Reusing subview

Question

Hi

I have a UIImageView object that is just a plain black rectangle. This is what i use to select a button in the view.

Problem is, I have 49 of these buttons in my view, and all of them can be selected at the same time.

What I use for adding a subview to a button is:

UIImageView* selectedSquareView = [[UIImageView alloc] initWithFrame:CGRectMake(0,0,40,40)];
[selectedSquareView setImage:[UIImage imageNamed:@"SelectedSquare.png"]];
[button addSubview: selectedSquareView];

I would like the selectedSquareView to be reused multiple times as subviews for the other buttons, but only keep one allocation of it. I would prefer not having 49 UIImageViews created at the same time just for this purpose. Is this possible?

If not, should I store them in a NSMutableArray for easy removal later?

Regards

-Raymond

Answer 1

You will need 49 UIImageViews, you only need 1 UIImage. The UIImageViews contain position, size, is_higlighted, etc information for each button.

That being said even if you had a lot of UIImage's, UIImage is supposed to be pretty intelligent about these things as Apple describes in their documentation:

In low-memory situations, image data may be purged from a UIImage object to free up memory on the system. This purging behavior affects only the image data stored internally by the UIImage object and not the object itself. When you attempt to draw an image whose data has been purged, the image object automatically reloads the data from its original file. This extra load step, however, may incur a small performance penalty.

You should avoid creating UIImage objects that are greater than 1024 x 1024 in size. Besides the large amount of memory such an image would consume, you may run into problems when using the image as a texture in OpenGL ES or when drawing the image to a view or layer. This size restriction does not apply if you are performing code-based manipulations, such as resizing an image larger than 1024 x 1024 pixels by drawing it to a bitmap-backed graphics context. In fact, you may need to resize an image in this manner (or break it into several smaller images) in order to draw it to one of your views.

Alternatively if you really feel like you need to do delete the UIImageView's when not in use you can do as you suggest store them in an array and release them on viewDidDisappear and then recreate them all on viewWillAppear.

Answer 2

Each UIView appears only once, so you will definitely need to create 49 copies of it.

Your current code is probably fine, since UIImage will probably cache the image, but you might like to create the image only once and then set it each time, something like:

UIImageView* selectedSquareView = [[UIImageView alloc] initWithFrame:CGRectMake(0,0,40,40)];
static UIImage* kSelectedSquareImage = [UIImage imageNamed:@"SelectedSquare.png"] retain];
[selectedSquareView setImage:kSelectedSquareImage];

If not, should I store them in a NSMutableArray for easy removal later?

It depends if there are any other views in the container view - if not, then there is no need to store them in an NSMutableArray as you can just use container.subviews to get an array of views. Otherwise, sure, you could store them in an NSMutableArray and remove them that way (just make sure you remove them from the array or release the array as well, otehrwise they will remain in memory simply because they are stored in the array).