I have a horizontal UIScrollView that contains UIButtons (though it could be any object). I would like to only download images for these buttons as the user scrolls and they are seen. I would also like an activityindicator running on each button while the images are downloading. Here's what I've tried with results:
Check content offset of uiscrollview and download images for visible buttons. Problem: issues getting activity view shown within subclassed UIButton and not desired usability since images are only downloaded once scrolling stops.
Add subviews (not buttons) to the UIScrollview hoping to use the view's ViewController to initiate a downloaded on viewDidAppear. Problem: viewDidAppear never seems to get called (because I am using addSubView?).
I've seen other apps that do this kind of loading but need some guidance on the typical set-up. Ebay's app is a great example. All items have a default image until the item comes into view, then an activityindicator runs and the items unique image is downloaded.
Thanks for assistance.
Without actually trying it out, here's my idea:
You can use the UIScrollViewDelegate method scrollViewDidScroll: to find out when your scroll view finished scrolling and then use [UIScrollView contentOffset] to determine which part of the scroll view is visible and (CGRect)convertRect:(CGRect)rect fromView:(UIView *)view to determine the button's position within the visible part of the scroll view. Have a method that checks if the button has had its image loaded yet and if not, load it and refresh the view if necessary.
Most such apps are using a UITableView, not a plain scroll view. Joe Hewitt's Three20 framework has a TTImageView class that can do lazy image loading and is pretty well suited to this kind of use.
Related
I'm trying to load a table view with cells that contain images downloaded off the internet. I'd like to only perform the download operation on cells that are currently visible instead of downloading every image as the user scrolls down quickly.
I found the indexPathsForVisibleRows method and that should fit my needs nicely but I need a way to figure out when the table view is no longer scrolling. My idea is to use the indexPathsForVisibleRows when the table view isn't scrolling so that all of the images currently on screen are loaded without unnecessarily loading images.
Are there any methods that I can override or notifications to subscribe to that will alert me when the table view is no longer scrolling?
Thanks!
Take a look at the methods listed here:
http://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIScrollViewDelegate_Protocol/Reference/UIScrollViewDelegate.html
If your view controller is the delegate of the UITableView, these will also be fired for it, as the UITableView is a subclass of UIScrollView.
Particularly, pay attention to: scrollViewDidEndDecelerating. This is the one I used to do pretty much what you're asking.
I have a UIScrollView with textviews as subviews. Now in my app there are multiple UIScrollViews like these. And depending on the selection I display the appropriate UIScrollView on top of the previous view. This works fine in all cases except when the previous view has been a UIScrollView as well. In this case the behavior I get is of two UIScrollViews stacked on top of each other and both the views capture the scrolling events. The textViews from previous scrollView is also visible (not editable though) and overlaps and causes all sorts of issues. The thing is a full screen UIScrollView placed as subview to a previous view causes problems when the previous view is a full screen UIScrollView as well.
Any pointers on how to overcome this would be great. Is there anyway to notify the parent scrollview of the child's scroll events and move it the exact same way so this mess is masked?
Thanks!
UIScrollView documentation says:
Important: You should not embed UIWebView or UITableView objects in UIScrollView objects. If you do so, unexpected behavior can result because touch events for the two objects can be mixed up and wrongly handled.
As far as I know, it is generally not recommended to embed a UIScrollView subclass instance in a UIScrollView. In one of my projects, which is an IM application, I have a UIScrollView that contains many UITableViews, it works when you try enough but it really takes a lot of effort to handle subtle bugs and make it work properly.
It is really hard to say something useful for your problem without diving in to code, but i can recommend you to not add UIScrollViews one on to another like a stack. I'd try doing it by allowing only one UIScrollView at top, and removing others. When you need to show another one, remove the top one from view hierarchy and add the new one. I wish this helps, good luck.
