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lightning-sunbird  0.9+nobinonly
Public Attributes
nsIXPCWrappedJSObjectGetter Interface Reference

This is a sort of a placeholder interface. More...

import "nsIXPConnect.idl";

Inheritance diagram for nsIXPCWrappedJSObjectGetter:
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Collaboration diagram for nsIXPCWrappedJSObjectGetter:
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List of all members.

Public Attributes

readonly attribute nsISupports neverCalled

Detailed Description

This is a sort of a placeholder interface.

It is not intended to be implemented. It exists to give the nsIXPCSecurityManager an iid on which to gate a specific activity in XPConnect.

That activity is...

When JavaScript code uses a component that is itself implemeted in JavaScript then XPConnect will build a wrapper rather than directly expose the JSObject of the component. This allows components implemented in JavaScript to 'look' just like any other xpcom component (from the perspective of the JavaScript caller). This insulates the component from the caller and hides any properties or methods that are not part of the interface as declared in xpidl. Usually this is a good thing.

However, in some cases it is useful to allow the JS caller access to the JS component's underlying implementation. In order to facilitate this XPConnect supports the 'wrappedJSObject' property. The caller code can do:

// 'foo' is some xpcom component (that might be implemented in JS). try { var bar = foo.wrappedJSObject; if(bar) { // bar is the underlying JSObject. Do stuff with it here. } } catch(e) { // security exception? }

Recall that 'foo' above is an XPConnect wrapper, not the underlying JS object. The property get "foo.wrappedJSObject" will only succeed if three conditions are met:

1) 'foo' really is an XPConnect wrapper around a JSObject. 2) The underlying JSObject actually implements a "wrappedJSObject" property that returns a JSObject. This is called by XPConnect. This restriction allows wrapped objects to only allow access to the underlying JSObject if they choose to do so. Ususally this just means that 'foo' would have a property tht looks like: this.wrappedJSObject = this. 3) The implemementation of nsIXPCSecurityManager (if installed) allows a property get on the interface below. Although the JSObject need not implement 'nsIXPCWrappedJSObjectGetter', XPConnect will ask the security manager if it is OK for the caller to access the only method in nsIXPCWrappedJSObjectGetter before allowing the activity. This fits in with the security manager paradigm and makes control over accessing the property on this interface the control factor for getting the underlying wrapped JSObject of a JS component from JS code.


a) If 'foo' above were the underlying JSObject and not a wrapper at all, then this all just works and XPConnect is not part of the picture at all. b) One might ask why 'foo' should not just implement an interface through which callers might get at the underlying object. There are three reasons: i) XPConnect would still have to do magic since JSObject is not a scriptable type. ii) JS Components might use aggregation (like C++ objects) and have different JSObjects for different interfaces 'within' an aggregate object. But, using an additional interface only allows returning one underlying JSObject. However, this allows for the possibility that each of the aggregte JSObjects could return something different. Note that one might do: this.wrappedJSObject = someOtherObject; iii) Avoiding the explicit interface makes it easier for both the caller and the component.

Anyway, some future implementation of nsIXPCSecurityManager might want do special processing on 'nsIXPCSecurityManager::CanGetProperty' when the interface id is that of nsIXPCWrappedJSObjectGetter.

Definition at line 371 of file nsIXPConnect.idl.

Member Data Documentation

Definition at line 373 of file nsIXPConnect.idl.

The documentation for this interface was generated from the following file: