Android碎片实现问题

Question

这是我片段的 XML 代码：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:tools="http://schemas.android.com/tools"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            tools:context="com.thenewjonathan.foodtracker.fragments.FoodItemDisplay"
    >
<TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textSize="50dp"
        android:textStyle="bold"
        android:text="blank"
        android:id="@+id/foodItemNameView"
        android:gravity="center"
        android:layout_gravity="top|left|center"
        android:textAlignment="center"/>
<ImageView
        android:contentDescription="@string/foodPicture"
        android:layout_width="241dp"
        android:layout_height="241dp"
        android:layout_gravity="left|center_vertical"
        android:background="@color/material_blue_grey_800"
        android:layout_below="@id/foodItemNameView"
        android:id="@+id/foodPic"
        />
<RelativeLayout
        android:layout_width="140dp"
        android:layout_height="241dp"
        android:layout_alignParentRight="true"
        android:layout_below="@id/foodItemNameView"
        android:layout_gravity="end|center_vertical"
        android:gravity="center"
        android:id="@+id/checkBoxGroup"
        >
    <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Monday"
            android:id="@+id/mondayBox"
            android:layout_alignParentTop="true"
            android:layout_alignParentStart="true"/>
    <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Tuesday"
            android:id="@+id/tuesdayBox"
            android:layout_below="@id/mondayBox"
            android:layout_alignParentStart="true"/>
    <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Wednesday"
            android:id="@+id/wednesdayBox"
            android:layout_below="@id/tuesdayBox"
            android:layout_alignParentStart="true"/>
    <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Thursday"
            android:id="@+id/thursdayBox"
            android:layout_below="@id/wednesdayBox"
            android:layout_alignParentStart="true"/>
    <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Friday"
            android:id="@+id/fridayBox"
            android:layout_below="@id/thursdayBox"
            android:layout_alignParentStart="true"/>
    <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Saturday"
            android:id="@+id/saturdayBox"
            android:layout_below="@id/fridayBox"
            android:layout_alignParentStart="true"/>
    <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Sunday"
            android:id="@+id/sundayBox"
            android:layout_below="@id/saturdayBox"
            android:layout_alignParentStart="true"/>
</RelativeLayout>

这是我的Java文件，用于片段（FoodItem只是我用来存储每个食品项目信息的自定义对象）：

package com.thenewjonathan.foodtracker.fragments;
import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.CheckBox;
import android.widget.ImageView;
import android.widget.TextView;
import com.thenewjonathan.foodtracker.R;
import com.thenewjonathan.foodtracker.enums.GoodBad;
import com.thenewjonathan.foodtracker.objects.FoodItem;

public class FoodItemDisplay extends Fragment
{
    private static final String FOOD_ITEM_NAME = "blank";

    private FoodItem foodItem;
    private String foodItemName;

    private ImageView image;
    private TextView itemNameView;
    private CheckBox mondayBox;
    private CheckBox tuesdayBox;
    private CheckBox wednesdayBox;
    private CheckBox thursdayBox;
    private CheckBox fridayBox;
    private CheckBox saturdayBox;
    private CheckBox sundayBox;

    public FoodItemDisplay()
    {
        setFoodItem(new FoodItem("blank", GoodBad.HEALTHY, "blank"));
    }



public static FoodItemDisplay newInstance(String inFoodItemName)
    {
        FoodItemDisplay fragment = new FoodItemDisplay();
        Bundle args = new Bundle();
        args.putString(FOOD_ITEM_NAME, inFoodItemName);
        fragment.setArguments(args);
        return fragment;
    }



@Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        if (getArguments() != null)
        {
            foodItemName = getArguments().getString(FOOD_ITEM_NAME);
        }
        itemNameView = (TextView) itemNameView.findViewById(R.id.foodItemNameView);
        image = (ImageView) image.findViewById(R.id.foodPic);
        mondayBox = (CheckBox) mondayBox.findViewById(R.id.mondayBox);
        tuesdayBox = (CheckBox) tuesdayBox.findViewById(R.id.tuesdayBox);
        wednesdayBox = (CheckBox) wednesdayBox.findViewById(R.id.wednesdayBox);
        thursdayBox = (CheckBox) thursdayBox.findViewById(R.id.thursdayBox);
        fridayBox = (CheckBox) fridayBox.findViewById(R.id.fridayBox);
        saturdayBox = (CheckBox) saturdayBox.findViewById(R.id.saturdayBox);
        sundayBox = (CheckBox) sundayBox.findViewById(R.id.sundayBox);

