【Android面试】为什么要用newInstance来实例化F

Android日常研发中不可避免的肯定要用到Fragment，你如何使用的呢？Compare the two methods of use，是否觉得第二种更加简洁。这时很多人肯定提出疑问：这两种使用方式有何区别，我的代码中到底使用哪种方式更好一些，以及为什么要使用这种方式 and so on，各位看官稍安勿躁，且听老衲娓娓道来。

Usage 1：

@Override
    public void initView(Bundle savedInstanceState) {
        BlankFragment mFragment = new BlankFragment();
        Bundle bundle = new Bundle();
        bundle.putString("arg1", "a");
        bundle.putString("arg2", "b");
        bundle.putString("arg3", "c");
        mFragment.setArguments(bundle);
        getFragmentManager().beginTransaction().replace(R.id.frame, mFragment).commit();
    }

Usage 2：

@Override
    public void initView(Bundle savedInstanceState) {
        getFragmentManager().beginTransaction().replace(R.id.frame, BlankFragment.newInstance("a", "b")).commit();
    }

首先我们新建一个fragment，我们一起来看一下android建议的fragment如何编写（请严格按照截图的来步步创建哦）

Create BlankFragment.png

package com.itbird.utils;

import android.content.Context;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.itbird.base.R;

/**
 * A simple {@link Fragment} subclass.
 * Activities that contain this fragment must implement the
 * {@link BlankFragment.OnFragmentInteractionListener} interface
 * to handle interaction events.
 * Use the {@link BlankFragment#newInstance} factory method to
 * create an instance of this fragment.
 */
public class BlankFragment extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    private OnFragmentInteractionListener mListener;

    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);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_blank, container, false);
    }

    // TODO: Rename method, update argument and hook method into UI event
    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnFragmentInteractionListener) {
            mListener = (OnFragmentInteractionListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

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。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。

实践是检验真理的唯一标准，我们一起通过一个样例来实际操作一番
fragment_main.xml

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

fragment_main.png

由图和代码可知，我们在xml中定义两个FrameLayout，平分整个屏幕高度

MainActivity.java

package com.itbird.myapplication;

import android.os.Bundle;
import android.support.v4.app.FragmentTransaction;


public class MainActivity extends BaseActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_main);
        if (savedInstanceState == null) {
            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
            transaction.add(R.id.layout_top, new BlankFragment("顶部的Fragment", "test"));
            transaction.add(R.id.layout_bottom, BlankFragment.newInstance("底部的Fragment", "test"));
            transaction.commit();
        }
    }
}

BlankFragment.java

package com.itbird.myapplication;

import android.annotation.SuppressLint;
import android.content.Context;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class BlankFragment extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;


    public BlankFragment() {
        // Required empty public constructor
    }

    @SuppressLint("ValidFragment")
    public BlankFragment(String mParam1, String mParam2) {
        this.mParam1 = mParam1;
        this.mParam2 = mParam2;
    }
    /**
     * 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);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_blank, container, false);
        TextView textView = view.findViewById(R.id.text);
        textView.setText(mParam1 + mParam2);
        return view;
    }
}

fragment_blank.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/text"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center" />

</FrameLayout>

通过阅读代码可知，我们通过两种不同的方式创建fragment，同样在其中心textview中展示相应拼接字段。

运行效果.png

嗯，效果如预期的一样完美，此时，我们把屏幕横过来，看看会出现怎样的状况

横屏时.png

My god，顶部的fragment 文本内容咋都变成null了。。。
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);
        }
        mFragments.dispatchCreate();
        getApplication().dispatchActivityCreated(this, savedInstanceState);
        if (mVoiceInteractor != null) {
            mVoiceInteractor.attachActivity(this);
        }
        mCalled = true;
    }

显而易见，fragment的重建是在restoreAllState方法中，跟进
FragmentController.java

 /**
     * Restores the saved state for all Fragments. The given FragmentManagerNonConfig are Fragment
     * instances retained across configuration changes, including nested fragments
     *
     * @see #retainNestedNonConfig()
     */
    public void restoreAllState(Parcelable state, FragmentManagerNonConfig nonConfig) {
        mHost.mFragmentManager.restoreAllState(state, nonConfig);
    }

继续跟进
FragmentManager.java

 void restoreAllState(Parcelable state, FragmentManagerNonConfig nonConfig) {
        // If there is no saved state at all, then there can not be
        // any nonConfig fragments either, so that is that.
        if (state == null) return;
        FragmentManagerState fms = (FragmentManagerState)state;
        if (fms.mActive == null) return;

        List<FragmentManagerNonConfig> childNonConfigs = null;

        // First re-attach any non-config instances we are retaining back
        // to their saved state, so we don't try to instantiate them again.
        ...
        
        // Build the full list of active fragments, instantiating them from
        // their saved state.
        mActive = new ArrayList<>(fms.mActive.length);
        if (mAvailIndices != null) {
            mAvailIndices.clear();
        }
        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);
                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<>();
                }
                if (DEBUG) Log.v(TAG, "restoreAllState: avail #" + i);
                mAvailIndices.add(i);
            }
        }
        
        // Update the target of all retained fragments.
       ...

        // Build the list of currently added fragments.
       ...
        
        // Build the back stack.
        ...
    }

通过阅读， 找到关键代码

 Fragment f = fs.instantiate(mHost, mParent, childNonConfig);

然后锲而不舍跟进
FragmentManager.java

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);

            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;
    }

跟进到这里，终于有点头绪了，至少看到fragment实例化的地方了，迫不及待的再次点击去view一下下
Fragment.java

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);
                if (!Fragment.class.isAssignableFrom(clazz)) {
                    throw new InstantiationException("Trying to instantiate a class " + fname
                            + " that is not a Fragment", new ClassCastException());
                }
                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);
        }
    }

山重水复疑无路，柳暗花明又一村
原来Fragment对象被反射创建之后，会调用这么一句代码
f.mArguments = args;
哦，なるほど(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。

总结

1.通过对比两种使用方式，我们知道两种方式别无其他，只是事关风格而已（代码”整”“洁”之道）
2.使用Fragment过程中在涉及到传参时，千万不要通过构造方法或者setParam方式直接赋值传入参数，必须使用setArguments来传参，否则程序在某些应用情景下，会丢参
强烈建议：两者虽无严格的对错之分，都可以使用，但是newInstance方式无论从代码整洁之道还是程序规范的稳定性而言，都是每个程序员应该学习使用的方式。