java - 使用带有 FragmentActivity 的 SimpleCursorAdapter 为 SMS URI 返回空

Question

我遵循了有效的导航示例。构建一个新的应用程序。基本上我想要的是每条短信都应该出现在我的应用程序的新屏幕( fragment )上，并且可以滚动，就像 Gmail 应用程序中的 Gmail 消息一样

这是相同的代码

package com.example.android.effectivenavigation;

import java.sql.Date;
import java.text.SimpleDateFormat;
import java.util.Locale;

import android.app.ActionBar;
import android.content.Intent;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.app.LoaderManager;
import android.support.v4.app.NavUtils;
import android.support.v4.app.TaskStackBuilder;
import android.support.v4.content.CursorLoader;
import android.support.v4.content.Loader;
import android.support.v4.view.ViewPager;
import android.support.v4.widget.SimpleCursorAdapter;
import android.view.LayoutInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class CollectionDemoActivity extends FragmentActivity implements LoaderManager.LoaderCallbacks<Cursor> {

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide fragments representing each object in a
     * collection. We use a {@link android.support.v4.app.FragmentStatePagerAdapter} derivative, which will destroy and
     * re-create fragments as needed, saving and restoring their state in the process. This is important to conserve
     * memory and is a best practice when allowing navigation between objects in a potentially large collection.
     */
    DemoCollectionPagerAdapter mDemoCollectionPagerAdapter;

    // This is the Adapter being used to display the list's data.
    SimpleCursorAdapter mAdapter;

    /**
     * The {@link android.support.v4.view.ViewPager} that will display the object collection.
     */
    ViewPager mViewPager;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_collection_demo);

        //-----------------------THIS BLOCK WORK JUST FINE--------------------------
        Cursor cursor = getContentResolver().query(Uri.parse("content://sms/inbox"), null, null, null, null);

        cursor.moveToFirst();
        do {
            String msgData = "a";
            for (int idx = 0; idx < cursor.getColumnCount(); idx++) {
                msgData += " " + cursor.getColumnName(idx) + ":" + cursor.getString(idx);
            }
            Date date = new Date(Long.parseLong(cursor.getString(5)));
            SimpleDateFormat sdf = new SimpleDateFormat("E yyyy.MM.dd 'at' hh:mm:ss a zzz",Locale.ENGLISH);
            System.out.println("Cursor123"+msgData + "\r Dateeeee" + sdf.format(date));
        } while (cursor.moveToNext());
        //-----------------------------END BLOCK ----------------------------------


        // Create an adapter that when requested, will return a fragment representing an object in
        // the collection.
        //
        // ViewPager and its adapters use support library fragments, so we must use
        // getSupportFragmentManager.
        mDemoCollectionPagerAdapter = new DemoCollectionPagerAdapter(getSupportFragmentManager());

        // Set up action bar.
        final ActionBar actionBar = getActionBar();

        // Specify that the Home button should show an "Up" caret, indicating that touching the
        // button will take the user one step up in the application's hierarchy.
        actionBar.setDisplayHomeAsUpEnabled(true);

        // Set up the ViewPager, attaching the adapter.
        mViewPager = (ViewPager) findViewById(R.id.pager);

        getSupportLoaderManager().initLoader(0, null, this);
        mAdapter = new SimpleCursorAdapter(this, R.layout.activity_collection_demo, null, null, null, 0);
        Cursor cursor1 = mAdapter.getCursor();
        mDemoCollectionPagerAdapter.setCount(cursor1.getCount());
        mViewPager.setAdapter(mDemoCollectionPagerAdapter);


    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case android.R.id.home:

            Intent upIntent = new Intent(this, MainActivity.class);
            if (NavUtils.shouldUpRecreateTask(this, upIntent)) {
                // This activity is not part of the application's task, so create a new task
                // with a synthesized back stack.
                TaskStackBuilder.from(this)
                // If there are ancestor activities, they should be added here.
                        .addNextIntent(upIntent).startActivities();
                finish();
            } else {
                // This activity is part of the application's task, so simply
                // navigate up to the hierarchical parent activity.
                NavUtils.navigateUpTo(this, upIntent);
            }
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A {@link android.support.v4.app.FragmentStatePagerAdapter} that returns a fragment representing an object in the
     * collection.
     */
    public static class DemoCollectionPagerAdapter extends FragmentStatePagerAdapter {

        public DemoCollectionPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        private int count;
        @Override
        public Fragment getItem(int i) {
            Fragment fragment = new DemoObjectFragment();
            Bundle args = new Bundle();
            args.putInt(DemoObjectFragment.ARG_OBJECT, i + 1); 
            fragment.setArguments(args);
            return fragment;
        }

        @Override
        public int getCount() {
            // temporarily setting count to 5
            return 5;
        }

        public void setCount( int iCount) {
            this.count=iCount;
        }
        @Override
        public CharSequence getPageTitle(int position) {
            return "OBJECT " + (position + 1);
        }
    }

    /**
     * A dummy fragment representing a section of the app, but that simply displays dummy text.
     */
    public static class DemoObjectFragment extends Fragment {

        public static final String ARG_OBJECT = "object";
        Cursor cursor;

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_collection_object, container, false);
            Bundle args = getArguments();
            ((TextView) rootView.findViewById(android.R.id.text1)).setText(Integer.toString(args.getInt(ARG_OBJECT)));

            // cursor.move(args.getInt(ARG_OBJECT));

            TextView tv1 = (TextView) getActivity().findViewById(R.id.editText1);
            tv1.setText("Test");


            return rootView;
        }

    }

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {

        Uri baseUri = Uri.parse("content://sms/inbox");

        return new CursorLoader(this, baseUri, null, null, null, null);

    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        mAdapter.swapCursor(data);

    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {
        mAdapter.swapCursor(null);
    }
}

这是异常的堆栈跟踪:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.effectivenavigation/com.example.android.effectivenavigation.CollectionDemoActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
    at android.app.ActivityThread.access$600(ActivityThread.java:123)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4424)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at com.example.android.effectivenavigation.CollectionDemoActivity.onCreate(CollectionDemoActivity.java:108)
    at android.app.Activity.performCreate(Activity.java:4465)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
    ... 11 more

108行对应

"mDemoCollectionPagerAdapter.setCount(cursor1.getCount());"

Answer 1

你的 SimpleCursorAdapter 启动(并且一直到 Loader 完成它的工作)在那一刻有一个空的 Cursor 所以:

mDemoCollectionPagerAdapter.setCount(cursor1.getCount());

将抛出该异常，因为 cursor1 为 null。相反，您应该在 onLoadFinished 回调中更新 ViewPager 的 count 变量，因为此时 Cursor 应该是有效，您可以获取其中的行数。