        if (foodItem != null)
        {
            setValues();
        }
    }

    public void setValues()
    {
        if(itemNameView == null)
        {
            return;
        }
        itemNameView.setText(foodItem.getName());
        image.setImageResource(image.getResources()
                .getIdentifier(foodItem.getImgName(), "drawable", this.getClass().getPackage().toString()));
        mondayBox.setChecked(foodItem.isMonday());
        tuesdayBox.setChecked(foodItem.isTuesday());
        wednesdayBox.setChecked(foodItem.isWednesday());
        thursdayBox.setChecked(foodItem.isThursday());
        fridayBox.setChecked(foodItem.isFriday());
        saturdayBox.setChecked(foodItem.isSaturday());
        sundayBox.setChecked(foodItem.isSunday());
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        return inflater.inflate(R.layout.food_item_display, container, false);
    }

    @Override
    public void onAttach(Activity activity)
    {
        super.onAttach(activity);
    }

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

    public FoodItem getFoodItem()
    {
        return foodItem;
    }

    public void setFoodItem(FoodItem foodItem)
    {
        this.foodItem = foodItem;
    }

    public String getFoodItemName()
    {
        return foodItemName;
    }

    public void setFoodItemName(String foodItemName)
    {
        this.foodItemName = foodItemName;
    }
}

这是我用来放置片段的类:

public class HealthyFoods extends Activity
{
    private ArrayList<FoodItem> foodItemList;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.healthy_foods);

        foodItemList = new ArrayList<FoodItem>();
        foodItemList.add(new FoodItem("Turkey Burger", GoodBad.HEALTHY, "burger"));

        FragmentManager fm = getFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();
        for(FoodItem fi : foodItemList)
        {
            FoodItemDisplay fid = FoodItemDisplay.newInstance(fi.getName());
            fid.setFoodItem(fi);
            fid.setValues();
            ft.add(R.id.healthyTab, fid);
        }
        ft.commit();
    }
}

当我运行它时，会得到以下堆栈跟踪：

java.lang.NullPointerException
    at com.android.ddmlib.JdwpPacket.writeAndConsume(JdwpPacket.java:213)
    at com.android.ddmlib.Client.sendAndConsume(Client.java:675)
    at com.android.ddmlib.HandleHello.sendHELO(HandleHello.java:195)
    at com.android.ddmlib.HandleHello.sendHelloCommands(HandleHello.java:66)
    at com.android.ddmlib.Client.getJdwpPacket(Client.java:772)
    at com.android.ddmlib.MonitorThread.processClientActivity(MonitorThread.java:317)
    at com.android.ddmlib.MonitorThread.run(MonitorThread.java:263)

我在这个项目中做得很好，因为我可以看到代码堆栈跟踪并查找错误。但当我遇到这个问题时，它会给我一些我没有编写的代码的堆栈跟踪。我知道这表明我设置错了什么，但谷歌在这方面没有提供太多帮助，我已经查看了几次这段代码，并且根据每个对象和类的文档，我没有发现任何错误......有人以前遇到过这种情况吗？也许有一些要查看的想法？或者直接有答案？哈哈。我正在使用intellij-idea（而不是android studios）。

谢谢！ Jon

* 来自下面的评论 * 以下是adb日志输出：

DeviceMonitor: Adb rejected connection to client '27925': closed
DeviceMonitor: Adb rejected connection to client '27952': closed
DeviceMonitor: Adb rejected connection to client '28560': closed
DeviceMonitor: Adb rejected connection to client '31166': closed
DeviceMonitor: Adb rejected connection to client '31790': closed
DeviceMonitor: Adb rejected connection to client '32138': closed
DeviceMonitor: Adb rejected connection to client '578': closed
DeviceMonitor: Adb rejected connection to client '578': closed
ddms: null
java.lang.NullPointerException
    at com.android.ddmlib.JdwpPacket.writeAndConsume(JdwpPacket.java:213)
    at com.android.ddmlib.Client.sendAndConsume(Client.java:675)
    at com.android.ddmlib.HandleHello.sendHELO(HandleHello.java:195)
    at com.android.ddmlib.HandleHello.sendHelloCommands(HandleHello.java:66)
    at com.android.ddmlib.Client.getJdwpPacket(Client.java:772)
    at com.android.ddmlib.MonitorThread.processClientActivity(MonitorThread.java:317)
    at com.android.ddmlib.MonitorThread.run(MonitorThread.java:263)

