Question

When subclassing UIView, I usually place all my initialisation and layout code in its init method. But I’m told that the layout code should be done by overriding layoutSuviews. There’s a post on SO that explains when each method gets called, but I’d like to know how to use them in practice.

I currently put all my code in the init method, like this:

MyLongView.m

- (id)initWithHorizontalPlates:(int)theNumberOfPlates
{
    self = [super initWithFrame:CGRectMake(0, 0, 768, 1024)];

    if (self) {
        // Initialization code
        _numberOfPlates = theNumberOfPlates;

        UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame:self.frame];
        [scrollView setContentSize:CGSizeMake(self.bounds.size.width* _numberOfPlates, self.bounds.size.height)];
        [self addSubview:scrollView];

        for(int i = 0; i < _numberOfPlates; i++){
            UIImage *img = [UIImage imageNamed:[NSString stringWithFormat:@"a1greatnorth_normal_%d.jpg", i+1]];
            UIImageView *plateImage = [[UIImageView alloc] initWithImage:img];
            [scrollView addSubview:plateImage];
            plateImage.center = CGPointMake((plateImage.bounds.size.width/2) + plateImage.bounds.size.width*i, plateImage.bounds.size.height/2);
        }

    }
    return self;
}

It’s the usual tasks: setting up the view’s frame, initialising an ivar, setting up a scrollview, initialising UIImages, placing them in UIImageViews, laying them out.

My question is: which of these should be done in init, and which of these should be done in layoutSubviews?

Answer 1

Your init should create all the objects, with the required data. Any frame you pass to them in init should ideally be their starting positions.

Then, within layoutSubviews:, you change the frames of all your elements to place them where they should go. No alloc’ing or init’ing should take place in layoutSubviews:, only the changing of their positions, sizes etc…