ITKeyword,专注技术干货聚合推荐

注册 | 登录

解决android - How does CursorLoader with LoaderManager know to send the cursor to a CursorAdapter?

itPublisher 分享于

2021腾讯云限时秒杀,爆款1核2G云服务器298元/3年!(领取2860元代金券),
地址https://cloud.tencent.com/act/cps/redirect?redirect=1062

2021阿里云最低价产品入口+领取代金券(老用户3折起),
入口地址https://www.aliyun.com/minisite/goods

推荐:深入源码解析Android中Loader、AsyncTaskLoader、CursorLoader、LoaderManager

如果对Loader、AsyncTaskLoader、CursorLoader、LoaderManager等概念不明白或不知道如何使用Loader机制,可参见博文Android中Loader及LoaderManager的使用(附源

I was going through some of my code and I realized I don't actually know how a CursorLoader and LoaderManager combination used with a CursorAdapter connect. Heres the part that I am confused in.

agendaAdapter = new MyAgendaAdapter(this, null);

makeProviderBundle(new String[] {"_id", "event_name", "start_date", "start_time",
    "end_date", "end_time", "location"}, "date(?) >= start_date and date(?) <= end_date", 
new String[]{getChosenDate(), getChosenDate()}, null);

getLoaderManager().initLoader(0, myBundle, MainDisplayActivity.this);
list.setAdapter(agendaAdapter);

So how does the query() method from my custom ContentProvider know to send it to that specific CursorAdapter? I just don't see the connection. I understand everything else in that but what this question is on. Oh and I should mention, the code works fine.

android android-contentprovider android-loadermanager android-cursorloader
|
  this question
edited Jun 22 '12 at 14:18 Alex Lockwood 65.6k 28 167 219 asked Jun 22 '12 at 5:15 Andy 4,725 13 59 99 1   I am beginning to suspect that you are asking these questions just to screw with me, knowing that I can't help but answer (even when I am at work lol) –  Alex Lockwood Jun 22 '12 at 14:18      idk what you are talking about :) but if you're there... –  Andy Jun 22 '12 at 16:22

 | 

1 Answers
1

解决方法

First of all, check out the code sample at this post and this post for an even more in-depth look into how the process works.

And now, to answer your questions...

How does the query() method from my custom ContentProvider...?

Well, first remember that getContentResolver().query() doesn't invoke the content provider's query method directly. You are invoking the content resolver's query method, which parses the Uri, determines the provider you wish to invoke, and then calls your provider's query method.

How does the query get sent to that specific CursorAdapter?

I'll walk you through the process using the API Demos as an example. Note that the API demos uses a ListFragment instead of a ListActivity (the difference is not important in the context of this question).

  1. First, create (and set up) the CursorAdapter.

    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);
    

    After this statement is executed, the SimpleCursorAdapter knows how it should associate the cursor data with your views. Whatever data is in the cursor's Contacts.DISPLAY_NAME column will be associated with the view with id android.R.id.text1, etc.

    Note that you have passed a null cursor as the third argument to the constructor. This is very important, as we have not queried any data yet (this is the LoaderManager and CursorLoader's job).

  2. Next, initialize the loader.

    getLoaderManager().initLoader(0, null, this);
    

    This tells the LoaderManager to create and start the Loader corresponding to id 0.

  3. The LoaderManager calls onCreateLoader(int id, Bundle args).

    onCreateloader returns a subclass of the Loader<Cursor> interface (i.e. a CursorLoader, in this case). This CursorLoader will perform the initial query and will update itself when the data changes.

    If your activity/fragment has more than one loader, then you'd use switch(id) to determine the specific loader that has been instructed to begin the loading process.

  4. The queried cursor is passed to onLoadFinished().

    Immediately after the CursorLoader is instantiated and returned in step 3, the CursorLoader performs the initial query on a separate thread and a cursor is returned. When the CursorLoader finishes the query, it returns the newly queried cursor to the LoaderManager, which then passes the cursor to the onLoadFinished method. From the documentation, "the onLoadFinished method is called when a previously created loader has finished its load."

  5. The queried data is associated with the CursorAdapter.

    mAdapter.swapCursor(data);
    

    Note that onLoadFinished is also typically where you update the activity/fragment's UI with the queried data. This isn't necessary in this case, as we have previously called setListAdapter(mAdapter). The ListFragment knows how to use the CursorAdapter (see step 1)... all we need to do is pass the adapter the cursor with swapCursor, and the ListFragment will take care of displaying the data on the screen for us.

Let me know if you have any questions (or if there are any typos, etc.).

TL;DR

The cursor that contains your queried data is associated with the CursorAdapter in onLoadFinished. This is typically done by calling mAdapter.swapCursor(data).


|
  this answer
edited Apr 21 '16 at 21:22 Matt ♦ 54k 12 103 151 answered Jun 22 '12 at 15:04 Alex Lockwood 65.6k 28 167 219 2   Fantastic run through on loaders –  SeanPONeil Jun 22 '12 at 15:13 13   Don't thank me... thank all of the crappy, poorly-developed apps on the Android market. They are the reason why I feel responsible for explaining this process in excessive detail. :) –  Alex Lockwood Jun 22 '12 at 15:42 1   Hey @AlexLockwood quick question. If I am using a CursorLoader with a LoaderManager I do not need to worry about closing the database or cursors right. But what about when I just use it directly like getContentResolver().query(). In that case do I have to explicitly close them, or does all that still get handled by the class itself? –  Andy Jul 21 '12 at 20:59 2   Then, in these methods the CursorLoader will close its cursors accordingly (i.e. in onReset, etc.). In other words, it's not the LoaderManager closing the cursors... it's the LoaderManager telling the CursorLoader to close the cursors. –  Alex Lockwood Jul 21 '12 at 21:42 1   @IgorGanapolsky I remember discussing this with some other Android devs here. I always initialize loaders in onActivityCreated(). I agree with you that Google's documentation is a little vague about this point... but I have never had any issues with initializing Loaders in onActivityCreated(). –  Alex Lockwood Jan 23 '14 at 22:11  |  show more comments


相关阅读排行


相关内容推荐

最新文章

×

×

请激活账号

为了能正常使用评论、编辑功能及以后陆续为用户提供的其他产品,请激活账号。

您的注册邮箱: 修改

重新发送激活邮件 进入我的邮箱

如果您没有收到激活邮件,请注意检查垃圾箱。