ios - Why changing xib's constraint is invalid in url request's success block? -

i have written viewcontroller using interface building: top part tableview, bottom part uiview(called bottomview).the tableview's bottom bottomview's top, , add height constraint in bottomview control height of tableview.then link bottomview's height constraint viewcontroller's property. let's see xib picture viewcontroller. 　

the constraints in viewcontroller in picture followed(the first picture tableview's, second bottombackview's):

　 in viewcontroller, use iboutlet corresponding link this:
@property (strong, nonatomic) iboutlet nslayoutconstraint *bottomviewheightconstraint;
change bottomviewheightconstraint's constant in url request's success block this:

[getcontacts startwithparams:@{@"bizid":self.bizid, @"bizcode":@(self.bizcode)} success:^(id responseobject) { self.bottomviewheightconstraint.constant = self.bottomcontentview.height; [self.view setneedslayout]; } 

first time, value of self.bottomviewheightconstraint.constant 70 here, , result right. second time value 0, expected result this(table view can scrolled screen bottom):
result this(the tableview can't scrolled screen bottom):

according comment,i used gcd this:

        dispatch_queue_t mainqueue = dispatch_get_main_queue();     dispatch_async(mainqueue, ^{         self.bottomcontentview.frame = cgrectmake(0.0f, 0.0f, screenwidth, 0.0f);         [self.bottomcontentview configviewwithmodel:model];         self.bottomviewheightconstraint.constant = self.bottomcontentview.height;         [self.view setneedslayout];         [self.view layoutifneeded];     }); 

it doesn't work too. there 1 point troubled me too:when change constraint's constant in method invoked in method 'viewdidload', doesn't work if don't call method 'layoutifneeded' code(i think call method 'setneedslayout' enough):

- (void)viewdidload {     [super viewdidload];      [self addbottomview]; } - (void)addbottomview {     self.bottomcontentview.delegate = self;     self.bottomcontentview.frame = cgrectmake(0.0f, 0.0f, screenwidth, 0.0f);     self.bottomviewheightconstraint.constant = 0.0f;     [self.view setneedslayout];     [self.view layoutifneeded];     [self.bottombackview addsubview:self.bottomcontentview]; }