ios - Auto Layout with UITextField causes exception -

i'd position uitextfield inside uiviewcontroller view using auto layout (dynamic resizing , 5 pt margin between textfield , superview). create view programmatically using loadview method.

header of uiviewcontroller

@property (weak, nonatomic) uitextfield *textfield;

implementation

-(void)loadview{      //frame     uiview *frame = [[uiview alloc]initwithframe:cgrectmake(0, 0, 100, 40)];     [self setview:frame];     [self.view setbackgroundcolor:[uicolor redcolor]];      //textfield     [self.textfield setdelegate:self];     [self.textfield settranslatesautoresizingmaskintoconstraints:no];     [self.textfield setbackgroundcolor:[uicolor bluecolor]];     [frame addsubview:self.textfield];      [frame addconstraints:[nslayoutconstraint constraintswithvisualformat:@"h:|-(5@50)-[textfield(==10@10)]-(5@50)-|"                                                                   options:0                                                                   metrics:nil                                                                     views:@{@"textfield" : self.textfield}]];      [frame addconstraints:[nslayoutconstraint constraintswithvisualformat: @"v:|-(5@50)-[textfield(==10@10)]-(5@50)-|"                                                                   options:0                                                                   metrics:nil                                                                     views:@{@"textfield" : self.textfield}]];  //    //test uiview instead //    uiview *sub = [[uiview alloc]init]; //    [sub settranslatesautoresizingmaskintoconstraints:no]; //    [sub setbackgroundcolor:[uicolor greencolor]]; //    [frame addsubview:sub]; //     //    [frame addconstraints:[nslayoutconstraint constraintswithvisualformat:@"h:|-(5@50)-[sub(==10@10)]-(5@50)-|" //                                                                  options:0 //                                                                  metrics:nil //                                                                    views:@{@"sub" : sub}]]; //     //    [frame addconstraints:[nslayoutconstraint constraintswithvisualformat: @"v:|-(5@50)-[sub(==10@10)]-(5@50)-|" //                                                                  options:0 //                                                                  metrics:nil //                                                                    views:@{@"sub" : sub}]]; 

running throws exception. curiously using uiview ("sub") subview instead (the part commented out) work expected.

ouput:

shouldn't textfield behave uiview? how come textfield causes exception?

---

new finding: app crashes when use addconstraints on self.textfield. if remove 2 calls spare crash, textfield doesn't show either.

nslayoutconstraint part of ios 6. if try work code other ios6 ( such ios 5, ios 4.3) highly runtime error.