On 30 Jul 2012, at 19:40, David Duncan wrote:
You are going to have to define what you mean by start & end, as it
is not clear in context.
If you mean a visual location, then neither of these methods are
going to do what you want. You would put something visually at the
start/end of the view by setting its location (via frame.origin or
center) not via the add subview method you use.
If you mean a layering order, then the insertSubview:atIndex:
method may attempt to do what you want, but given that a scroll
view has other implicit subviews you would need to be more careful
if you need an exact location. It may make more sense to use an
intermediate subview that you place content into and which is a
child of the scroll view in that case.
--
David Duncan
Hi David,
It's probably easier if I try to describe what I am trying to do!
Basically I have process that generates UIImage's and each of these
images needs to be presented in a Scroll View. However there can be a
large number of images generated and each image is pretty big too, so
obviously, it can't hold them all in RAM. My idea is to keep a number
of images (say 5) cached and as the user scrolls, generate the next
or previous image and discard the old one.
In this document:
http://developer.apple.com/library/ios/DOCUMENTATION/WindowsViews/
Conceptual/UIScrollView_pg/UIScrollView_pg.pdf
I found the following:
Configuring Subviews of a Paging Scroll View
The subviews of a paging scroll view can be configured in one of two
ways. If the content is small, you could
draw the entire contents at once, in a single view that is the size
of the scroll view’s contentSize. While this
is the easiest to implement, it is not efficient when dealing with
large content areas, or page content that takes
time to draw.
When your application needs to display a large number of pages or
drawing the page content can take some
time, your application should use multiple views to display the
content, one view for each page. This is more
complicated, but can greatly increase performance and allows your
application to support much larger display
sets. The PageControl example uses this multiple view technique. By
examining the sample code, you can see
exactly how this technique can be implemented.
Supporting a large number of pages in a paging scroll view can be
accomplished using only three view instances,
each the size of the device screen: one view displays current page,
another displays the previous page, and
third displays the next page. The views are reused as the user
scrolls through the pages.
When the scroll view controller is initialized, all three views are
created and initialized. Typically the views are
a custom subclass of UIView, although an application could use
instances of UIImageView if appropriate. The
views are then positioned relative to each so that when the user
scrolls, the next or previous page is always in
place and the content is ready for display. The controller is
responsible for keeping track of which page is the
current page.
To determine when the pages need to be reconfigured because the user
is scrolling the content, the scroll
view requires a delegate that implements the scrollViewDidScroll:
method. The implementation of this
method should track the contentOffset of the scroll view, and when it
passes the mid point of the current
view’s width, the views should be reconfigured, moving the view that
is no longer visible on the screen to the
position that represents the next and previous page (depending on the
direction the user scrolled). The delegate
should then inform the view that it should draw the content
appropriate for the new location it the represents.
By using this technique, you can display a large amount of content
using a minimum of resources.
If drawing the page content is time consuming, your application could
add additional views to the view pool,
positioning those as pages on either side of the next and previous
pages as scrolling occurs, and then draw
the page content of those additional pages when the current content
scrolls.
--------------------------------------------------
The above is basically what I'd like to do, hold a number of pages/
images "around" the current page and when the user Scrolls left or
right replace the appropriate pages/images with newly rendered versions.
I reckon that holding to 2 or 3 pages/images to the left and right of
the current image should be enough to allow the user to scroll and
not know the pages are being generated on the fly.
I've looked for the Sample Code mention is the above document, but I
can't find it anywhere, I found a sample project on the Apple
Developer Site but it doesn't seem to the the one they are talking
about.
Any ideas or sample code on how to do this gratefully appreciated!
Thanks in Advance
Dave
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