How to implement UIScrollView with 1000+ subviews?

Question

I am struggling with writing portion of an app which should behave like the native iphone photo app. Looked at iphone sdk app development book from Orielly which gave an example code for implementing this so-called page-flicking. The code there first created all subviews and then hide/unhide them. At a given time only 3 subviews are visible rest are hidden. After much effort I got it working with app which at that time had only around 15 pages. As soon as I added 300 pages, it became clear that there are performance/memory issues with that approach of pre-allocating so many subviews. Then I thought may be for my case I should just allocate 3 subviews and instead of hide/unhide them. May be I should just remove/add subviews at runtime. But can't figure out whether UIScrollView can dynamically update contents. For example, at the start there are 3 frames at different x-offsets ( 0, 320, 640 ) from the screen as understood by UIScrollView. Once user moves to 3rd page how do I make sure I…

Accepted Answer

UIScrollView is just a subclass of UIView so it's possible to add and remove subviews at runtime. Assuming you have fixed width photos (320px) and there are 300 of them, then your main view would be `300 * 320` pixels wide. When creating the scroll view, initialize the frame to be that wide.

So the scroll view's frame would have the dimensions (0, 0) to (96000, 480). Whenever you are adding a subview, you will have to change it's frame so it fits in the correct position in its parent view.

So let's say, we are adding the 4th photo to the scroll view. It's frame would be from (960, 480) to (1280, 480). That is easily to calculate, if you can somehow associate an index with each picture. Then use this to calculate the picture's frame where indexes start at 0:

```
Top-Left -- (320 * (index - 1), 0)
```

to

```
Bottom-Right -- (320 * index, 480)
```

Removing the first picture/subview should be easy. Keep an array of the 3 subviews currently on-screen. Whenever you are adding a new subview to the screen, also add it to the end of this array, and then remove the first subview in this array from the screen too.

How to implement UIScrollView with 1000+ subviews?

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