为什么ViewHolder模式能够工作？

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问题描述

我今天学会了Android的阵列，发现有一个孔模式，它使用视图持有人保存视图的参考，而不是每次调用findviewbyid.

但是它如何工作?适配器通常用于显示视图列表(组)S，如果我缓存视图，为什么它们都不全部参考最旧的一个?

其他推荐答案

对不起，但丹尼斯的回答可能是错误的. 实际上，视图实例(和视图持有人)是的数量，就像您的屏幕可以显示.

如果您的屏幕看起来像:

[list view]
the first item
the second item
the third item
the fourth item

您将有4个意见实例.如果滚动屏幕，则第一个将消失，但可以通过convertView将其传递给您创建第五项.

因此，您可以使用第一视图持有人中的参考.

其他推荐答案

我相信列表视图下面的工作就是这样(考虑到我们只有一个项目视图类型):

做一次:

inflate item view from layout, cache it

重复每个项目:

ask adapter to fill the data into the view
draw the view on the screen
move to next item

因此，您具有从XML布局中夸大的视图，可以重复使用以绘制多个列表项目.查看持有人通过保存getViewById查找来加快速度.

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问题描述

I learned Android's ArrayAdapter today, and find there is a commom pattern which uses a ViewHolder to hold Views' reference instead of calling findViewById everytime.

But how does it work? Adapter is usually used to display a list of View(Group)s, If I cache the View, why don't they all reference to the oldest one?

其他推荐答案

Sorry but denis' answer may be wrong. In fact, the view instances(and ViewHolders) are as many as your screen can display.

If your screen looks like:

[list view]
the first item
the second item
the third item
the fourth item

You will have 4 instances of views. If you scroll screen, the first will disappear but be pass to getItem() as convertView for you to create the fifth item.

So you can use the references in first ViewHolder.

其他推荐答案

I believe the work beneath the list view is something like this (considering we have only one item view type):

do once:

inflate item view from layout, cache it

repeat for every item:

ask adapter to fill the data into the view
draw the view on the screen
move to next item

so you have the view which is inflated from xml layout and can be reused for drawing multiple list items. ViewHolder speeds it up a bit more by saving getViewById lookups.

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