Angular2 change detection: ngOnChanges not firing for nested object

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:

1. Implement ngDoCheck() and perform your own change detection logic to determine if the array contents have changed. (The Lifecycle Hooks doc has an example.)

2. 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() – 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.

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 @GünterZöchbauer could chime in.

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}; //for Object, change detection will happen

rawLapsData = [...somevalue]; //for Array, change detection will happen

and ngOnChanges will called in child component