Android Studio教程09-加载器Loader的使用

目录

• 1.加载器特征
• 2. loader api
• 3. 在应用中使用loader
• 4. 实例： 访问用户联系人

1.加载器特征

• 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结果。

2. loader api

3. 在应用中使用loader

主要步骤

1. activity 或 fragment
2. loadermanager 的实例
3. 一个 cursorloader，用于加载由contentprovider 支持的数据。您也可以实现自己的 loader或 asynctaskloader 子类，从其他源中加载数据。
4. 一个 loadermanager.loadercallbacks 实现。您可以使用它来创建新加载器，并管理对现有加载器的引用。
5. 一种显示加载器数据的方法，如 simplecursoradapter。
6. 使用 cursorloader 时的数据源，如 contentprovider。

3.1. 启动加载器

在 activity 的 oncreate() 方法或片段的onactivitycreated() 方法内初始化 loader

// prepare the loader.  either re-connect with an existing one,
// or start a new one.
// 标识id + 构建时给加载器的可选参数 + loadermanager.loadercallbacks 实现
getloadermanager().initloader(0, null, this);
// 如果 id 指定的加载器已存在，则将重复使用上次创建的加载器。
// 如果 id 指定的加载器不存在，则 initloader() 将触发 loadermanager.loadercallbacks 方法 oncreateloader()

3.2. 重启加载器

public boolean onquerytextchanged(string newtext) {
    // called when the action bar search text has changed.  update
    // the search filter, and restart the loader to do a new query
    // with this filter.
    mcurfilter = !textutils.isempty(newtext) ? newtext : null;
    getloadermanager().restartloader(0, null, this);
    return true;
}

3.3. 使用loadermanager回调

loadermanager.loadercallbacks 是一个支持客户端与loadermanager 交互的回调接口。

• 加载器（特别是 cursorloader）在停止运行后，仍需保留其数据。这样，应用即可保留 activity 或片段的 onstop()和 onstart() 方法中的数据。当用户返回应用时，无需等待它重新加载这些数据。您可使用 loadermanager.loadercallbacks 方法了解何时创建新加载器，并告知应用何时停止使用加载器的数据。
  
• loadermanager.loadercallbacks 包括以下方法：
  • oncreateloader()：针对指定的 id 进行实例化并返回新的 loader
  • onloadfinished()：将在先前创建的加载器完成加载时调用
  • onloaderreset()：将在先前创建的加载器重置且其数据因此不可用时调用

4. 实例： 访问用户联系人

public static class cursorloaderlistfragment extends listfragment
        implements onquerytextlistener, loadermanager.loadercallbacks<cursor> {

    // this is the adapter being used to display the list's data.
    simplecursoradapter madapter;

    // if non-null, this is the current filter the user has provided.
    string mcurfilter;

    @override public void onactivitycreated(bundle savedinstancestate) {
        super.onactivitycreated(savedinstancestate);

        // give some text to display if there is no data.  in a real
        // application this would come from a resource.
        setemptytext("no phone numbers");

        // we have a menu item to show in action bar.
        sethasoptionsmenu(true);

        // create an empty adapter we will use to display the loaded data.
        madapter = new simplecursoradapter(getactivity(),
                android.r.layout.simple_list_item_2, null,
                new string[] { contacts.display_name, contacts.contact_status },
                new int[] { android.r.id.text1, android.r.id.text2 }, 0);
        setlistadapter(madapter);

        // prepare the loader.  either re-connect with an existing one,
        // or start a new one.
        getloadermanager().initloader(0, null, this);
    }

    @override public void oncreateoptionsmenu(menu menu, menuinflater inflater) {
        // place an action bar item for searching.
        menuitem item = menu.add("search");
        item.seticon(android.r.drawable.ic_menu_search);
        item.setshowasaction(menuitem.show_as_action_if_room);
        searchview sv = new searchview(getactivity());
        sv.setonquerytextlistener(this);
        item.setactionview(sv);
    }

    public boolean onquerytextchange(string newtext) {
        // called when the action bar search text has changed.  update
        // the search filter, and restart the loader to do a new query
        // with this filter.
        mcurfilter = !textutils.isempty(newtext) ? newtext : null;
        getloadermanager().restartloader(0, null, this);
        return true;
    }

    @override public boolean onquerytextsubmit(string query) {
        // don't care about this.
        return true;
    }

    @override public void onlistitemclick(listview l, view v, int position, long id) {
        // insert desired behavior here.
        log.i("fragmentcomplexlist", "item clicked: " + id);
    }

    // these are the contacts rows that we will retrieve.
    static final string[] contacts_summary_projection = new string[] {
        contacts._id,
        contacts.display_name,
        contacts.contact_status,
        contacts.contact_presence,
        contacts.photo_id,
        contacts.lookup_key,
    };
    public loader<cursor> oncreateloader(int id, bundle args) {
        // this is called when a new loader needs to be created.  this
        // sample only has one loader, so we don't care about the id.
        // first, pick the base uri to use depending on whether we are
        // currently filtering.
        uri baseuri;
        if (mcurfilter != null) {
            baseuri = uri.withappendedpath(contacts.content_filter_uri,
                    uri.encode(mcurfilter));
        } else {
            baseuri = contacts.content_uri;
        }

        // now create and return a cursorloader that will take care of
        // creating a cursor for the data being displayed.
        string select = "((" + contacts.display_name + " notnull) and ("
                + contacts.has_phone_number + "=1) and ("
                + contacts.display_name + " != '' ))";
        return new cursorloader(getactivity(), baseuri,
                contacts_summary_projection, select, null,
                contacts.display_name + " collate localized asc");
    }

    public void onloadfinished(loader<cursor> loader, cursor data) {
        // swap the new cursor in.  (the framework will take care of closing the
        // old cursor once we return.)
        madapter.swapcursor(data);
    }

    public void onloaderreset(loader<cursor> loader) {
        // this is called when the last cursor provided to onloadfinished()
        // above is about to be closed.  we need to make sure we are no
        // longer using it.
        madapter.swapcursor(null);
    }
}