ddms: null
java.lang.NullPointerException
    at com.android.ddmlib.JdwpPacket.writeAndConsume(JdwpPacket.java:213)
    at com.android.ddmlib.Client.sendAndConsume(Client.java:675)
    at com.android.ddmlib.HandleHello.sendHELO(HandleHello.java:195)
    at com.android.ddmlib.HandleHello.sendHelloCommands(HandleHello.java:66)
    at com.android.ddmlib.Client.getJdwpPacket(Client.java:772)
    at com.android.ddmlib.MonitorThread.processClientActivity(MonitorThread.java:317)
    at com.android.ddmlib.MonitorThread.run(MonitorThread.java:263)

DeviceMonitor: Adb rejected connection to client '832': closed
DeviceMonitor: Adb rejected connection to client '3083': closed
DeviceMonitor: Adb rejected connection to client '5816': closed
DeviceMonitor: Adb rejected connection to client '5847': closed
DeviceMonitor: Adb rejected connection to client '6144': closed
PropertyFetcher: AdbCommandRejectedException getting properties for device LGUS990611b931f: device offline
PropertyFetcher: AdbCommandRejectedException getting properties for device LGUS990611b931f: device unauthorized. Please check the confirmation dialog on your device.
PropertyFetcher: AdbCommandRejectedException getting properties for device LGUS990611b931f: device unauthorized. Please check the confirmation dialog on your device.
PropertyFetcher: AdbCommandRejectedException getting properties for device LGUS990611b931f: device unauthorized. Please check the confirmation dialog on your device.

一些重新配置后现在出现了新错误: 好的...现在有一个新错误 ->

Caused by: java.lang.IllegalArgumentException: No view found for id 0x7f09005a (com.thenewjonathan.foodtracker:id/healthyTab) for fragment FoodItemDisplay{b5abff #0 id=0x7f09005a}
            at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:886)
            at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
            at android.app.BackStackRecord.run(BackStackRecord.java:833)
            at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1452)
            at android.app.Activity.performStart(Activity.java:6035)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269)
            at android.app.ActivityThread.startActivityNow(ActivityThread.java:2128)
            at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:135)
            at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:347)
            at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:756)
            at android.widget.TabHost.setCurrentTab(TabHost.java:420)
            at android.widget.TabHost.addTab(TabHost.java:247)
            at com.thenewjonathan.foodtracker.MainMenu.onCreate(MainMenu.java:30)
            at android.app.Activity.performCreate(Activity.java:6020)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2259)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2368)
            at android.app.ActivityThread.access$800(ActivityThread.java:149)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1284)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5299)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)

这是在我的主菜单的oncreate中发生的，当我尝试将"healthy"标签添加到tabhost时：

public class MainMenu extends ActivityGroup
{
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.main_menu);

        TabHost container = (TabHost) findViewById(R.id.mainTabHost);
        container.setup(this.getLocalActivityManager());
        TabSpec healthy = container.newTabSpec("Healthy Foods").setContent(new Intent(this, HealthyFoods.class))
                .setIndicator("Healthy Foods");
        TabSpec unHealthy = container.newTabSpec("Unhealthy Foods").setContent(new Intent(this, UnHealthyFoods.class))
                .setIndicator("Unhealthy Foods");

        container.addTab(healthy);
        container.addTab(unHealthy);
    }
}

- Jon Sansoucie

---

我认为你在那个logcat中缺少了几行，请添加完整的错误信息。 - Evan Bashir

最终，logcat 将会给出你代码中导致错误的那一行，即使它发生在你导入的库中（看起来就是这种情况）。 - AdamMc331

看看你获取视图引用的方式。itemNameView = (TextView) itemNameView...会抛出NPE，因为itemNameView从未被实例化。 - AdamMc331

