Question

我正在通过Android为初学者预订。尽管它于2015年12月发布，但当然有各个部分已经过时了。

我正在开发第一个使用导航抽屉的应用程序。原始实现要求使用ListView和Adapter显示导航项。切换片段是通过将整数值传递给方法来处理的：

mNavDrawerList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
     @Override
     public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
     // pass the position of the clicked item to switchFragment
     switchFragment(position);
}

然而，我发现虽然这种方法有效，但设置导航抽屉的旧方法意味着它有显示问题，因此抽屉出现在操作栏下方。在研究了这个问题后，我发现在Lollipop中有一种处理导航抽屉的新方法。所以我现在已经将代码切换到该方法：

mNavigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
     @Override
     public boolean onNavigationItemSelected(MenuItem item) {
            switchFragment(item);
            return true;
       }
});

以下是switchFragment()：

private void switchFragment(MenuItem item){

    Fragment fragment = null;
    String fragmentID = "";

    // handle the choice the user presses in the drawer
    switch(item.getItemId()){
        case R.id.nav_titles:
            fragmentID = "TITLES";
            Bundle args = new Bundle();
            args.putString("Tag","_NO_TAG");
            fragment = new TitlesFragment();
            fragment.setArguments(args);
            break;

        case R.id.nav_tags:
            fragmentID = "TAGS";
            fragment = new TagsFragment();
            break;

        case R.id.nav_capture:
            fragmentID = "CAPTURE";
            fragment = new CaptureFragment();
            break;

        default:
            break;
    }

    // set up a fragment manager and open the fragment
    FragmentManager fragmentManager = getFragmentManager();
    fragmentManager.beginTransaction()
            .replace(R.id.fragmentHolder, fragment, fragmentID).commit();

    // Close the drawer
    mDrawerLayout.closeDrawer(mNavigationView);
}

标题片段是默认视图，因此在我的MainActivity中的一些位置，例如在onCreate()方法的末尾，按下后退按钮，我最初使用了{{1自动加载此视图。但是，现在我已经切换到NavigationView版本，我自然不能这样做。

我已尝试将成员变量设置并绑定到默认项目：

switchFragment(0)

（其中mDefaultFrag = (MenuItem) findViewById(R.id.nav_titles);是nav_titles中的项目），并将其传递给：{/ p>

menu.xml

但是会引发错误：

switchFragment(mDefaultFrag)

我猜因为它不是一个视图。那么如何抓取默认视图选项以传递给Attempt to invoke interface method 'int android.view.MenuItem.getItemId()' on a null object reference？

编辑：

导航菜单项xml（存储在菜单＆gt; nav.xml中）

switchFragment

content_main.xml中的DrawerLayout：

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_titles"

            android:title="Titles" />
        <item
            android:id="@+id/nav_tags"

            android:title="Tags" />

        <item
            android:id="@+id/nav_capture"
            android:title="Capture" />
    </group>
</menu>

OnCreateOptionsMenu：

 <android.support.design.widget.NavigationView
            android:id="@+id/navigation"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:layout_gravity="start"
            app:menu="@menu/nav" /> <!-- The items to display -->

添加之后，我意识到正在膨胀的菜单是设置菜单，而不是导航抽屉，所以当然它不会找到@Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.menu_main, menu); mDefaultFrag = menu.findItem(R.id.nav_titles); switchFragment(mDefaultFrag); return true; }中的项目。

Answer 1

Menu次观看是在与onCreate()不同的阶段创建的。请尝试以下代码来获取您的菜单ID：

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(yourMenuID, menu);

    mDefaultFrag = menu.findItem(R.id.nav_titles);
    return true;
}

更新

你让我想到了......为什么你不能在onCreate()中自己用默认的片段替换片段？所以，只需在onCreate()：

中执行此操作即可

FragmentManager fragmentManager = getFragmentManager();
fragmentManager.beginTransaction()
               .replace(R.id.fragmentHolder, new TitlesFragment(), "TITLES")
               .commit();

这将为您提供所需的默认片段。

Answer 2

使用MenuItems实现导航抽屉。

这是一个工作样本。你可以随心所欲地更新它。

创建activity_main.xml

<?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"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">

    <include
        layout="@layout/app_bar_main"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <android.support.design.widget.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_main"
        app:menu="@menu/activity_main_drawer" />

</android.support.v4.widget.DrawerLayout>

然后创建app_bar_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="com.root5solutions.myapplication.MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>



    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:src="@android:drawable/ic_dialog_email" />

</android.support.design.widget.CoordinatorLayout>

然后创建nav_header_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="@dimen/nav_header_height"
    android:background="@drawable/side_nav_bar"
    android:gravity="bottom"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:theme="@style/ThemeOverlay.AppCompat.Dark">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/nav_header_vertical_spacing"
        android:src="@android:drawable/sym_def_app_icon" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/nav_header_vertical_spacing"
        android:text="Android Studio"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="android.studio@android.com" />

</LinearLayout>

然后创建一个菜单xml并将其命名为res-＆gt;中的activity_main_drawer。菜单 - ＆gt; activity_main_drawer。

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_camera"
            android:icon="@drawable/ic_menu_camera"
            android:title="Import" />
        <item
            android:id="@+id/nav_gallery"
            android:icon="@drawable/ic_menu_gallery"
            android:title="Gallery" />
        <item
            android:id="@+id/nav_slideshow"
            android:icon="@drawable/ic_menu_slideshow"
            android:title="Slideshow" />
        <item
            android:id="@+id/nav_manage"
            android:icon="@drawable/ic_menu_manage"
            android:title="Tools" />
    </group>

    <item android:title="Communicate">
        <menu>
            <item
                android:id="@+id/nav_share"
                android:icon="@drawable/ic_menu_share"
                android:title="Share" />
            <item
                android:id="@+id/nav_send"
                android:icon="@drawable/ic_menu_send"
                android:title="Send" />
        </menu>
    </item>

</menu>

的strings.xml

<resources>
    <string name="app_name">My Application</string>

    <string name="navigation_drawer_open">Open navigation drawer</string>
    <string name="navigation_drawer_close">Close navigation drawer</string>

    <string name="action_settings">Settings</string>
</resources>

您的MainActivity.java

中的

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_camera) {
            // Handle the camera action
        } else if (id == R.id.nav_gallery) {

        } else if (id == R.id.nav_slideshow) {

        } else if (id == R.id.nav_manage) {

        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_send) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
}

如何在导航抽屉中传递默认的MenuItem？

2 个答案: