加载器: Android 3.0 中引入了加载器，支持轻松在Activity或者片段中异步加载数据。

加载器的特征：

• 可用于每个 Activity和 Fragment。
• 支持异步加载数据。
• 监控其数据源并在内容变化时传递新的结果。
• 在某一配置更改后重建加载器时，会自动重新连接上一个加载器的游标。 因此，它们无需重新查询其数据。

好了，加载器的基本介绍就到这里，下面是本文的参考博客和资料链接。

• 官网api指南
• 官方示例demo
• 参考博客(包含完整示例代码)

下面就以两个例子的代码对加载器来进行说明。
例子一，用listView显示通话记录。其主要源码代参考博客,具体移步去参考。这里我就直接贴代码了。

activity的布局文件：

<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=".MainActivity" >

    <TextView
        android:id="@+id/tv_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="学习加载其的使用" />

    <Button
        android:id="@+id/btn_all"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/tv_text"
        android:text="所有通话记录" />

    <Button
        android:id="@+id/btn_incoming"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/tv_text"
        android:layout_marginLeft="20dp"
        android:layout_toRightOf="@id/btn_all"
        android:text="拨入记录" />

    <Button
        android:id="@+id/btn_outcoming"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/btn_all"
        android:layout_marginTop="10dp"
        android:text="播出记录" />

    <Button
        android:id="@+id/btn_missed"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@id/btn_incoming"
        android:layout_below="@id/btn_all"
        android:layout_marginTop="10dp"
        android:layout_toRightOf="@id/btn_outcoming"
        android:text="未接记录" />
    <Button
        android:layout_below="@id/btn_all"
        android:layout_toRightOf="@id/btn_missed"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="nextView"
        android:text="简单版示例"/>

    <View
        android:id="@+id/line_layout"
        android:layout_width="match_parent"
        android:layout_height="2dp"
        android:layout_below="@id/btn_missed"
        android:background="#000000" >
    </View>

    <ListView
        android:id="@+id/lv_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/line_layout"
        android:clipToPadding="false"
        android:divider="#ff553311"
        android:dividerHeight="2dp"
        android:fadingEdge="none"
        android:paddingTop="10dp" />

</RelativeLayout>

listView的子布局文件，item_load.xml

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

    <ImageView
        android:id="@+id/bt_icon"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:layout_marginLeft="10dp"
        android:background="#00000000"
        android:contentDescription="yi"
        android:src="@drawable/ic_playlist_play_black_24dp" />

    <TextView
        android:id="@+id/tv_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="2dp"
        android:layout_toRightOf="@id/bt_icon"
        android:textSize="23sp" />

    <TextView
        android:id="@+id/tv_number"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@id/tv_name"
        android:layout_below="@id/tv_name"
        android:layout_marginBottom="2dp"
        android:textColor="#7f7f7f"
        android:textSize="16sp" />

    <TextView
        android:id="@+id/tv_date"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignTop="@id/tv_number"
        android:layout_marginLeft="10dp"
        android:layout_toRightOf="@id/tv_number"
        android:ellipsize="end"
        android:textColor="#7f7f7f"
        android:textSize="16sp" />

    <ImageButton
        android:id="@+id/btn_delete"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_centerVertical="true"
        android:layout_marginRight="10dp"
        android:background="#00000000"
        android:contentDescription="er"
        android:focusableInTouchMode="false"
        android:src="@drawable/ic_delete_forever_black_24dp" />

