[Blazor] Accessing collection of changed parameters previous values

[daniel-p-tech]

Hi,

This is a follow-up to an issue that was somewhat prematurely closed:
#32163

In short, I would like to access previous values of changed parameters without implementing my own solution. Inspecting the source code, it appears that authors of Blazor at some point already contemplated this feature:

aspnetcore/src/Components/Components/src/RenderTree/RenderTreeDiffBuilder.cs

Lines 526 to 547 in b1c34c8

	// As an important rendering optimization, we want to skip parameter update
	// notifications if we know for sure they haven't changed/mutated. The
	// "MayHaveChangedSince" logic is conservative, in that it returns true if
	// any parameter is of a type we don't know is immutable. In this case
	// we call SetParameters and it's up to the recipient to implement
	// whatever change-detection logic they want. Currently we only supply the new
	// set of parameters and assume the recipient has enough info to do whatever
	// comparisons it wants with the old values. Later we could choose to pass the
	// old parameter values if we wanted. By default, components always rerender
	// after any SetParameters call, which is safe but now always optimal for perf.
	// When performing hot reload, we want to force all components to re-render.
	// We do this using two mechanisms - we call SetParametersAsync even if the parameters
	// are unchanged and we ignore ComponentBase.ShouldRender
	var oldParameters = new ParameterView(ParameterViewLifetime.Unbound, oldTree, oldComponentIndex);
	var newParametersLifetime = new ParameterViewLifetime(diffContext.BatchBuilder);
	var newParameters = new ParameterView(newParametersLifetime, newTree, newComponentIndex);
	if (!newParameters.DefinitelyEquals(oldParameters) || diffContext.Renderer.IsHotReloading)
	{
	componentState.SetDirectParameters(newParameters);
	}

Thank you for your consideration.

[javiercn]

@daniel-p-tech thanks for contacting us.

This is not likely something we plan to do in the immediate future. You'll have to capture the values yourself in your own component for the time being.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[wstaelens]

Now we have custom logic in parameters set [Parameter] public Foo Bar {get; set { /* custom logic*/ }; } which is not recommended.

The OnParametersSetAsync does not tell you which parameters have changed, meaning that we have to double all code / parameters to keep track ourselves of previous values just to know which properties have changed in order to be able to execute custom logic.

Any plans that this will be added in near future?

Update: the reason why we have custom logic in Parameter setter is because in the past they did not need to be auto properties (see: #45152 ).
If you look at this: #45152 (comment) this feels like a bad smell and so much unneeded work.