Android：多片段设计，每个片段中的标签数量不同

Question

我有一个由一个活动组成的应用程序。所有其他屏幕都使用片段实现。根据导航抽屉中选择的项目，将加载相应的片段。其中一个视图（片段）有另一种做事方式。扫描QR码或手动填写表格。我不知道它是否违反了设计最佳做法，因为此视图应包含制表符，但其他视图则不包含。

http://cd8ba0b44a15c10065fd-24461f391e20b7336331d5789078af53.r23.cf1.rackcdn.com/xamarin.vanillaforums.com/FileUpload/d6/40b7173a09619d47c144ebf444feeb.png http://cd8ba0b44a15c10065fd-24461f391e20b7336331d5789078af53.r23.cf1.rackcdn.com/xamarin.vanillaforums.com/FileUpload/d3/f50fe292739faa65f70fef82a46a34.png

我也使用AppBarLayout，因为此控件实现了开箱即用的材质设计（高程）。如果我在TabLayout中实施PairingFragment，那么在视觉上标签会被放置在工具栏的阴影下。所以我已将TabLayout移至MainActivity。

MainLayout.axml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/MainDrawerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true">
    <android.support.design.widget.CoordinatorLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true">
        <android.support.design.widget.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">
          <android.support.v7.widget.Toolbar
              android:id="@+id/MainToolbar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:minHeight="?attr/actionBarSize" />
          <android.support.design.widget.TabLayout
                  android:id="@+id/MainTabLayout"
                  android:layout_width="match_parent"
                  android:layout_height="wrap_content" />
        </android.support.design.widget.AppBarLayout>
        <RelativeLayout
            android:id="@+id/MainContentRelativeLayout"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior" />
    </android.support.design.widget.CoordinatorLayout>
    <android.support.design.widget.NavigationView
        android:id="@+id/MainDrawerNavigationView"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:menu="@menu/main_drawerlayout_menu" />
</android.support.v4.widget.DrawerLayout>

注意：这是一个Xamarin项目。我不认为Java开发人员很难读取C＃代码。

MainActivity.cs

using Support4 = Android.Support.V4;
using Support7 = Android.Support.V7;

[Activity(MainLauncher = true, Icon = "@drawable/icon")]
public class MainActivity : Support7.App.AppCompatActivity, NavigationView.IOnNavigationItemSelectedListener
{
    private TabLayout mainTabLayout;
    private RelativeLayout mainRelativeLayout;
    private NavigationView mainDrawerNavigationView;

    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        this.SetContentView(Resource.Layout.MainLayout);

        this.mainTabLayout = this.FindViewById<TabLayout>(Resource.Id.MainTabLayout);
        this.mainRelativeLayout = this.FindViewById<RelativeLayout>(Resource.Id.MainContentRelativeLayout);
        this.mainDrawerNavigationView = this.FindViewById<NavigationView>(Resource.Id.MainDrawerNavigationView);

        this.mainDrawerNavigationView.SetNavigationItemSelectedListener(this);

        if (savedInstanceState == null)
        {
            this.ShowFragment(new DefaultFragment(this.mainTabLayout));
        }
        else
        {
            // Configuration change. The last fragment was recreated by the runtime
            // using the fragment's default constructor.
        }
    }

    public bool OnNavigationItemSelected(IMenuItem menuItem)
    {
        if (menuItem.ItemId == Resource.Id.DefaultDrawerMenuItem)
        {
            this.ShowFragment(new DefaultFragment(this.mainTabLayout));
        }
        else if (menuItem.ItemId == Resource.Id.PairingDrawerMenuItem)
        {
            this.ShowFragment(new PairingFragment(this.mainTabLayout));
        }

        this.mainDrawerLayout.CloseDrawer(Support4.View.GravityCompat.Start);
        return true;
    }

    private void ShowFragment(Support4.App.Fragment fragment)
    {
        Support4.App.FragmentManager fm = this.SupportFragmentManager;
        fm.BeginTransaction().Replace(this.mainRelativeLayout.Id, fragment).Commit();
    }
}

PairingFragment.cs

public class PairingFragment : Support4.App.Fragment
{
    private TabLayout tabLayout;

    public PairingFragment()
    {
        // This constructor is needed by the runtime because of
        // calling base.OnCreate(savedInstanceState) within
        // MainActivity.OnCreate(Bundle savedInstanceState) calls
        // this constructor.
    }

    public PairingFragment(TabLayout tabLayout)
    {
        this.tabLayout = tabLayout;
    }

    public override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
    }

    public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        this.tabLayout.Visibility = ViewStates.Visible;

        View view = inflater.Inflate(Resource.Layout.PairingFragment, container, false);

        Support4.View.ViewPager pairingViewPager = view.FindViewById<Support4.View.ViewPager>(Resource.Id.PairingViewPager);

        pairingViewPager.Adapter = new PairingPagerAdapter(this.ChildFragmentManager);
        this.tabLayout.SetupWithViewPager(pairingViewPager);
        return view;
    }
}

public class PairingPagerAdapter : Support4.App.FragmentStatePagerAdapter
{
    public PairingPagerAdapter(Support4.App.FragmentManager fragmentManager)
        : base(fragmentManager)
    {
    }

    // Removed code for readability
}

正如您所见，TabLayout通过构造函数注入到片段中。没有标签的片段会将注入的TabLayout的可见性设置为ViewStates.Gone。

只要没有配置更改，这就足够了。如果有，那么活动将被重新创建。在这种情况下，片段不会通过构造函数注入，代码会因NullReferenceException而中断。

我该怎么办？我应该改变应用程序的设计吗？我不认为这是一个设计缺陷。如果有配置更改，我应该获取实际片段，然后注入TabLayout via方法或属性？这将在今天工作，因为总有一个片段。但是当我明天需要多个片段时会发生什么？检查每个片段＆＃34;是否可以处理TabLayout＆＃34;似乎不对。

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