addSubview change the index of one object in scroll.subviews - ios

Question

I add objects in UIScrollView inside cycle FOR, but after the second interaction the index of subviews change.
When i create the ScrollView:
scroll.subviews :(
"<UIImageView: 0x147aeee0; frame = (1017 186; 7 5); alpha = 0; opaque = NO; autoresize = TM; userInteractionEnabled = NO; layer = <CALayer: 0x147aef50>>",
"<UIImageView: 0x147aefa0; frame = (1019 184; 5 7); alpha = 0; opaque = NO; autoresize = LM; userInteractionEnabled = NO; layer = <CALayer: 0x147af010>>"
)
After i have the cycle FOR with [scroll addSubview:btn];
First
scroll.subviews :(
"<UIImageView: 0x147aeee0; frame = (1017 186; 7 5); alpha = 0; opaque = NO; autoresize = TM; userInteractionEnabled = NO; layer = <CALayer: 0x147aef50>>",
"<UIImageView: 0x147aefa0; frame = (1019 184; 5 7); alpha = 0; opaque = NO; autoresize = LM; userInteractionEnabled = NO; layer = <CALayer: 0x147af010>>",
"<ContentButton: 0x13fdc610; baseClass = UIButton; frame = (15 10; 221 173); opaque = NO; tag = 1; layer = <CALayer: 0x1034cc00>>"
)
Second (change last object to index 0)
scroll.subviews :(
"<ContentButton: 0x13fdc610; baseClass = UIButton; frame = (15 10; 221 173); opaque = NO; tag = 1; layer = <CALayer: 0x1034cc00>>",
"<UIImageView: 0x147aefa0; frame = (1019 184; 5 7); alpha = 0; opaque = NO; autoresize = LM; userInteractionEnabled = NO; layer = <CALayer: 0x147af010>>",
"<UIImageView: 0x147aeee0; frame = (1017 186; 7 5); alpha = 0; opaque = NO; autoresize = TM; userInteractionEnabled = NO; layer = <CALayer: 0x147aef50>>",
"<ContentButton: 0x105ddf70; baseClass = UIButton; frame = (246 10; 221 173); opaque = NO; tag = 149; layer = <CALayer: 0x105523e0>>"
)
Third and so on, the object will add normally (in last index)
scroll.subviews :(
"<ContentButton: 0x13fdc610; baseClass = UIButton; frame = (15 10; 221 173); opaque = NO; tag = 1; layer = <CALayer: 0x1034cc00>>",
"<UIImageView: 0x147aefa0; frame = (1019 184; 5 7); alpha = 0; opaque = NO; autoresize = LM; userInteractionEnabled = NO; layer = <CALayer: 0x147af010>>",
"<UIImageView: 0x147aeee0; frame = (1017 186; 7 5); alpha = 0; opaque = NO; autoresize = TM; userInteractionEnabled = NO; layer = <CALayer: 0x147aef50>>",
"<ContentButton: 0x105ddf70; baseClass = UIButton; frame = (246 10; 221 173); opaque = NO; tag = 149; layer = <CALayer: 0x105523e0>>",
"<ContentButton: 0x13f1dc80; baseClass = UIButton; frame = (477 10; 221 173); opaque = NO; tag = 3; layer = <CALayer: 0x13f1dd20>>"
)
The problem is in the second interaction, so why the last object (ContentButton) was in the index 2 and change into index 0?
for (obj * objScroll in arrayChaObj)
{
// UIButton
ContentButton * btn = [[ContentButton alloc] initWithFrame:CGRectMake(btContentPosXChaObj,
10,
imgButtonScroll.size.width,
imgButtonScroll.size.height)];
btContentPosXChaObj += 10 + imgButtonScroll.size.width;
[scroll addSubview:btn];
NSLog(#"scroll.subviews :%#",scroll.subviews);
[scroll setContentSize:CGSizeMake(btContentPosXChaObj, imgButtonScroll.size.height+10)];
}

Answer 1

This is just a wild guess.
What happens when you call setContentSize: for the first time? Well, the two image views end up outside the content region. They won't be ever displayed.
I guess the scrollview then optimizes the order of subviews and puts the invisible ones to the end which can probably improve its clipping (redrawing) functionality.
Currently I don't have the time to check this and experiment but it's a reasonable explanation for me.