introduce SignalsMountState

Summary:
## What?
The current approach for mount requires all render units to update their binders and defer skipping of content binding to their individual binders. This is not only time consuming but also inefficient and wasteful memory wise as we cache dummy evaluation results for all mount signals (extensions and mount binders).

To make mount more efficient, we must be able to skip updates for render units that do not have any transitive signal dependencies.

## How?
We extend `MountState` and track invocations of the major mount operations:
- initial mount
- updates
- unmounts

Tracking the first two operations gives us the opportunity to figure the set of render units that access signals so that we limit future updates to them. Tracking the last operation allows us to trim the tracked render units as needed.

Doing all of these makes updates faster because we skip a good portion of the tree on updates and only restrict updates to render units that access signals which have changed or render units that have new binders or are affected by changed layout data. It also helps that the dependency tree for render units is shallow too so we spend less time on updates.

Reviewed By: rooju

Differential Revision: D62671753

fbshipit-source-id: 7edf36c40d66371e476d9b7e15a7fc84dc57f4f0

litho-rendercore/src/main/java/com/facebook/rendercore/MountState.kt

@@ -40,7 +40,7 @@ import com.facebook.rendercore.utils.isEqualOrEquivalentTo
 import java.util.ArrayList
 import java.util.HashMap
 
-class MountState
+open class MountState
 @JvmOverloads
 constructor(
     private val _rootHost: Host,
@@ -501,7 +501,7 @@ constructor(
 
   private fun isMounted(id: Long): Boolean = idToMountedItemMap[id] != null
 
-  private fun mountRenderUnit(renderTreeNode: RenderTreeNode) {
+  protected open fun mountRenderUnit(renderTreeNode: RenderTreeNode) {
     if (renderTreeNode.renderUnit.id == ROOT_HOST_ID) {
       mountRootItem(renderTreeNode)
       return
@@ -845,8 +845,7 @@ constructor(
     currentMountItem.update(renderTreeNode)
     currentRenderUnit.onStartUpdateRenderUnit()
     mountDelegate?.startNotifyVisibleBoundsChangedSection()
-    if (currentRenderUnit.shouldUpdate(renderUnit) ||
-        !isEqualOrEquivalentTo(currentLayoutData, newLayoutData)) {
+    if (shouldUpdateMountItem(currentRenderUnit, renderUnit, currentLayoutData, newLayoutData)) {
       val traceIdentifier =
           DebugEventDispatcher.generateTraceIdentifier(DebugEvent.RenderUnitUpdated)
       if (traceIdentifier != null) {
@@ -865,16 +864,13 @@ constructor(
       if (isTracing) {
         tracer.beginSection("UpdateItem: ${renderUnit.description}")
       }
-      renderUnit.updateBinders(
-          context,
+      updateRenderUnitBinders(
           content,
+          renderUnit,
           currentRenderUnit,
           currentLayoutData,
           newLayoutData,
-          mountDelegate,
-          currentMountItem.bindData,
-          currentMountItem.isBound,
-          tracer)
+          currentMountItem)
       if (isTracing) {
         tracer.endSection()
       }
@@ -896,6 +892,36 @@ constructor(
     currentRenderUnit.onEndUpdateRenderUnit()
   }
 
+  protected open fun shouldUpdateMountItem(
+      currentRenderUnit: RenderUnit<*>,
+      renderUnit: RenderUnit<*>,
+      currentLayoutData: Any?,
+      newLayoutData: Any?,
+  ): Boolean {
+    return currentRenderUnit !== renderUnit ||
+        !isEqualOrEquivalentTo(currentLayoutData, newLayoutData)
+  }
+
+  protected open fun updateRenderUnitBinders(
+      content: Any,
+      renderUnit: RenderUnit<Any>,
+      currentRenderUnit: RenderUnit<Any>,
+      currentLayoutData: Any?,
+      newLayoutData: Any?,
+      currentMountItem: MountItem,
+  ) {
+    renderUnit.updateBinders(
+        context,
+        content,
+        currentRenderUnit,
+        currentLayoutData,
+        newLayoutData,
+        _mountDelegate,
+        currentMountItem.bindData,
+        currentMountItem.isBound,
+        tracer)
+  }
+
   private fun maybeEnsureParentIsMounted(node: RenderTreeNode, parent: RenderUnit<*>) {
     if (ensureParentMounted && !isMounted(parent.id)) {
       mountRenderUnit(node)