Angular2 change detection ngOnChanges not firing for nested object

Question

I know I am not the first to ask about this  but I can t find an answer in the previous questions  I have this in one component   lt div class  col-sm-5  gt       lt laps          lapsData   rawLapsData           selectedTps   selectedTps           lapsHandler   lapsHandler  event   gt       lt  laps gt   lt  div gt    lt map      lapsData   rawLapsData      class  col-sm-7  gt   lt  map gt    In the controller rawLapsdata gets mutated from time to time   In laps  the data is output as HTML in a tabular format  This changes whenever rawLapsdata changes   My map component needs to use ngOnChanges as a trigger to redraw markers on a Google Map  The problem is that ngOnChanges does not fire when rawLapsData changes in the parent  What can I do   import  Component  Input  OnInit  OnChanges  SimpleChange  from  angular2 core     Component       selector   map       templateUrl     components edMap edMap html       styleUrls      components edMap edMap css      export class MapCmp implements OnInit  OnChanges        Input   lapsData  any      map  google maps Map       ngOnInit                            ngOnChanges changes     propName  string   SimpleChange              console log  ngOnChanges      changes  lapsData             if  this map  this drawMarkers            Update  ngOnChanges is not working  but it looks as though lapsData is being updated  In the ngInit is an event listener for zoom changes that also calls this drawmarkers  When I change the zoom I do indeed see a change in markers  So the only issue is that I don t get the notification at the time the input data changes   In the parent  I have this line   Recall that the change is reflected in laps  but not in map    this rawLapsData   deletePoints this rawLapsData  this selectedTps     And note that this rawLapsData is itself a pointer to the middle of a large json object  this rawLapsData   this main data TrainingCenterDatabase Activities 0  Activity 0  Lap

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When you are manipulating the data like  this data profiles i  icon url        Then you should use in order to detect changes  let array   this data profiles map x   gt  Object assign     x       It will detect changes  Since angular ngOnchanges not be able to detect changes in array  objects then we have to assign a new reference  Works everytime

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rawLapsData continues to point to the same array  even if you modify the contents of the array  e g   add items  remove items  change an item     During change detection  when Angular checks components  input properties for change  it uses  essentially      for dirty checking   For arrays  this means the array references  only  are dirty checked   Since the rawLapsData array reference isn t changing  ngOnChanges   will not be called    I can think of two possible solutions    Implement ngDoCheck   and perform your own change detection logic to determine if the array contents have changed    The Lifecycle Hooks doc has an example   Assign a new array to rawLapsData whenever you make any changes to the array contents  Then ngOnChanges   will be called because the array  reference  will appear as a change    In your answer  you came up with another solution   Repeating some comments here on the OP      I still don t see how laps can pick up on the change  surely it must be using something equivalent to ngOnChanges   itself   while map can t     In the laps component your code template loops over each entry in the lapsData array  and displays the contents  so there are Angular bindings on each piece of data that is displayed  Even when Angular doesn t detect any changes to a component s input properties  using     checking   it still  by default  dirty checks all of the template bindings  When any of those change  Angular will update the DOM   That s what you are seeing  The maps component likely doesn t have any bindings in its template to its lapsData input property  right  That would explain the difference    Note that lapsData in both components and rawLapsData in the parent component all point to the same one array   So even though Angular doesn t notice any  reference  changes to the lapsData input properties  the components  get  see any array contents changes because they all share reference that one array   We don t need Angular to propagate these changes  like we would with a primitive type  string  number  boolean    But with a primitive type  any change to the value would always trigger ngOnChanges    ndash  which is something you exploit in your answer solution   As you probably figured out by now object input properties have the same behavior as array input properties

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Not the cleanest approach  but you can just clone the object each time you change the value      rawLapsData   Object assign     rawLapsData     I think I would prefer this approach over implementing your own ngDoCheck   but maybe someone like  G  nterZ  chbauer could chime in

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suppose you have a nested object  like var obj     quot parent quot     quot child quot            If you passed the reference of the complete object  like  wholeObj     quot obj quot  In that case  you can t detect the changes in the child objects  so to overcome this problem you can also pass the reference of the child object through another property  like  wholeObj     quot obj quot   childObj     quot obj parent child quot  So you can also detect the changes from the child objects too  ngOnChanges changes  SimpleChanges         if  changes childObj      your logic here

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If the data comes from an external library you might need to run the data upate statement within zone run       Inject zone  NgZone for this  If you can run the instantiation of the external library within zone run   already  then you might not need zone run   later

