blocks|key|212348|text|假设您的ListView显示了一些存储在ArrayList中的数据。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|212349|在更改了ArrayList的内容之后，您需要告诉list数据源已经更改，并且它需要重新绘制自身以显示新数据。|212350|所以，这就是notifyDatasetChanged()的用武之地。它告诉ListView数据已经被修改；为了显示新数据，必须重新绘制ListView。|BOLD|212351|entityMap^0|4|8|K|9|0|4|9|0|6|M|6|M|11|8|1V|8|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|U|8|@$9|V|A|W|B|J]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|K|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|L|$]]

Suppose your <code>ListView</code> displays some data stored in an <code>ArrayList</code>.

After you change the contents of the <code>ArrayList</code>, you need to tell the list that the source of the data had changed and it needs to redraw itself to show the new data.

So, that is where <code>notifyDatasetChanged()</code> comes in. It tells the <code>ListView</code> that the data has been modified; and to show the new data, the <code>ListView</code> must be redrawn.

blocks|key|2859751|text|BaseAdapter可以被其他类“观察”。当你调用适配器的方法时，它会调用ListView的setAdapter()的registerDataSetObserver。因此，适配器知道谁对新数据感兴趣。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2859752|您可以查看BaseAdapter+here的源代码。它很小。|2859753|notifyDataSetChanged+是，你基本上不应该覆盖它(因为它不做任何特殊的事情，所以你可以在你自己的类中重用它)。|BOLD|2859754|entityMap|0|LINK|mutability|MUTABLE|url|http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/2.1_r2/android/widget/BaseAdapter.java#BaseAdapter.notifyDataSetChanged%2528%2529^0|0|B|12|8|1B|C|1O|N|0|5|B|H|4|0|0|0|K|L|1|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@$9|14|A|15|1|16]]|E|$]]|$1|H|3|I|5|6|7|17|8|@$9|18|A|19|B|C]|$9|1A|A|1B|B|J]]|D|@]|E|$]]|$1|K|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

<code>BaseAdapter</code> can be "observed" by other classes. When you're calling method of <code>ListView</code> <code>setAdapter()</code> it's calls <code>registerDataSetObserver</code> of adapter. So, adapter knows who are interested in new data.

You can check the source of <code>BaseAdapter</code> <a href="http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/2.1_r2/android/widget/BaseAdapter.java#BaseAdapter.notifyDataSetChanged%28%29">here</a>. It's pretty small.

<code>notifyDataSetChanged</code> is available for you and you're basically shouldn't override it (because it's not doing anything special, so you can just reuse it in your own class).