    <ImageButton
        android:id="@+id/btn_call"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:layout_marginRight="5dp"
        android:layout_toLeftOf="@id/btn_delete"
        android:background="#00000000"
        android:contentDescription="san"
        android:focusableInTouchMode="false"
        android:src="@mipmap/ic_launcher" />
</RelativeLayout>

activity的源代码：

package reoger.hut.openfiretest.activty;

import android.Manifest;
import android.content.CursorLoader;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.database.Cursor;
import android.os.Bundle;
import android.provider.CallLog;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.ListView;

import reoger.hut.openfiretest.R;

public class TestLoaderActivity extends AppCompatActivity implements View.OnClickListener {

    private static final String[] CALLLOG_PROJECTION = new String[]{
            CallLog.Calls._ID, CallLog.Calls.NUMBER, CallLog.Calls.CACHED_NAME,
            CallLog.Calls.TYPE, CallLog.Calls.DATE};


    private ListView mListView;
    private MyCursorAdapter mAdapter;

    private MyLoaderListener mLoader = new MyLoaderListener();


    private static final int ALL = 0; // 默认显示所有
    private static final int INCOMING = CallLog.Calls.INCOMING_TYPE; // 来电
    private static final int OUTCOMING = CallLog.Calls.OUTGOING_TYPE; // 拔号
    private static final int MISSED = CallLog.Calls.MISSED_TYPE; // 未接


    private int mCallLogShowType = ALL;
    private boolean m_FinishLoaderFlag = false; // 第一次加载完成

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

        getLoaderManager().initLoader(0, null, mLoader);
    }

    private void initWidgets() {
        mListView = (ListView) findViewById(R.id.lv_list);
        Button btn = (Button) findViewById(R.id.btn_all);
        btn.setOnClickListener(this);
        btn = (Button) findViewById(R.id.btn_incoming);
        btn.setOnClickListener(this);
        btn = (Button) findViewById(R.id.btn_outcoming);
        btn.setOnClickListener(this);
        btn = (Button) findViewById(R.id.btn_missed);
        btn.setOnClickListener(this);
        mAdapter = new MyCursorAdapter(TestLoaderActivity.this, null);
        mListView.setAdapter(mAdapter);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btn_all:
                allCalllog();
                break;
            case R.id.btn_incoming:
                incomingCalllog();
                break;
            case R.id.btn_outcoming:
                outcomingCalllog();
                break;
            case R.id.btn_missed:
                missedCalllog();
                break;
            default:
                break;
        }
    }

    private void missedCalllog() {
        mCallLogShowType = MISSED;
        String selection = CallLog.Calls.TYPE + "=?";
        String[] selectionArgs = new String[]{"3"};
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.READ_CALL_LOG) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        Cursor missedCursor = getContentResolver().query(
                CallLog.Calls.CONTENT_URI, CALLLOG_PROJECTION, selection,
                selectionArgs, CallLog.Calls.DEFAULT_SORT_ORDER);
        mAdapter.swapCursor(missedCursor);
    }

    private void allCalllog() {
        mCallLogShowType = ALL;
        String selection = null;
        String[] selectionArgs = null;
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.READ_CALL_LOG) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        Cursor allCursor = getContentResolver().query(
                CallLog.Calls.CONTENT_URI, CALLLOG_PROJECTION, selection,
                selectionArgs, CallLog.Calls.DEFAULT_SORT_ORDER);
        mAdapter.swapCursor(allCursor);
    }

    private void incomingCalllog() {
        mCallLogShowType = INCOMING;
        String selection = CallLog.Calls.TYPE + "=?";
        String[] selectionArgs = new String[]{"1"};
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.READ_CALL_LOG) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        Cursor incomingCursor = getContentResolver().query(
                CallLog.Calls.CONTENT_URI, CALLLOG_PROJECTION, selection,
                selectionArgs, CallLog.Calls.DEFAULT_SORT_ORDER);
        mAdapter.swapCursor(incomingCursor);
    }


    private void outcomingCalllog() {
        mCallLogShowType = OUTCOMING;
        String selection = CallLog.Calls.TYPE + "=?";
        String[] selectionArgs = new String[]{"2"};
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.READ_CALL_LOG) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        Cursor outcomingCursor = getContentResolver().query(
                CallLog.Calls.CONTENT_URI, CALLLOG_PROJECTION, selection,
                selectionArgs, CallLog.Calls.DEFAULT_SORT_ORDER);
        mAdapter.swapCursor(outcomingCursor);
    }


    private class MyLoaderListener implements android.app.LoaderManager.LoaderCallbacks<Cursor> {