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In Case of Arrays you can do it like this   In  ts file  Parent component  where you are updating your rawLapsData do it like this   rawLapsData   somevalue     change detection will not happen     Solution     rawLapsData       somevalue     change detection will happen   and ngOnChanges will called in child component

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Change detection is not triggered when you change a property of an object  including nested object   One solution would be to reassign a new object reference using  lodash  clone   function   import   as   from  lodash    this foo     clone this foo

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ok so my solution for this was   this arrayWeNeed DoWhatWeNeedWithThisArray    const tempArray       arrayWeNeed   this arrayWeNeed       this arrayWeNeed   tempArray    And this trigger me ngOnChanges

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Use ChangeDetectorRef detectChanges   to tell Angular to run a change detection when you edit a nested object  that it misses with its dirty checking

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As an extension to Mark Rajcok s second solution     Assign a new array to rawLapsData whenever you make any changes to the   array contents  Then ngOnChanges   will be called because the array    reference  will appear as a change   you can clone the contents of the array like this   rawLapsData   rawLapsData slice 0     I am mentioning this because      rawLapsData   Object assign     rawLapsData     didn t work for me  I hope this helps

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Here s a hack that just got me out of trouble with this one   So a similar scenario to the OP - I ve got a nested Angular component that needs data passed down to it  but the input points to an array  and as mentioned above  Angular doesn t see a change as it does not examine the contents of the array   So to fix it I convert the array to a string for Angular to detect a change  and then in the nested component I split      the string back to an array and its happy days again

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My  hack  solution is      lt div class  col-sm-5  gt           lt laps              lapsData   rawLapsData               selectedTps   selectedTps               lapsHandler   lapsHandler  event   gt           lt  laps gt       lt  div gt       lt map          lapsData   rawLapsData           selectedTps   selectedTps        lt --------         class  col-sm-7  gt       lt  map gt    selectedTps changes at the same time as rawLapsData and that gives map another chance to detect the change through a simpler object primitive type  It is NOT elegant  but it works

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In my case it was changes in object value which the ngOnChange was not capturing  A few object values are modified in response of api call  Reinitializing the object fixed the issue and caused the ngOnChange to trigger in the child component  Something like  this pagingObj   new Paging      This line did the magic  this pagingObj pageNumber   response PageNumber

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I have 2 solutions to resolve your problem    Use ngDoCheck to detect object data changed or not  Assign object to a new memory address by object   Object create object  from parent component

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I had to create a hack for it -   I created a Boolean Input variable and toggled it whenever array changed  which triggered change detection in the child component  hence achieving the purpose

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I stumbled upon the same need  And I read a lot on this so  here is my copper on the subject   If you want your change detection on push  then you would have it when you change a value of an object inside right   And you also would have it if somehow  you remove objects   As already said  use of changeDetectionStrategy onPush  Say you have this component you made  with changeDetectionStrategy onPush    lt component  collection   myCollection  gt  lt  component gt    Then you d push an item and trigger the change detection     myCollection push anItem   refresh      or you d remove an item and trigger the change detection     myCollection splice 0 1   refresh      or you d change an attrbibute value for an item and trigger the change detection     myCollection 5  attribute    new value   refresh      Content of refresh     refresh     void       this myCollection   this myCollection slice        The slice method returns the exact same Array  and the       sign make a new reference to it  triggering the change detection every time you need it  Easy and readable     Regards

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Here s an example using IterableDiffer with ngDoCheck  IterableDiffer is especially useful if you need to track changes over time as it lets you do things like iterate over only added changed removed values etc  A simple example not using all advantages of IterableDiffer  but it works and shows the principle  export class FeedbackMessagesComponent implements DoCheck      Input     messages  UserFeedback              Example UserFeedback instance   message    Ooops   type   Notice       HostBinding  style display     display    none      private  iterableDiffer  IterableDiffer lt UserFeedback gt      constructor private  iterableDiffers  IterableDiffers        this  iterableDiffer   this  iterableDiffers find     create null          ngDoCheck    void       const changes   this  iterableDiffer diff this messages        if  changes             Here you can do stuff like changes forEachRemovedItem             We know contents of this messages was changed so update visibility        this display   this messages length  gt  0    block     none                This will now automatically show hide depending on myMessagesArray count   x000D   x000D   lt app-feedback-messages    messages   myMessagesArray   gt  lt  app-feedback-messages gt  x000D   x000D   x000D

[angular] Angular2 change detection: ngOnChanges not firing for nested object

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