Found a crude solution by presenting an empty view before I present the next scrollView. Added the scrollView as subview to this empty view (with a BG image) and added the to-be presented scrollView as a subview to it and then presenting this on top of the previous scrollView.
I am trying to implement a filmstrip-like UIScrollView that will be populated with thumbnails of catalog pages. Selecting a thumbnail image will cause the main UIScrollView to move to the selected page. The Catalog may contain 100 - 200 pages, and I want to load them lazily only when required.
I have done this in a UIScrollView with paging enabled, but haven't seen anything on the best way to do this in a non-paging scenario. There will be 6 thumbnails visible in the UIScrollView (+ 1 when the view is being scrolled) at any one time. I want to dequeue and reuse the thumbnail's UIView when the view is scrolled, as I am doing in the main UIScrollView (which is a paging scroll view).
Thanks -
Jk
I am also going to suggest you take a look at some sample code of Apple, that is, Photo Scroller. If you are a registered iOS developer, you should also take a look at the WWDC10 session about scroll views in iPhone applications.
http://developer.apple.com/library/ios/#samplecode/PhotoScroller/Listings/main_m.html
What you need to do is mimic the behavior of a table view (which is nothing more than a subclass of UIScrollView). What you should mimic is the reuse of the cells. It is pretty easy to implement and will dramatically reduce the memory foot print of your application since you only load the content that is currently visible in the scroll view.
I hope this helps.
Check out the scrollview suite sample code from apple. The tiled example can probably be repurposed very easily.
http://developer.apple.com/library/ios/#samplecode/ScrollViewSuite/
Check out this class..it may proove helpful..
VSScroller
I was wondering, what would be the best way, to implement an 'extra' scrollbar, showing thumbnail versions of the pages, you are currently viewing.
In my case, I would like to have maybe around 20 images, which I would animate with Cover Flow Layers and while I 'scroll' up an down to view them, a scrollbar appears at the side, showing the smaller thumbnail versions of all the pages while the thumbnail currently visible is rendered slightly highlighted.
Is there anything specific I need to keep in mind? Thanks in advance for any ideas!
You are not giving much info to work with:)
Build a UIScrollView and a UIView, stick all the thumbnails into the UIView, stick the UIView into the UIScrollView.
If you have a lot of images, consider loading them in as "dummies/blank image" and have an NSOperation load them in the background. Consider releasing images that are outside of the screen, so that if 3-7 is on the screen then you only load in 2-8 and release everything else.
Regarding the logistics of it:
Build a viewController for holding everything. Then build a thumbnail viewController "component" that has a delegate method for setting the "displayed" image and maybe one for scroll and click.
Build a FullSize viewController component for handling the full size images. Have that also implement some delegate methods to communicate to the top most viewController. Add the two components to the top viewController, hook up the logic and you are done.
Make sure there exists only one model array/dictionary in the top most viewController and "feed" that downwards to the thumbnail and fullsize viewController, when some one clicks or scrolls, inform the top most viewController and have that update the other viewController.
Was that the kind of answers you were looking for?
:)
For a vertical scroller are there any advantages to using UIScrollView over UITableView?
I ask because I am currently using two vertical UIScrollViews with UIImageViews inside of them and am having memory issues and poor scrolling performance. I am not doing much with the scrollers, only highlighting images as they scroll into the center of the scrollviews and adding a delete button above an image if the user wants to remove it. I've started to look at lazy image loading/reuse and it seems that most of these issues have already been resolved in the UITableView class, so I'm wondering if there's any reason to stick with UIScrollView?
You should be able to use a UIScrolLView with no problem if you just, like you said, lazy load the controllers, when scrolling (assuming your views are full size) you dont need to have more than 3 views loaded at any one time, all you need is the current view, the view that goes behind it and infront of it, as you scroll your view you can unload and load the appropriate views. You should look into Page Control sample application, it does exactly this and you can pretty much get all the lazy loading code from there. Link is here https://developer.apple.com/iphone/library/samplecode/PageControl/index.html