        @Override
        public android.content.Loader<Cursor> onCreateLoader(int id, Bundle args) {
            m_FinishLoaderFlag = false;
            CursorLoader cursor = new CursorLoader(TestLoaderActivity.this,
                    CallLog.Calls.CONTENT_URI, CALLLOG_PROJECTION, null, null,
                    CallLog.Calls.DEFAULT_SORT_ORDER);

            return cursor;
        }

        @Override
        public void onLoadFinished(android.content.Loader<Cursor> loader, Cursor data) {
            if (data == null)
                return;
            Cursor tempData = data;
            if (tempData.getCount() == 0) {
                mAdapter.swapCursor(null);
                return;
            }

            if (m_FinishLoaderFlag) {
                tempData = null;
                String selection = null;
                String[] selectionArgs = null;
                if (mCallLogShowType == INCOMING) {
                    selection = CallLog.Calls.TYPE + "=?";
                    selectionArgs = new String[]{"1"};
                } else if (mCallLogShowType == OUTCOMING) {
                    selection = CallLog.Calls.TYPE + "=?";
                    selectionArgs = new String[]{"2"};
                } else if (mCallLogShowType == MISSED) {
                    selection = CallLog.Calls.TYPE + "=?";
                    selectionArgs = new String[]{"3"};
                }

                if (ActivityCompat.checkSelfPermission(TestLoaderActivity.this, Manifest.permission.READ_CALL_LOG) != PackageManager.PERMISSION_GRANTED) {
                    return;
                }

                tempData = getContentResolver().query(
                        CallLog.Calls.CONTENT_URI, CALLLOG_PROJECTION,
                        selection, selectionArgs,
                        CallLog.Calls.DEFAULT_SORT_ORDER);
            }
            mAdapter.swapCursor(tempData);
            m_FinishLoaderFlag = true;
        }

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



    public void nextView(View view){
        startActivity(new Intent(TestLoaderActivity.this,NextActivity.class));
    }
}

最后是适配器的代码：

package reoger.hut.openfiretest.activty;

import android.Manifest;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.database.Cursor;
import android.net.Uri;
import android.provider.CallLog;
import android.support.v4.app.ActivityCompat;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.CursorAdapter;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.TextView;

import java.util.Date;

import reoger.hut.openfiretest.R;

import static android.content.ContentValues.TAG;

/**
 * Created by 24540 on 2017/6/2.
 *
 */

public class MyCursorAdapter extends CursorAdapter {
    private final Context mContext;

    public MyCursorAdapter(Context context, Cursor c) {
        this(context, c, true);
    }

    public MyCursorAdapter(Context context, Cursor c, boolean autoRequery) {
        super(context, c, autoRequery);
        mContext = context;
    }

    public MyCursorAdapter(Context context, Cursor c, int flags) {
        super(context, c, flags);
        mContext = context;
    }


    @Override
    public View newView(Context context, Cursor cursor, ViewGroup parent) {
        LayoutInflater inflater = LayoutInflater.from(context);
        return inflater.inflate(R.layout.item_load, parent, false);
    }

