Updating multiple view constraints at the same time

Question

Is there a way to tell the auto layout engine, at runtime, that I'm going to change multiple constraints on a view at the same time? I keep running into a situation where the order in which I update the constraints matters because even though when all is said and done all of the constraint conditions are satisfied, changing the constant on one before the other throws a LAYOUT_CONSTRAINTS_NOT_SATISFIABLE exception (or whatever the exception is actually called...) Here's an example. I create and add a view to my current view and set some constraints on it. I'm saving the leading and trailing edge constraints so I can change their constants later. ``` - (MyView*)createMyView { MyView* myView = [[MyView alloc init]; [myView setTranslatesAutoresizingMaskIntoConstraints:NO]; [self.mainView addSubview:myView]; NSDictionary* views = @{@"myView" : myView}; NSLayoutConstraint* leading = [NSLayoutConstraint constraintWithItem:myView attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEq…

Accepted Answer

First, you seem to have over-constrained `myView` relative to its superview such that the constraints can't allow it to slide. You've pinned its leading and trailing edges plus, when adding the vertical constraint, you've pinned its center, too, by specifying `NSLayoutFormatAlignAllCenterX`.

That aside, perhaps the better way to think about it is that having two constraints that you have to update in sync suggests you're using the wrong constraints. You could have a constraint for the leading edge and then a constraint making `myView`'s width equal to the width of `self.view`. That way, you only have to adjust the leading constraint's constant and both edges move as a consequence.

Also, you're using `-setFrame:`, which is a no-no with auto layout. You should be animating the constraints. You should set up constraints on `newView` such that its leading or trailing edge (as appropriate) equals the the neighboring edge of `myView` and its width equals that of `self.view`. Then, in the animation block, change the constant of the constraint determining the placement of `myView` (and, indirectly, `newView`) to slide them together. In the completion handler, install a constraint on `newView` to keep its leading edge coincident with `self.view`'s, since it's the new `myView`.

```
BOOL left = (swipeDirection == UISwipeGestureRecognizerDirectionLeft);
float width = self.myView.frame.size.width;
float finalX = left ? -width : width;
NSString* layout = left ? @"[myView][newView(==mainView)]" : @"[newView(==mainView)][myView]";
NSDictionary* views = NSDictionaryOfVariableBindings(newView, myView, mainView);

[self.mainView addSubview:newView];
[self.mainView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:layout
                                                                      options:0
                                                                      metrics:nil
                                                                        views:views]];
[self.mainView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|newView|"
                                                                      options:0
                                                                      metrics:nil
                                                                        views:views]];
[self.mainView layoutIfNeeded];

// Slide old view out and new view in
[UIView animateWithDuration:0.4f
                      delay:0.0f
                    options:UIViewAnimationOptionCurveEaseInOut | UIViewAnimationOptionAllowUserInteraction
                 animations:^
        {
            self.myView.leadingConstraint.constant = finalX;
            [self.myView layoutIfNeeded];
        }
                 completion:^(BOOL finished)
        {
            [self.myView removeFromSuperview];
            self.myView = newView;
            NSLayoutConstraint* leading = [NSLayoutConstraint constraintWithItem:newView
                                                                       attribute:NSLayoutAttributeLeading
                                                                       relatedBy:NSLayoutRelationEqual
                                                                          toItem:self.view
                                                                       attribute:NSLayoutAttributeLeading
                                                                      multiplier:1.0f
                                                                        constant:0];
            self.myView.leadingConstraint = leading;
            [self.mainView addConstraint:leading];
        }];
```

Edited to answer the question more directly: No, there's no way in the public API to batch constraint updates so that the engine doesn't check them one-at-a-time as they're added or mutated by setting their constant.

Updating multiple view constraints at the same time

Solution

Problem