How to swap UIScrollViews? - objective-c

Question

Let's imagine that I have 2 UIScrollViews with different UIImageViews inside. When I trigger an action I would like the contents, parameters etc (besides the location) of the 2nd UIScrollView to be passed onto the 1st UIScrollView.
So, here's the code that I've come up with:
-(void) someAction {
UIScrollView * scroll = [[UIScrollView alloc] init]; // create an intermediary scrollView
scroll = secondScroll; // here I intent to pass the content of secondScroll to scroll
scroll.frame = firstScroll.frame; // here I assign the frame of firstScroll to scroll
so that it doesn't take the frame of secondScroll;
firstScroll =scroll; // and then pass all the contents of scroll to firstScroll;
}
But when I execute that and trigger the action, the firstScroll seems to take the content of secondScroll, but secondScroll seems to be deleted. I need it to stay as is.
Any help?
Thanks

Answer 1

-(void) someAction {
UIScrollView * scroll = [[UIScrollView alloc] init]; // create an intermediary scrollView
That's WRONG. You'll be leaking memory with it, since you're losing the pointer to that scroll view after assigning it to a different object. You'd better simply declare UIScrollView *scroll; for that purpose.
scroll = secondScroll; // here I intent to pass the content of secondScroll to scroll
But that's not what happens. The scroll views don't magically copy themselves; if you assign one to another, you only assign the pointer.
scroll.frame = firstScroll.frame; // here I assign the frame of firstScroll to scroll
// so that it doesn't take the frame of secondScroll;
So, from now on, if you operate on scroll, it will effect secondScroll.
firstScroll = scroll; // and then pass all the contents of scroll to firstScroll;
And with this, you assign firstScroll to SecondScroll. So firstScroll will now essentially be secondScroll, losing it (leaking memory again and again) and having done nothing useful.
}
What on Earth are you trying to do with this?
Assuming you want to exchange the two views, you should do something like this instead:
BOOL firstScrollVisible = NO; // in -init or whatever
- (void) someAction
{
if (fistScrollVisible)
[self.view bringSubviewToFront:secondScroll];
else
[self.view bringSubviewToFront:firstScroll];
firstScrollVisible = !firstScrollVisible;
}

Answer 2

Try doing the following:
- (void)someAction {
[firstScroll removeFromSuperview]; // So that we do not leave the old firstScroll as a subview
firstScroll = [secondScroll copy]; // Makes a copy of secondScroll and stores it in firstScroll
[self addSubview:firstScroll]; // Add the new firstScroll to the view
}
Important Note(s):
This code will leak memory, as -copy returns an object with a reference count of +1, but we do not release firstScroll. I highly suggest you make firstScroll and secondScroll into nonatomic, retain properties (#property (nonatomic, retain) UIScrollView *firstScroll). If you want more info on this, just ask. Furthermore, you will need to make sure that firstScroll and secondScroll are never garbage values when this code gets run (it will crash) - depending on when and how it is called you may be fine.

Answer 3

If you want to clone a UI element, you use NSCoder (just like the nib file loader does).
NSData *scrollViewData = [NSKeyedArchiver archivedDataWithRootObject:self.firstScroll];
CGRect oldFrame = self.secondScroll.frame;
self.secondScroll = [NSKeyedUnarchiver unarchiveObjectWithData:scrollViewData];
self.secondScroll.frame = oldFrame;
This serializes the UIView and all its subviews into an NSData and then creates a complete copy of those from the NSData.

Answer 4

#jrtc27 Thanks for your answer. I think I've found a solution and it seems to accomplish what I'm looking for. But I'm not sure if it's the right way or not. Here's what I do:
- (void)someAction {
[[firstScroll subviews] makeObjectsPerformSelector:#selector(removeFromSuperView:);//here I sort of blank out the content of the firstScroll
NSArray * array = [secondScroll subviews];//place all subview of the secondScroll in array
int i = 0;
int nrOfSubviews = [array count];
for (i=0;i<nrOfSubviews;i++) { //appoint them one by one to firstScroll
[firstScroll addSubview:[array objectAtIndex:i]];
}
}