Catch UIViewAlertForUnsatisfiableConstraints in production

Question

Is it possible to catch autolayout constraint ambiguities in production – the equivalent of a UIViewAlertForUnsatisfiableConstraints breakpoint but for production apps?

My goal would be to add a global handler that would report such errors to a logging system.

Answer 1

The symbol UIViewAlertForUnsatisfiableConstraints actually is a function :

_UIViewAlertForUnsatisfiableConstraints(NSLayoutConstraint* unsatisfiableConstraint, NSArray<NSLayoutConstraint*>* allConstraints).

It is private, so you can't replace it.

But it is called from private method -[UIView engine:willBreakConstraint:dueToMutuallyExclusiveConstraints:], which can be swizzled. This method has approximately this content:

void -[UIView engine:willBreakConstraint:dueToMutuallyExclusiveConstraints:] {
  if ([self _isUnsatisfiableConstraintsLoggingSuspended]) {
    [self _recordConstraintBrokenWhileUnsatisfiableConstraintsLoggingSuspended:$arg4]; // add constraint to some pool
  }
  else {
    if (__UIConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints) {
      // print something in os_log
    }
    else {
      _UIViewAlertForUnsatisfiableConstraints($arg4, $arg5);
    }
  }
}

If I understand correctly from this article, __UIConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints will always return NO on iOS, so all you need to do is to check private bool property called _isUnsatisfiableConstraintsLoggingSuspended and call original method then.

This is result code example:

#import <objc/runtime.h>

void SwizzleInstanceMethod(Class classToSwizzle, SEL origSEL, Class myClass, SEL newSEL) {
  Method methodToSwizzle = class_getInstanceMethod(classToSwizzle, origSEL);
  Method myMethod = class_getInstanceMethod(myClass, newSEL);
  class_replaceMethod(classToSwizzle, newSEL, method_getImplementation(methodToSwizzle), method_getTypeEncoding(methodToSwizzle));
  class_replaceMethod(classToSwizzle, origSEL, method_getImplementation(myMethod), method_getTypeEncoding(myMethod));
}

@interface InterceptUnsatisfiableConstraints : NSObject
@end

@implementation InterceptUnsatisfiableConstraints

+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    SEL willBreakConstantSel = NSSelectorFromString(@"engine:willBreakConstraint:dueToMutuallyExclusiveConstraints:");
    SwizzleInstanceMethod([UIView class], willBreakConstantSel, [self class], @selector(pr_engine:willBreakConstraint:dueToMutuallyExclusiveConstraints:));
  });
}

- (void)pr_engine:(id)engine willBreakConstraint:(NSLayoutConstraint*)constraint dueToMutuallyExclusiveConstraints:(NSArray<NSLayoutConstraint*>*)layoutConstraints {
  BOOL constrainsLoggingSuspended = [[self valueForKey:@"_isUnsatisfiableConstraintsLoggingSuspended"] boolValue];
  if (!constrainsLoggingSuspended) {
    NSLog(@"_UIViewAlertForUnsatisfiableConstraints would be called on next line, log this event");
  }
  [self pr_engine:engine willBreakConstraint:constraint dueToMutuallyExclusiveConstraints:layoutConstraints];
}

@end

It works on iOS 8.2/9/10 (it doesn't work in iOS 8.1, so be careful), but I can't give any guarantee. Also, it catches constraint problems in system components, such as keyboard/video player/etc. This code is fragile (it can lead to crash on any system version update, parameters change, etc) and I will not recommend to use it in production (guess that it will not pass even automated review process). You have the last word, but you are warned.

However I think that you can use it in builds for internal/external testers to fix bugs in autolayout before production.

Noticed that you are using swift: you can add this code to your swift project with bridging header file.