {
final Context context = host.getContext();
if (mArguments != null) {
mArguments.setClassLoader(context.getClassLoader());
}
mInstance = Fragment.instantiate(context, mClassName, mArguments);//创建Fragment对象的地方
if (mSavedFragmentState != null) {
mSavedFragmentState.setClassLoader(context.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.mHidden = mHidden;
mInstance.mFragmentManager = host.mFragmentManager;
if (FragmentManagerImpl.DEBUG) Log.v(FragmentManagerImpl.TAG,
"Instantiated fragment " + mInstance);
}
mInstance.mChildNonConfig = childNonConfig;
return mInstance;
}继续跟进mInstance = Fragment.instantiate(context, mClassName, mArguments);看看里面的真正实现：
public static Fragment instantiate(Context context, String fname, @Nullable 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;//将之前设置的参数保存在自己身上
}
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);
}
}终于走到了Fragment最终被实例化创建的地方，我们可以看到Fragment对象被反射创建之后，会调用这么一句代码：f.mArguments = args; 哦，原来如此，Fragment在重新创建的时候只会调用无参的构造方法，并且如果之前通过fragment.setArguments(bundle)这种方式设置过参数的话，Fragment重建时会得到这些参数，所以，在onCreate中我们可以通过getArguments()的方式拿到我们之前设置的参数。同时由于Fragment在重建时并不会调用我们自定义的带参数的构造方法，所以我们传递的参数它也就获取不到了，这就是为什么会出现上述情况的原因。
细心的童鞋可以发现，上面的代码在catch语句当中抛出了几个异常，意思是：在Fragment重建过程中，确保你的Fragment的类是public的，并且带有一个public的空参的构造器，否则就让你崩溃~~~
好了，抛弃之前那些不好的代码习惯吧，支持谷歌，拥抱变化。