Introduction

Within the framework reconciliation is enabled by default.

Note: As of January 2020 reconciliation is only implemented for state updates.

Reconciliation is an implementation detail in Litho’s layout calculation process. Instead of recreating the entire layout tree for every update reconciliation enables Litho to clone the unmodified subtrees and only recreate the subtree which has changed.

When using Litho, the component hierarchy is determined by what is returned by the OnCreateLayout method. On the next state or prop update, the OnCreateLayout may return a different tree of components. Litho uses reconciliation to figure out which components should be re-created and which can be re-used (read cloned).

In essence, this means Litho will not call (most) lifecycle methods of components which were unaffected by the update. Reconciliation doubles down on the requirement that component specs must be immutable and should not rely on side effects to achieve any desired behaviour.

Enable or disable reconciliation

There are various integration points.

ComponentTree

The ComponentTree builder exposes an API to enable or disable reconciliation. (As of January 2020) Reconciliation is enabled by default in Litho.

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ComponentTree.create(context)
  .isReconciliationEnabled(true)
  .build();

Note: In this integration the config is not passed into any Sections within the component hierarchy. It needs to be explicitly set in the Sections (see below). There are plans to enable propagation over the Section boundary in future milestones.

RecyclerCollectionComponent

The RecyclerBinderConfiguration and the RecyclerBinder builders both expose an API to enable or disable reconciliation.

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RecyclerBinderConfiguration.create()
  .isReconciliationEnabled(false)
  .build();


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new RecyclerBinder.Builder()
  .isReconciliationEnabled(true)
  .build(context);

Sections

The ComponentRenderInfo builder exposes an API to enable or disable reconciliation for individual Sections in a GroupSectionSpec.

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ComponentRenderInfo.create()
  .component(component)
  .customAttribute(ComponentRenderInfo.RECONCILIATION_ENABLED, false)
  .build();

Tradeoffs

It is important to remember that the reconciliation algorithm is an implementation detail. Litho could re-create the whole hierarchy on every action; the end result would be the same. Just to be clear, re-creation in this context means calling OnCreateLayout for the modified component, it doesn’t mean Litho will unmount and remount them. Litho will only mount and unmount items will which have changed.

In order to reconcile changes, Litho keeps the previous component hierarchy in memory. This increases memory usage and can potentially increase OOMs.

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