blocks|key|215397|text|我已经弄明白了，。我不明白这个该死的适配器是怎么启动的，它怎么知道从哪里获取数据的。当我扩展BaseAdapter类时，我在该类的构造函数中初始化了希望在ListView中看到的项目列表。但我不知道这些值将如何使用以及何时使用。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|215398|是这样的！:|BOLD|215399|在BaseAdapter中，有一些方法需要被覆盖。其中就有getCount()。|215400|当创建ListView时，它调用getCount()。如果返回的值不是0(我返回的是之前在构造函数中初始化的ArrayList的大小)，那么它调用getView()的次数就足以填满屏幕上的项。以为例，我用20个项目初始化了ArrayList。因为最初只有8个项目可以显示在屏幕上，所以getView()被调用了8次，每次都要求我返回它所需的位置(更准确地说，它想知道该行在列表中该特定位置上的外观，以及它需要包含哪些数据)。如果我向下滚动列表，getView()会被一次又一次地调用，直到我到达列表的末尾，在我的例子中是20项/行。|215401|notifyDataSetChanged()做的是..。当被调用时，它会查看调用时屏幕上显示了哪些项目(更准确地说，是哪一行索引)，并使用这些位置调用getView()。|215402|，即如果你显示列表中的前8个项目(这些是在屏幕上可见的项目)，你在列表中的第二个和第三个项目之间添加另一个项目，然后调用notifyDataSetChanged()，那么getView()将被调用8次，位置从0开始，以7结束，因为在getView()方法中，你从ArrayList获取数据，所以它将自动返回插入列表中的前8个项目中的7个(7而不是8，因为最后一个项目向下了一个位置，所以它不再可见)。然后ListView将使用这些项重新绘制或执行其他操作。|215403|此外，需要指定的重要一点是，如果正确实现了getView()，则最终将回收已显示的项(对象)(而不是创建新的项)。请在12:00分钟左右查看，了解实现getView()的正确方法|215404|通过在每个方法中调用LogCat并跟踪发生的事情，我已经弄清楚了这一切。|215405|希望这篇文章能帮助那些刚刚开始了解ListView的工作原理的人。|215406|另外，也帮助我理解了很多。|215407|更新|215408|如今，ListViews已经不再被真正使用了。安卓推出了RecyclerView，它可以为你回收视图，但了解ListView的基础知识有助于理解RecyclerView。|215409|这里有一个供参考的链接：https://developer.android.com/guide/topics/ui/layout/recyclerview|215410|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.android.com/guide/topics/ui/layout/recyclerview^0|1A|B|25|8|0|0|6|0|1|B|T|A|0|3|8|G|A|21|9|33|9|3Y|9|66|9|0|0|M|23|9|0|0|2|1O|M|2D|9|38|9|3N|9|5M|8|0|L|9|23|9|0|A|6|0|H|8|0|0|0|2|0|3|9|S|C|1I|8|20|C|0|C|1T|0|0^^$0|@$1|2|3|4|5|6|7|1C|8|@$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1H|8|@$9|1I|A|1J|B|H]]|D|@]|E|$]]|$1|I|3|J|5|6|7|1K|8|@$9|1L|A|1M|B|C]|$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|K|3|L|5|6|7|1P|8|@$9|1Q|A|1R|B|C]|$9|1S|A|1T|B|C]|$9|1U|A|1V|B|C]|$9|1W|A|1X|B|C]|$9|1Y|A|1Z|B|C]|$9|20|A|21|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|22|8|@$9|23|A|24|B|C]|$9|25|A|26|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|27|8|@$9|28|A|29|B|H]|$9|2A|A|2B|B|C]|$9|2C|A|2D|B|C]|$9|2E|A|2F|B|C]|$9|2G|A|2H|B|C]|$9|2I|A|2J|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|2K|8|@$9|2L|A|2M|B|C]|$9|2N|A|2O|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|2P|8|@$9|2Q|A|2R|B|C]]|D|@]|E|$]]|$1|U|3|V|5|6|7|2S|8|@$9|2T|A|2U|B|C]]|D|@]|E|$]]|$1|W|3|X|5|6|7|2V|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|2W|8|@$9|2X|A|2Y|B|H]]|D|@]|E|$]]|$1|10|3|11|5|6|7|2Z|8|@$9|30|A|31|B|C]|$9|32|A|33|B|C]|$9|34|A|35|B|C]|$9|36|A|37|B|C]]|D|@]|E|$]]|$1|12|3|13|5|6|7|38|8|@]|D|@$9|39|A|3A|1|3B]]|E|$]]|$1|14|3|-4|5|6|7|3C|8|@]|D|@]|E|$]]]|15|$16|$5|17|18|19|E|$1A|1B]]]]

I've figured it out. I couldn't understand how the hell the adapter started and how did it know where to get the data from. When i extended the <code>BaseAdapter</code> class, in the constructor of that class I initialized the list of items that I wanted to see in the <code>ListView</code>. But I couldn't figure out how these values would be used and when.

So here's the thing !!! :

In the <code>BaseAdapter</code> there are some methods that need to be overridden. Among these, there is <code>getCount()</code>.

