为什么要用newInstance来实例化Fragment？

看了些文章，感觉讲的有疑问，看源码研究了下。先说结论，Android 在恢复Fragment时，调用的是无参的构造函数，如果使用new的方式构造Fragment，Fragment重新构建会丢失状态。使用newInstance方式会将参数存储在mArguments中，在Fragment恢复时，会从mArguments中取得参数恢复状态。
过程分两步：Fragment销毁和Fragment恢复。
1、Fragment销毁
在屏幕旋转等状态下，FragmentActivity会调用onSaveInstanceState方法:

protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        this.markFragmentsCreated();
        //取得Fragment相关的参数
        Parcelable p = this.mFragments.saveAllState();
        if (p != null) {
            //参数存Bundle，与之对应，之后在FragmentActivity的onCreate里取得Bundle
            outState.putParcelable("android:support:fragments", p);
        }
         ...................
}

接着看看mFragments.saveAllState()，通过FragmentController中转了下，最后调用FragmentManager的saveAllState方法：

//忽略不相关代码，只看参数保存部分
Parcelable saveAllState() {
    //N代表Fragment的个数
    FragmentState[] active = new FragmentState[N];
    //取得Fragment对象，i是遍历列表的索引
     Fragment f = (Fragment)this.mActive.valueAt(i);
    //重点在这里，将该Fragment的mArguments赋值给新建的
    //FragmentState对象，new FragmentState只是简单的数据copy
     FragmentState fs = new FragmentState(f);
     active[i] = fs;
    //构建FragmentManagerState类型的对象fms，mActive参数赋值为
    //FragmentState，这样fms将会有Fragment的mArguments参数。
     FragmentManagerState fms = new FragmentManagerState();
     fms.mActive = active;
     return fms;
}

总结上面，就是在FragmentActivity销毁前，调用onSaveInstanceState方法，里面遍历包含的Fragment，将Fragment的mArguments一层层封装，最后保存在outState的Bundle对象里用于Fragment恢复使用。
2、Fragment恢复
首先FragmentActivity的调用生命周期方法：

@Override
protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      if (savedInstanceState != null) {
           //取得之前销毁时调用onSaveInstanceState保存的Bundle
            Parcelable p =savedInstanceState.getParcelable("android:support:fragments");
 //恢复Fragment状态
 this.mFragments.restoreAllState(p, nc != null ? nc.fragments : null);
    }
}

同样通过FragmentController中转了下，最后调用FragmentManager的restoreAllState方法：

    void restoreAllState(Parcelable state, FragmentManagerNonConfig nonConfig) {
     //将state转化成之前保存mArguments的类型FragmentManagerState
     FragmentManagerState fms = (FragmentManagerState)state;
    //很熟悉了，fs里面有mArguments
     FragmentState fs = fms.mActive[index];
//构建Fragment
 Fragment f = fs.instantiate(this.mHost, this.mContainer, this.mParent, childNonConfig, viewModelStore);
}

看看fs.instantiate做了什么:

public Fragment instantiate(FragmentHostCallback host, FragmentContainer container, Fragment parent, FragmentManagerNonConfig childNonConfig, ViewModelStore viewModelStore) {
//mInstance就是Fragment对象，mArguments就是之前保存的参数
this.mInstance = Fragment.instantiate(context, this.mClassName, this.mArguments);
}

终于到了最后一步了，Fragment对象的类名知道，mArguments也知道了，可以创建对象，并恢复参数了，看看Fragment.instantiate：

public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
        try {
            Class<?> clazz = (Class)sClassMap.get(fname);
            if (clazz == null) {
                clazz = context.getClassLoader().loadClass(fname);
                sClassMap.put(fname, clazz);
            }

            Fragment f = (Fragment)clazz.getConstructor().newInstance();
            //恢复参数的位置在这里
            if (args != null) {
                args.setClassLoader(f.getClass().getClassLoader());
                f.setArguments(args);
            }

            return f;
        } catch (ClassNotFoundException var5) {
            throw new Fragment.InstantiationException("Unable to instantiate fragment " + fname + ": make sure class name exists, is public, and has an" + " empty constructor that is public", var5);
        } catch (java.lang.InstantiationException var6) {
            throw new Fragment.InstantiationException("Unable to instantiate fragment " + fname + ": make sure class name exists, is public, and has an" + " empty constructor that is public", var6);
        } catch (IllegalAccessException var7) {
            throw new Fragment.InstantiationException("Unable to instantiate fragment " + fname + ": make sure class name exists, is public, and has an" + " empty constructor that is public", var7);
        } catch (NoSuchMethodException var8) {
            throw new Fragment.InstantiationException("Unable to instantiate fragment " + fname + ": could not find Fragment constructor", var8);
        } catch (InvocationTargetException var9) {
            throw new Fragment.InstantiationException("Unable to instantiate fragment " + fname + ": calling Fragment constructor caused an exception", var9);
        }
    }

可以看到，反射调用构建出Fragment后，在通过setArguments恢复参数。