For each key defined for the current context (and it's parent) call the provided callback function with each key name and it's value. This will only call the callback function once for each name even when the same name is presen in the current instance and it's parent.
The callback function to call
Return the named context value from this instance or it's parent if present.
The name of the value to return
Optional dfValue: TIf the named values does not exist in this instance or it's parent then return this value. The default will not be returned even if the value for the key is undefined or null.
The value contained in the context or the default
Return whether the named context value is available from this instance ir it's parent.
The name of the value to check if it's available
true if it's present otherwise false
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The generic context used for passing