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Angular DevTools now supports mutating objects underneath signals in the property explorer view.
This is done by performing an "immutable update" by recursively copying objects underneath a signal and overwriting the one property specified. For example, if the user attempted to set `foo.bar.baz[2].hello = 'world'` and `bar` was a signal, this would effectively become:
```typescript
foo.bar.set({
...foo.bar(),
baz: [
...foo.bar().baz.slice(0, 2),
{
...foo.bar().baz[2],
hello: 'world',
},
...foo.bar().baz.slice(3),
],
})
```
The motivation for immutable updates is because signals and Angular change detection don't really like interior mutability of signal values. If we didn't do this, any kind of comparison or dirty check would prevent the UI from updating. If an application attempts to change a deeply nested property inside a signal, it doesn't work today. DevTools should generally be limited to operations an application could do itself, and the recommended approach to make such a change like this is an immutable update. Creating entirely new objects intentionally breaks referential equality such that the application can properly react to the change.
Unfortunately, it is not possible to make immutable updates in a truly generic sense. You can't just copy a class for instance `({...new MyFoo()}).doSomething()`. We could do something fancier like manually copying over the prototype or something like that, but there is no way to do this without breaking class semantics (ex. the class might reasonably rely on the constructor being called). Therefore we instead reject any mutations to non-primitive objects. In the future, we might expand the set of "primitives" to include other built-ins and well-known objects like `URL` or `Element`, but those are out of scope for now.
I opted to ban mutating the result of a readonly/computed signal. While the mutation is likely to succeed, a subsequent rerun of the `computed` will immediately drop the change. However, I opted to allow mutating the result of a getter property. This has a similar problem because it might be returning a synthetic object which will be invalidated on the next execution, but it is possible and reasonable for a getter to return the same object multiple times such that a mutation may reasonably survive other updates. DevTools can't easily know whether a getter actually will return the same object on each execution or not, so we optimistically assume the reference is stable. If it isn't, the mutation will be lost whenever the getter is re-executed.
PR Close #60381
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