ios - Adding constraint crash down the application -

i have custom uiview designing uitableviewcell, every things work perfect it's not resize properly. , when trying add constraint crashed. used below code.

- (uitableviewcell *)tableview:(uitableview *)tableview cellforrowatindexpath:(nsindexpath *)indexpath {      uitableviewcell *cell = [tableview dequeuereusablecellwithidentifier:@"identifier"];     if(!cell) {         cell = [[uitableviewcell alloc] initwithstyle:uitableviewcellstyledefault reuseidentifier:@"identifier"];         uiview *vbackview = [[uiview alloc] initwithframe:aframe];         [[cell contentview] addsubview:vbackview];          [vbackview addconstraint:[nslayoutconstraint constraintwithitem:[cell contentview]                                                               attribute:nslayoutattributetop                                                               relatedby:nslayoutrelationequal                                                                  toitem:vbackview                                                               attribute:nslayoutattributetop                                                              multiplier:1.0                                                                constant:0.0]];          [vbackview addconstraint:[nslayoutconstraint constraintwithitem:[cell contentview]                                                               attribute:nslayoutattributeleading                                                               relatedby:nslayoutrelationequal                                                                  toitem:vbackview                                                               attribute:nslayoutattributeleading                                                              multiplier:1.0                                                                constant:0.0]];          [vbackview addconstraint:[nslayoutconstraint constraintwithitem:[cell contentview]                                                               attribute:nslayoutattributebottom                                                               relatedby:nslayoutrelationequal                                                                  toitem:vbackview                                                               attribute:nslayoutattributebottom                                                              multiplier:1.0                                                                constant:0.0]];          [vbackview addconstraint:[nslayoutconstraint constraintwithitem:[cell contentview]                                                               attribute:nslayoutattributetrailing                                                               relatedby:nslayoutrelationequal                                                                  toitem:vbackview                                                               attribute:nslayoutattributetrailing                                                              multiplier:1.0                                                                constant:0.0]];     }      // doing other stuff      return cell; } 

and in log it's shows

2015-06-13 12:51:57.482 digitalboard[5314:70921] view hierarchy not prepared constraint: <nslayoutconstraint:0x7fb508dc0ae0 uitableviewcell:0x7fb508dbb0a0'cellidentifier'.top == uiview:0x7fb508dc16f0.top>     when added view, constraint's items must descendants of view (or view itself). crash if constraint needs resolved before view hierarchy assembled. break on -[uiview _viewhierarchyunpreparedforconstraint:] debug.  2015-06-13 12:51:57.485 digitalboard[5314:70921] view hierarchy unprepared constraint.     constraint: <nslayoutconstraint:0x7fb508dc0ae0 uitableviewcell:0x7fb508dbb0a0'cellidentifier'.top == uiview:0x7fb508dc16f0.top>     container hierarchy:  <uiview: 0x7fb508dc16f0; frame = (0 0; 400 44); autoresize = rm+bm; tag = 1; layer = <calayer: 0x7fb508dc0720>>    | <uilabel: 0x7fb508dc5310; frame = (15 0; 105 44); text = 'meeting #'; opaque = no; autoresize = rm+bm; userinteractionenabled = no; tag = 2; layer = <_uilabellayer: 0x7fb508dc54c0>>    | <uibutton: 0x7fb508dc5880; frame = (130 7; 80 30); opaque = no; autoresize = rm+bm; tag = 3; layer = <calayer: 0x7fb508da95d0>>    | <uibutton: 0x7fb508dc5fa0; frame = (220 7; 80 30); opaque = no; autoresize = rm+bm; tag = 4; layer = <calayer: 0x7fb508dc5c60>>    | <uibutton: 0x7fb508dc61c0; frame = (310 7; 80 30); opaque = no; autoresize = rm+bm; tag = 5; layer = <calayer: 0x7fb508dc5e90>>    | <uiimageview: 0x7fb508dc6540; frame = (0 42; 400 2); autoresize = rm+bm; userinteractionenabled = no; layer = <calayer: 0x7fb508dc6680>>     view not found in container hierarchy: <uitableviewcell: 0x7fb508dbb0a0; frame = (0 0; 320 44); layer = <calayer: 0x7fb508da98e0>>     view's superview: no superview  2015-06-13 12:51:57.498 digitalboard[5314:70921] *** terminating app due uncaught exception 'nsgenericexception', reason: 'unable install constraint on view.  constraint reference outside subtree of view?  that's illegal. constraint:<nslayoutconstraint:0x7fb508dc0ae0 uitableviewcell:0x7fb508dbb0a0'cellidentifier'.top == uiview:0x7fb508dc16f0.top> view:<uiview: 0x7fb508dc16f0; frame = (0 0; 400 44); autoresize = rm+bm; tag = 1; layer = <calayer: 0x7fb508dc0720>>' *** first throw call stack: ( etc.... 

can understand problem is.

you shouldn't add constraints vbackview. assume vbackview subview of cell.contentview in case should adding constraints cell.contentview instead.

the crash log tells why:

when added view, constraint's items must descendants of view (or view itself).

change each line of code begin: [cell.contentview addconstraint:....