Add Component Visualizer sub-documents

Component Visualizer - Handle input.md

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+
+# Component Visualizer - Handle input
+
+There is `HandleInputDelta` and `HandleInputKey`.
+When you have determined that a change is going to happen (all checks passed) then call `Modify` on the edited object, do the change, and then call `NotifyPropertyModified` and `GEditor->RedrawLevelEditingViewports`.
+
+```cpp
+bool FMyVisualizer::HandleInputDelta(
+	FEditorViewportClient* ViewportClient,
+    FViewport* Viewport,
+    FVector& DeltaTranslate,
+	FRotator& DeltaRotate,
+    FVector& DeltaScale)
+{
+    UMyComponent* My = GetSelectedMy();
+    if (My == nullptr)
+	{
+		return false;
+	}
+    if (DeltaTranslate.IsZero())
+    {
+        return true;
+    }
+    My->Modify();
+    My->MyProperty += DeltaRotate;
+    NotifyPropertyModified(
+        My,
+        FindFProperty<FProperty>(
+            Class, GET_MEMBER_NAME_CHECKED(UMyComponent, MyProperty)
+        )
+    );
+    GEditor->RedrawLevelEditingViewports()
+}
+```
+
+Getting the edited Component is complicated.
+The strategy I've seen uses Hit Proxies that store a pointer to the Component and a Hit Proxy callback on the Visualizer that read that pointer and remembers it for use in `HandleInputDelta`.
+
+[[2020-08-06_18:48:41]] [Component visualizer](./Component%20visualizer.md)  

Component Visualizer - Hit Proxies.md

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+
+# Component Visualizer - Hit Proxies
+
+[[2020-08-06_18:48:41]] [Component visualizer](./Component%20visualizer.md)  

Component Visualizer - Widget location.md

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+
+# Component Visualizer - Widget location
+
+Unreal Editor uses a Transform Widget to let the user transform objects.
+The transformed object can be an Actor in the level, a Component in an Actor, or a custom attribute of a Component.
+By "attribute" I mean anything that a Component might have, often a UProperty.
+For example one of the control points of a spline.
+
+![A transformation widget on a spline control point](Images/ComponentVisualiser_WidgetLocation_example.png)
+
+The editor provide code for transforming Actors and Components, but custom attributes require custom code.
+The custom code lives in the Component Visualizer associated with the Component type.
+The Component Visualizer has three virtual methods that deal with interactive transformations:
+- `GetWidgetLocation`
+- `HandleInputDelta`
+- `IsVisualizingArchetype`
+
+These functions are called by Unreal Editor from `FEditorViewportClient::Draw`.
+There is also `ISCSEditorCustomization` which seem related but I know nothing about it.
+
+`GetWidgetLocation` should provide the location, in world space, where the transformation widget should be placed.
+
+```cpp
+bool FMyVisualizer::GetWidgetLocation(
+	const FEditorViewportClient* ViewportClient, FVector& OutLocation) const
+{
+    UMyComponent* MyComponent = GetEditedMy();
+    if (MyComponent == nullptr)
+    {
+        return false;
+    }
+    OutLocation = MyComponent->MyVectorUProperty;
+    return true;
+}
+```
+
+Getting the edited Component is complicated.
+The strategy I've seen uses Hit Proxies that store a pointer to the Component and a Hit Proxy callback on the Visualizer that read that pointer and remembers it for use in `GetWidgetLocation`.
+
+[[2020-08-06_18:48:41]] [Component visualizer](./Component%20visualizer.md)