    @Override
    public void bindView(View view, Context context, Cursor cursor) {
        if (cursor == null)
            return;
        final String id = cursor.getString(0);
        String number = cursor.getString(1);
        String name = cursor.getString(2);
        int type = cursor.getInt(3);
        String date = cursor.getString(4);
        ImageView TypeView = (ImageView) view.findViewById(R.id.bt_icon);
        TextView nameCtrl = (TextView) view.findViewById(R.id.tv_name);
        if (name == null) {
            nameCtrl.setText(mContext.getString(R.string.name_unknown));
        } else {
            nameCtrl.setText(name);
        }
        TextView numberCtrl = (TextView) view.findViewById(R.id.tv_number);
        numberCtrl.setText(number);
        String value = ComputeDate(date);
        TextView dateCtrl = (TextView) view.findViewById(R.id.tv_date);
        dateCtrl.setText(value);
        switch (type) {
            case CallLog.Calls.INCOMING_TYPE:
                TypeView.setImageResource(R.drawable.ic_playlist_play_black_24dp);
                break;
            case CallLog.Calls.OUTGOING_TYPE:
                TypeView.setImageResource(R.drawable.chatright);
                break;
            case CallLog.Calls.MISSED_TYPE:
                TypeView.setImageResource(R.drawable.ic_delete_forever_black_24dp);
                break;
            case 4: // CallLog.Calls.VOICEMAIL_TYPE
                break;
            default:
                break;
        }
        ImageButton dailBtn = (ImageButton) view.findViewById(R.id.btn_call);
        dailBtn.setTag(number);

        dailBtn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // Intent.ACTION_CALL_PRIVILEGED 由于Intent中隐藏了，只能用字符串代替
                Intent intent = new Intent(Intent.ACTION_CALL, Uri.fromParts(
                        "tel", (String) v.getTag(), null));
                intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                if (ActivityCompat.checkSelfPermission(mContext, Manifest.permission.CALL_PHONE) != PackageManager.PERMISSION_GRANTED) {
                    return;
                }
                mContext.startActivity(intent);
            }
        });
        ImageButton deleteBtn = (ImageButton) view
                .findViewById(R.id.btn_delete);
        deleteBtn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                // 根据ID进行记录删除
                String where = CallLog.Calls._ID + "=?";
                String[] selectionArgs = new String[]{id};
                if (ActivityCompat.checkSelfPermission(mContext, Manifest.permission.WRITE_CALL_LOG) != PackageManager.PERMISSION_GRANTED) {
                    return;
                }
                int result = mContext.getContentResolver().delete(
                        CallLog.Calls.CONTENT_URI, where, selectionArgs);
                Log.d(TAG, "11result = " + result);
            }
        });


    }

    private String ComputeDate(String date) {
        long callTime = Long.parseLong(date);
        long newTime = new Date().getTime();
        long duration = (newTime - callTime) / (1000 * 60);
        String value;
        // SimpleDateFormat sfd = new
        // SimpleDateFormat("yyyy-MM-dd HH:mm:ss",
        // Locale.getDefault());
        // String time = sfd.format(callTime);
        // Log.d(TAG, "[MyCursorAdapter--ComputeDate] time = " + time);
        // 进行判断拨打电话的距离现在的时间，然后进行显示说明
        value=duration + "分钟前";
//        if (duration < 60) {
//            value = duration + "分钟前";
//        } else if (duration >= 60 && duration < MainActivity.DAY) {
//            SimpleDateFormat sdf = new SimpleDateFormat("HH:mm",
//                    Locale.getDefault());
//            value = sdf.format(new Date(callTime));
//
//            // value = (duration / 60) + "小时前";
//        } else if (duration >= MainActivity.DAY
//                && duration < MainActivity.DAY * 2) {
//            value = "昨天";
//        } else if (duration >= MainActivity.DAY * 2
//                && duration < MainActivity.DAY * 3) {
//            value = "前天";
//        } else if (duration >= MainActivity.DAY * 7) {
//            SimpleDateFormat sdf = new SimpleDateFormat("MM/dd",
//                    Locale.getDefault());
//            value = sdf.format(new Date(callTime));
//        } else {
//            value = (duration / MainActivity.DAY) + "天前";
//        }
        return value;
    }
}