恐怕我必须附上一个文件，才能给你完整的adb日志文本。 - Jon Sansoucie

如果文本内容太长无法在问题中发布，请使用Gist或PasteBin并分享链接。 - AdamMc331

Answer 1

在fragment类的onCreate()方法中，你引用views的方式是错误的。你应该在onCreateView()方法中获取它们的引用，像这样：

View view = inflater.inflate(R.layout.food_item_display, container, false);
itemNameView = (TextView) view.findViewById(R.id.foodItemNameView);
image = (ImageView) view.findViewById(R.id.foodPic);

按照您当前的写法，您试图在给这些视图赋值之前引用它们，因此会出现NPE（空指针异常）。如果food_item_display确实是所有这些项的父布局，则应该使用该View通过ID查找其他视图，就像我在这里展示的那样。

- AdamMc331

---

你是100%正确的..谢谢。我只是在不停地尝试让代码工作，没有考虑到为什么使用我的对象来查找视图...这很奇怪。然而，它并没有解决问题..我仍然遇到了同样的问题。我会把完整的adb日志文本放在上面那个人要求的评论中。 - Jon Sansoucie

Answer 2

你必须使用 onActivityCreated 或 onCreateView 来获取引用，如下面的例子所示。你不能使用 onCreate，因为该片段只会在 onCreateView 之后创建。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    return inflater.inflate(R.layout.food_item_display, container, false);
}

@Override
public void onActivityCreated(Bundle savedInstanceState){

    View _view = getView();

    itemNameView = (TextView) _view.findViewById(R.id.foodItemNameView);
    image = (ImageView) _view.findViewById(R.id.foodPic);
    mondayBox = (CheckBox) _view.findViewById(R.id.mondayBox);
    tuesdayBox = (CheckBox) _view.findViewById(R.id.tuesdayBox);
    wednesdayBox = (CheckBox) _view.findViewById(R.id.wednesdayBox);
    thursdayBox = (CheckBox) _view.findViewById(R.id.thursdayBox);
    fridayBox = (CheckBox) _view.findViewById(R.id.fridayBox);
    saturdayBox = (CheckBox) _view.findViewById(R.id.saturdayBox);
    sundayBox = (CheckBox) _view.findViewById(R.id.sundayBox);

}

另一个例子：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    View _view = inflater.inflate(R.layout.food_item_display, container, false);

    itemNameView = (TextView) _view.findViewById(R.id.foodItemNameView);
    image = (ImageView) _view.findViewById(R.id.foodPic);
    mondayBox = (CheckBox) _view.findViewById(R.id.mondayBox);
    tuesdayBox = (CheckBox) _view.findViewById(R.id.tuesdayBox);
    wednesdayBox = (CheckBox) _view.findViewById(R.id.wednesdayBox);
    thursdayBox = (CheckBox) _view.findViewById(R.id.thursdayBox);
    fridayBox = (CheckBox) _view.findViewById(R.id.fridayBox);
    saturdayBox = (CheckBox) _view.findViewById(R.id.saturdayBox);
    sundayBox = (CheckBox) _view.findViewById(R.id.sundayBox);

    return _view;
}

- Lennon Spirlandelli

---

在onCreateView()和onActivityCreated()中执行此操作是否有重大差异？ - AdamMc331

出现了相同的错误 -> ddms: null java.lang.NullPointerException at com.android.ddmlib.JdwpPacket.writeAndConsume(JdwpPacket.java:213) at com.android.ddmlib.Client.sendAndConsume(Client.java:675) at com.android.ddmlib.HandleHello.sendHELO(HandleHello.java:195) at com.android.ddmlib.HandleHello.sendHelloCommands(HandleHello.java:66) at com.android.ddmlib.Client.getJdwpPacket(Client.java:772) at com.android.ddmlib.MonitorThread.processClientActivity(MonitorThread.java:317) at com.android.ddmlib.MonitorThread.run(MonitorThread.java:263) - Jon Sansoucie

尝试将您的活动从简单的“Activity”扩展为“FragmentActivity”。 - Lennon Spirlandelli

在这种情况下，您将使用 FragmentManager fm = getSupportFragmentManager(); 而不是 FragmentManager fm = getFragmentManager();。 - Lennon Spirlandelli

我正在分析你的代码，如果我没错的话，你是想把 HealthyFoods 放到一个 Tab 里面，对吗？然后你的 MainMenu 是主活动，所以在我的看法中，HealthyFoods 应该是一个 fragment。你应该将一个 fragment 设置到 Tab 中，而不是 activity。 - Lennon Spirlandelli