Research the principle of metadata.generation value increase

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func (r *RuleReconciler) SetupWithManager(mgr ctrl.Manager) error {
	return ctrl.NewControllerManagedBy(mgr).
		For(&recommendationv1alpha1.Rule{}).
		Owns(&autoscalingv1.VerticalPodAutoscaler{},
			builder.WithPredicates(predicate.GenerationChangedPredicate{}).
		Complete(r)
}

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// PrepareForUpdate clears fields that are not allowed to be set by end users on update.
func (a customResourceStrategy) PrepareForUpdate(ctx context.Context, obj, old runtime.Object) {
	newCustomResourceObject := obj.(*unstructured.Unstructured)
	oldCustomResourceObject := old.(*unstructured.Unstructured)

	newCustomResource := newCustomResourceObject.UnstructuredContent()
	oldCustomResource := oldCustomResourceObject.UnstructuredContent()

	// If the /status subresource endpoint is installed, update is not allowed to set status.
    // a.status就是crd资源里的spec.versions[*].subresources.status字段
	if a.status != nil {
		_, ok1 := newCustomResource["status"]
		_, ok2 := oldCustomResource["status"]
		switch {
		case ok2:
			newCustomResource["status"] = oldCustomResource["status"]
		case ok1:
			delete(newCustomResource, "status")
		}
	}

	// except for the changes to `metadata`, any other changes
	// cause the generation to increment.
	newCopyContent := copyNonMetadata(newCustomResource)
	oldCopyContent := copyNonMetadata(oldCustomResource)
	if !apiequality.Semantic.DeepEqual(newCopyContent, oldCopyContent) {
		oldAccessor, _ := meta.Accessor(oldCustomResourceObject)
		newAccessor, _ := meta.Accessor(newCustomResourceObject)
		newAccessor.SetGeneration(oldAccessor.GetGeneration() + 1)
	}
}

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// PrepareForUpdate clears fields that are not allowed to be set by end users on update.
func (deploymentStrategy) PrepareForUpdate(ctx context.Context, obj, old runtime.Object) {
	newDeployment := obj.(*apps.Deployment)
	oldDeployment := old.(*apps.Deployment)
	newDeployment.Status = oldDeployment.Status

	pod.DropDisabledTemplateFields(&newDeployment.Spec.Template, &oldDeployment.Spec.Template)

	// Spec updates bump the generation so that we can distinguish between
	// scaling events and template changes, annotation updates bump the generation
	// because annotations are copied from deployments to their replica sets.
	if !apiequality.Semantic.DeepEqual(newDeployment.Spec, oldDeployment.Spec) ||
		!apiequality.Semantic.DeepEqual(newDeployment.Annotations, oldDeployment.Annotations) {
		newDeployment.Generation = oldDeployment.Generation + 1
	}
}

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// PrepareForUpdate clears fields that are not allowed to be set by end users on update.
func (daemonSetStrategy) PrepareForUpdate(ctx context.Context, obj, old runtime.Object) {
	newDaemonSet := obj.(*apps.DaemonSet)
	oldDaemonSet := old.(*apps.DaemonSet)

	dropDaemonSetDisabledFields(newDaemonSet, oldDaemonSet)
	pod.DropDisabledTemplateFields(&newDaemonSet.Spec.Template, &oldDaemonSet.Spec.Template)

	// update is not allowed to set status
	newDaemonSet.Status = oldDaemonSet.Status

	// update is not allowed to set TemplateGeneration
	newDaemonSet.Spec.TemplateGeneration = oldDaemonSet.Spec.TemplateGeneration

	// Any changes to the spec increment the generation number, any changes to the
	// status should reflect the generation number of the corresponding object. We push
	// the burden of managing the status onto the clients because we can't (in general)
	// know here what version of spec the writer of the status has seen. It may seem like
	// we can at first -- since obj contains spec -- but in the future we will probably make
	// status its own object, and even if we don't, writes may be the result of a
	// read-update-write loop, so the contents of spec may not actually be the spec that
	// the manager has *seen*.
	//
	// TODO: Any changes to a part of the object that represents desired state (labels,
	// annotations etc) should also increment the generation.
	if !apiequality.Semantic.DeepEqual(oldDaemonSet.Spec.Template, newDaemonSet.Spec.Template) {
		newDaemonSet.Spec.TemplateGeneration = oldDaemonSet.Spec.TemplateGeneration + 1
		newDaemonSet.Generation = oldDaemonSet.Generation + 1
		return
	}
	if !apiequality.Semantic.DeepEqual(oldDaemonSet.Spec, newDaemonSet.Spec) {
		newDaemonSet.Generation = oldDaemonSet.Generation + 1
	}
}

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// PrepareForUpdate clears fields that are not allowed to be set by end users on update.
func (podStrategy) PrepareForUpdate(ctx context.Context, obj, old runtime.Object) {
	newPod := obj.(*api.Pod)
	oldPod := old.(*api.Pod)
	newPod.Status = oldPod.Status

	podutil.DropDisabledPodFields(newPod, oldPod)
}