最后程序运行的截图类似于这种：

Paste_Image.png

要注意的一点就是，当写继承LoaderCallback的时候，不要倒错包了，导入的包是android.app.Load......的那个。如图；

Paste_Image.png

示例代码二，这里我只是将手机通讯录里的名字显示到了listView中。代码如下：

public class NextActivity extends AppCompatActivity implements LoaderCallbacks<Cursor> {

    private ListView mList;
    private final static int REQUEST_READ_CONTACTS = 0;

    static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
            ContactsContract.Contacts._ID,
            ContactsContract.Contacts.DISPLAY_NAME,
            ContactsContract.Contacts.CONTACT_STATUS,
            ContactsContract.Contacts.CONTACT_PRESENCE,
            ContactsContract.Contacts.PHOTO_ID,
            ContactsContract.Contacts.LOOKUP_KEY,
    };

    // If non-null, this is the current filter the user has provided.
    String mCurFilter;
    SimpleCursorAdapter mAdapter;


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

        mAdapter = new SimpleCursorAdapter(this,
                android.R.layout.simple_list_item_2, null,
                new String[] { ContactsContract.Contacts.DISPLAY_NAME, ContactsContract.Contacts.CONTACT_STATUS },
                new int[] { android.R.id.text1, android.R.id.text2 }, 0);


        mList = (ListView) findViewById(R.id.listView);
        mList.setAdapter(mAdapter);


    }

    private void populateAutoComplete() {
        if(!mayRequestContacts()){
            return;
        }
        getLoaderManager().initLoader(0,null,this);
    }

    /**
     * 获取联系人列表的权限
     * @return
     */
    private boolean mayRequestContacts() {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
            return true;
        }
        if (checkSelfPermission(READ_CONTACTS) == PackageManager.PERMISSION_GRANTED) {
            return true;
        }
        if (shouldShowRequestPermissionRationale(READ_CONTACTS)) {
            Snackbar.make(mList, R.string.permission_rationale, Snackbar.LENGTH_INDEFINITE)
                    .setAction(android.R.string.ok, new View.OnClickListener() {
                        @Override
                        @TargetApi(Build.VERSION_CODES.M)
                        public void onClick(View v) {
                            requestPermissions(new String[]{READ_CONTACTS}, REQUEST_READ_CONTACTS);
                        }
                    });
        } else {
            requestPermissions(new String[]{READ_CONTACTS}, REQUEST_READ_CONTACTS);
        }
        return false;
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        if(requestCode == REQUEST_READ_CONTACTS){
            if(grantResults.length == 1 && grantResults[0] ==PackageManager.PERMISSION_GRANTED){
                populateAutoComplete();
            }
        }
    }



    public void showContent(View view){
        populateAutoComplete();
    }


    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        Uri baseUri;
        if (mCurFilter != null) {
            baseUri = Uri.withAppendedPath(ContactsContract.Contacts.CONTENT_FILTER_URI,
                    Uri.encode(mCurFilter));
        } else {
            baseUri = ContactsContract.Contacts.CONTENT_URI;
        }

        String select = "((" + ContactsContract.Contacts.DISPLAY_NAME + " NOTNULL) AND ("
                + ContactsContract.Contacts.HAS_PHONE_NUMBER + "=1) AND ("
                + ContactsContract.Contacts.DISPLAY_NAME + " != '' ))";

        CursorLoader cursorLoader = new CursorLoader(this,baseUri,CONTACTS_SUMMARY_PROJECTION,select,null, ContactsContract.Contacts.DISPLAY_NAME+" COLLATE LOCALIZED ASC");

        return cursorLoader;
    }

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

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

对应的布局文件为：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    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:orientation="vertical"
    tools:context="reoger.hut.openfiretest.activty.NextActivity">
    
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="showContent"
        android:text="查看联系人"/>
    <ListView
        android:id="@+id/listView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        />
</LinearLayout>

最后显示的效果为：

device-2017-06-03-195754.png

最后，如果对Loader还有什么不懂的，强烈推荐看官方api指南。