Layout constraints for dynamically created buttons - ios

Question

I am trying to create a view with dynamically created buttons. I am finding it difficult to set the constraints for inner objects other than first one that's created. Where is the issue?
Create & Add buttons to view
-(void) createButton:(NSString *) btnText isButton:(BOOL) type phraseWidth:(NSInteger) width view:(UIView *) currentView {
if (!type) { // if it's a button then create label & button at same place else only create button
// align left to prev button, align baseline
if (prevX == 5) { // button left aligned to rowView, right align none
UIButton *btnView = [[UIButton alloc] init];
btnView.translatesAutoresizingMaskIntoConstraints=NO;
[currentView addSubview:btnView];
NSDictionary *dictScrollConst = NSDictionaryOfVariableBindings(btnView);
NSString *hConstraint = [NSString stringWithFormat:#"H:|-%f-[btnView]|",prevX];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:hConstraint options:0 metrics:nil views:dictScrollConst]];
NSString *vConstraint = #"V:|[btnView]|";
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:vConstraint options:0 metrics:nil views:dictScrollConst]];
prevObject = btnView;
}
else { // align new button to previous button
UIButton *btnView = [[UIButton alloc] init];
btnView.translatesAutoresizingMaskIntoConstraints=NO;
[currentView addSubview:btnView];
NSDictionary *dictScrollConst = NSDictionaryOfVariableBindings(prevObject,btnView);
NSString *hConstraint = [NSString stringWithFormat:#"H:[prevObject]-%d-[btnView]",kHorizontalSidePadding];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:hConstraint options:0 metrics:nil views:dictScrollConst]];
NSString *vConstraint = #"V:|[btnView]|";
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:vConstraint options:0 metrics:nil views:dictScrollConst]];
}
}
}
Doesn't allow constraint to be added with respect to previous button created. Throws up exception:
Impossible to set up layout with view hierarchy unprepared for constraint

Answer 1

There's still way too much code for me to work out what's going on, but this much is obvious:
NSString *vConstraint = #"V:|[btnView]|";
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:vConstraint options:0 metrics:nil views:dictScrollConst]];
[currentView addSubview:btnView];
Those lines are in the wrong order. You cannot add a constraint involving a view (here, btnView) at a time when that view is not in the view hierarchy. (That is exactly what the error message is telling you, though granted it phrases it in rather coy terminology.)
So, add the subview. Then add the constraint that affects it.
What I suggest you do is what I always do: start very simple and work your way up to the full extent of the actual problem. So, I suggest as an exercise that you begin with the second row of your layout, and see if you can do just this simple exercise: given the array of titles #[#"Yellow", #"Purple", #"Blue", #"Red"], can you use it to generate four buttons horizontally?
Here's my code for doing that. Notice how clear and simple it is - ruthlessly logical, spare, and plain. We can always add tweaks later, but this is the kind of simplicity you need to try to maintain and build upon, so that you don't confuse yourself:
NSArray* titles = #[#"Yellow", #"Purple", #"Blue", #"Red"];
UIView* previousButton = nil;
for (NSInteger i = 0; i < 4; i++) {
UIButton* b = [UIButton buttonWithType:UIButtonTypeSystem];
[b setTitle:titles[i] forState:UIControlStateNormal];
[b setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:b];
[self.view addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"V:|-(100)-[b]"
options:0 metrics:nil views:#{#"b":b}]];
if (i == 0) {
[self.view addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:|-(50)-[b]"
options:0 metrics:nil views:#{#"b":b}]];
} else {
[self.view addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:[p]-(20)-[b]"
options:0 metrics:nil views:#{#"b":b, #"p":previousButton}]];
}
previousButton = b;
}
Given this, we see at once one of the things wrong with your code: there is no evidence that you are setting the previous button (your prevObject) on any except the first pass, when of course you need to do it on every pass.
Once we have code that works, we can start to modify it to approach what you are wishing to do. For example, it is now easy to change the hard-coded spacing to use variables like yours instead:
NSArray* titles = #[#"Yellow", #"Purple", #"Blue", #"Red"];
UIView* previousButton = nil;
NSInteger initialX = 5; // *
NSInteger horizSpace = 10; // *
for (NSInteger i = 0; i < 4; i++) {
UIButton* b = [UIButton buttonWithType:UIButtonTypeSystem];
[b setTitle:titles[i] forState:UIControlStateNormal];
[b setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:b];
[self.view addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"V:|-(100)-[b]"
options:0 metrics:nil views:#{#"b":b}]];
if (i == 0) {
[self.view addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:|-(initialX)-[b]"
options:0 metrics:#{#"initialX":#(initialX)} views:#{#"b":b}]];
} else {
[self.view addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:[p]-(horizSpace)-[b]"
options:0 metrics:#{#"horizSpace":#(horizSpace)} views:#{#"b":b, #"p":previousButton}]];
}
previousButton = b;
And so forth. The point is: This is how I "grow my code", starting always with the simple and evolving, making sure it works at every iteration, until I reach the thing I'm really trying to do. Go ye and do likewise!