活动开始从片段导致NullPointerException异常 [英] Starting Activity from Fragment causes NullPointerException

问题描述

我在这里坚持了开始从片段内按钮的活动的一个问题。 这是我的code：

I'm stuck here with a problem starting an activity from a Button inside a Fragment. Here is my code:

ViewPagerAdapter.java

import java.util.List;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

public class ViewPagerAdapter extends FragmentPagerAdapter {

    private List<Fragment> fragments;

    public ViewPagerAdapter(FragmentManager fm, List<Fragment> fragments) {
        super(fm);
        this.fragments = fragments;
    }

    @Override
    public int getCount() {
        return this.fragments.size();
    }

    @Override
    public Fragment getItem(int position) {
        return this.fragments.get(position);
    }

}

ViewPagerActivity.java

import java.util.List;
import java.util.Vector;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.util.Log;
import com.package.here.Class1Overview;
import com.package.here.Class1Overview;
import com.package.here.Class1Overview;

public class ViewPagerActivity extends FragmentActivity{

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

        ViewPager pager = (ViewPager)super.findViewById(R.id.my_viewpager);
        pager.setAdapter(initialisePaging());
    }

    private ViewPagerAdapter initialisePaging() {

        List<Fragment> fragments = new Vector<Fragment>();
        fragments.add(Fragment.instantiate(this, Class1Overview.class.getName()));
        fragments.add(Fragment.instantiate(this, Class2Overview.class.getName()));
        fragments.add(Fragment.instantiate(this, Class3Overview.class.getName()));
        return new ViewPagerAdapter(super.getSupportFragmentManager(), fragments);          
    }

}

Class1Overview.java （这里是按钮）

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;
import com.package.here.R;

public class FuelOverview extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        if (container == null) {
            return null;
        }
        View v = (LinearLayout)inflater.inflate(R.layout.class_one_layout, container, false);
        View AddBtn = v.findViewById(R.id.btn_add);
        ((Button) AddBtn).setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Intent intent = new Intent();
                intent.setClass(getActivity(), ActivityToStart.class);
                startActivity(intent);
            }
        });

        return v;
    }
}

这是我的 LogCat中

01-05 19:39:00.913: E/AndroidRuntime(10557): Uncaught handler: thread main exiting due to uncaught exception
01-05 19:39:01.034: E/AndroidRuntime(10557): java.lang.RuntimeException: Unable to pause activity {com.package.here/com.package.here.ViewPagerActivity}: java.lang.NullPointerException
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3162)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3119)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:3102)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.access$2400(ActivityThread.java:119)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1870)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.os.Handler.dispatchMessage(Handler.java:99)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.os.Looper.loop(Looper.java:123)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.main(ActivityThread.java:4363)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at java.lang.reflect.Method.invokeNative(Native Method)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at java.lang.reflect.Method.invoke(Method.java:521)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at dalvik.system.NativeStart.main(Native Method)
01-05 19:39:01.034: E/AndroidRuntime(10557): Caused by: java.lang.NullPointerException
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.support.v4.app.FragmentManagerImpl.saveFragmentBasicState(FragmentManager.java:1576)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.support.v4.app.FragmentManagerImpl.saveAllState(FragmentManager.java:1617)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.support.v4.app.FragmentActivity.onSaveInstanceState(FragmentActivity.java:481)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.Activity.performSaveInstanceState(Activity.java:1022)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.Instrumentation.callActivityOnSaveInstanceState(Instrumentation.java:1180)
01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3144)
01-05 19:39:01.034: E/AndroidRuntime(10557):    ... 12 more
01-05 19:39:01.423: I/dalvikvm(10557): threadid=7: reacting to signal 3

ViewPager本身是工作的罚款。这似乎是一个错误暂停FragmentActivity，但我不能弄明白。我发现的唯一的事情是这个职位上谷歌<一href="http://$c$c.google.com/p/android/issues/detail?id=19917">http://$c$c.google.com/p/android/issues/detail?id=19917

我也得到了通过谷歌的例子code，以确保我主要是使用相同的技术......

I also got through the google example code to make sure I mainly use the same technique...

编辑：以确保ClickListener的作品，我想敬酒在onclick方法 - 完美的作品

to make sure the ClickListener works, I tried a Toast in the onClick method - works perfectly.

推荐答案

这是一个已知的问题，我很震惊地发现，没有一个人提出一个错误的公共bug跟踪系统这一点。

This is a known problem and I'm shocked to find that no one has filed a bug on the public bug tracker for this.

从此你的问题的根源：

if (f.mSavedViewState != null) {
    if (result == null) {
        result = new Bundle();
    }
    result.putSparseParcelableArray(
            FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState);
}
if (!f.mUserVisibleHint) {
    // Only add this if it's not the default value
    result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint);
}

您应该注意到，如果结果是空进入该code和第一个如果则不会触发，你会得到一个 NullPointerException异常如果我们进入第二个如果。

You should notice that if result is null coming into this code and the first if is not triggered you'll get a NullPointerException if we enter the second if.

它应该阅读：

if (f.mSavedViewState != null) {
    if (result == null) {
        result = new Bundle();
    }
    result.putSparseParcelableArray(
            FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState);
}
if (!f.mUserVisibleHint) {
    if (result == null) {
        result = new Bundle();
    }
    // Only add this if it's not the default value
    result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint);
}

我提交了一个补丁，这一个星期或两个前：<一href="https://android-review.googlesource.com/31261">https://android-review.googlesource.com/31261

这篇关于活动开始从片段导致NullPointerException异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！