When the <code>ListView</code> is created and whatnot, it calls <code>getCount()</code>. If this returns a value different than 0 (I returned the size of the ArrayList which I've previously initialized in the constructor), then it calls <code>getView()</code> enough times to fill the screen with items. For instance, I initialized the <code>ArrayList</code> with 20 items. Because only 8 items initially fit on the screen, <code>getView()</code> was called 8 times, each time asking for the position it required for me to return (more precisely it wanted to know how the row would look like in the list on that specific position, what data it needed to contain). If I scroll down the list, <code>getView()</code> gets called over and over again, 'til I hit the end of the list, in my case 20 items / rows.

What <code>notifyDataSetChanged()</code> does is ... when called, it looks at what items are displayed on the screen at the moment of its call (more precisely which row indexes ) and calls <code>getView()</code> with those positions. 

i.e. if you're displaying the first 8 items in the list (so those are the ones visible on the screen) and you add another item between the 2nd and 3rd item in the list and you call <code>notifyDataSetChanged()</code> then <code>getView()</code> is called 8 times, with positions starting from 0 and ending with 7, and because in the <code>getView()</code> method you're getting data from the <code>ArrayList</code> then it will automatically return the new item inserted in the list alongside 7 out of the previous 8 (7 and not 8 because the last item went one position down, so it is not visible anymore), and the <code>ListView</code> will redraw, or whatever, with these items. 

Also, important to specify is that if you've implemented <code>getView()</code> correctly, you'll end up recycling the items (the objects) already displayed (instead of creating new ones). See <a href="https://www.youtube.com/watch?v=wDBM6wVEO70" rel="noreferrer">this video</a> at around 12:00 minutes to see the correct way to implement <code>getView()</code>

I've figured all this out by placing calls to <code>LogCat</code> in every method and following what was going on.

Hope this helps someone who's just now starting to understand how <code>ListView</code>s work.

P.S. <a href="http://developerlife.com/tutorials/?p=327" rel="noreferrer">This example</a> also helped me a lot to understand.

UPDATE

Nowadays <code>ListViews</code> are not really used anymore. Android came out with the <code>RecyclerView</code> which does the recycling of the views for you, but knowing the basics of a <code>ListView</code> helps with understanding the <code>RecyclerView</code>. 

Here's a link for reference: <a href="https://developer.android.com/guide/topics/ui/layout/recyclerview" rel="noreferrer">https://developer.android.com/guide/topics/ui/layout/recyclerview</a>

blocks|key|212490|text|当你为了创建一个子类而扩展你自己的BaseAdapter时，你也会得到notifyDataSetChanged()方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|212491|entityMap^0|H|B|Z|M|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

As you are extending <code>BaseAdapter</code> for your own for creating a subclass, you will get <code>notifyDataSetChanged()</code> method as well.

I am trying to understand the <code>ListView</code> concept and how it works and I'm trying to create my own adapter which extends <code>BaseAdapter</code>. For <code>ArrayAdapter</code> for instance, there is the <code>notifyDataSetChanged()</code> method which should be called after you've updated the array list which holds all your data, in order to refresh the <code>ListView</code>. 

But I am creating my own subclass of <code>BaseAdapter</code>. That method is not available to me, or is it? How do i implement this method? Basically, what does that method do exactly, maybe I'll understand then.

In case of the <code>ArrayAdapter</code> i'm guessing it looks at what position the <code>ListView</code> is currently displaying and it checks if it's the same one as in the <code>ArrayList</code> after it was updated? Or...

It says that the method:

<blockquote>
 Notifies the attached observers that the underlying data has been
 changed and any View reflecting the data set should refresh itself.
</blockquote>

But how exactly does it refresh itself?

Can someone explain please?

Android. How does notifyDataSetChanged() method and ListViews work?

我正在尝试理解ListView的概念和它是如何工作的，并且我正在尝试创建我自己的扩展BaseAdapter的适配器。例如，对于ArrayAdapter，为了刷新ListView，应该在更新包含所有数据的数组列表之后调用notifyDataSetChanged()方法。但我正在创建自己的BaseAdapter子类。这种方...

问安卓。notifyDataSetChanged()方法和ListViews是如何工作的？
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