[CachedAccessor] for window.document.

third_party/WebKit/Source/bindings/IDLExtendedAttributes.md

 # Blink IDL Extended Attributes
 
 [TOC]
 
 ## Introduction
 
 The main interest in extended attributes are their _semantics_: Blink implements many more extended attributes than the Web IDL standard, to specify various behavior.
 
 The authoritative list of allowed extended attributes and values is [bindings/IDLExtendedAttributes.txt](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/IDLExtendedAttributes.txt). This is complete but not necessarily precise (there may be unused extended attributes or values), since validation is run on build, but coverage isn't checked.
 
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 Without `[NoImplHeader]`, the IDL compiler assumes that there is XXX.h in the impl side. With `[NoImplHeader]`, you can tell the IDL compiler that there is no XXX.h. You can use `[NoImplHeader]` when all of the DOM attributes and methods of the interface are implemented in Blink-in-JS and thus don't have any C++ header file.
 
 ## Temporary Blink-specific IDL Extended Attributes
 
 These extended attributes are _temporary_ and are only in use while some change is in progress. Unless you are involved with the change, you can generally ignore them, and should not use them.
 
 ### [LegacyTreatAsPartialInterface] _(i)_
 
 Summary: `[LegacyTreatAsPartialInterface]` on an interface that is the target of an `implements` statement means that the interface is treated as a partial interface, meaning members are accessed via static member functions in a separate class, rather than as instance methods on the instance object `*impl` or class methods on the C++ class implementing the (main) interface. This is legacy from original implementation of `implements`, and is being removed ([Bug 360435](https://crbug.com/360435), nbarth@).
 
+
+### [CachedAccessor] _(a)_
+
+Summary: Caches the accessor result in a private property (not directly accesible from JS). Improves accessor reads (getter) at the expense of extra memory and manual invalidation which should be trivial in most cases.
+
+
+*** note
+* The getter cannot have any side effects since calls to the getter will be replaced by a cheap property load.
+* It uses a **push approach**, so updates must be _pushed_ every single time, it **DOES NOT** invalidate/update the cache automatically.
+* Despite being cached, the getter can still be called on certain circumstances, consistency is a must.
+* The cache **MUST** be initialized before using it. There's no default value like _undefined_ or a _hole_.
+***
+
+
+Usage: `[CachedAccessor]` takes no arguments, can be specified on attributes.
+
+```webidl
+interface HTMLFoo {
+    [CachedAccessor] readonly attribute Bar bar;
+};
+```
+
+
+Register the required property in V8PrivateProperty.h.
+To update the cached value (e.g. for HTMLFoo.bar) proceed as follows:
+
+```c++
+V8PrivateProperty::getHTMLFooBarCachedAccessor().set(context, object, newValue);
+```
+
+
 ## Discouraged Blink-specific IDL Extended Attributes
 
 These extended attributes are _discouraged_ - they are not deprecated, but they should be avoided and removed if possible.
 
 ### [DoNotCheckConstants] _(i)_
 
 Summary: `[DoNotCheckConstants]` indicates that constant values in an IDL file can be different from constant values in Blink implementation.
 
 Usage: `[DoNotCheckConstants]` can be specified on interfaces:
 
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 ***