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MDQuerySetCreateResultFunction(_:_:_:_:)

Sets the function used to create the result objects of the MDQuery.

Declaration

func MDQuerySetCreateResultFunction(_ query: MDQuery!, _ func: MDQueryCreateResultFunction!, _ context: UnsafeMutableRawPointer!, _ cb: UnsafePointer<CFArrayCallBacks>!)

Parameters

  • query:

    The query.

  • func:

    The callback function the MDQuery will use to create its results, such as those returned by the function MDQueryGetResultAtIndex. This parameter may be NULL, in which case any previous result creation settings are cancelled and the MDQuery will subsequently produce MDItemRefs. If a function is specified and is not of type MDQueryCreateResultFunction or does not behave as a MDQueryCreateResultFunction must, the behavior is undefined.

  • context:

    A pointer-sized user-defined value, that is passed as the third parameter to the create function. MDQuery does not use this value, does not retain the context in any way, and requires that the context be valid for the lifetime of the query. If the context is not what is expected by the create function, the behavior is undefined.

  • cb:

    A pointer to a CFArrayCallBacks structure initialized with the callbacks for the query to use to manage the created result objects. A copy of the contents of the callbacks structure is made, so that a pointer to a structure on the stack can be passed in, or can be reused for multiple query creations. Only version 0 of the CFArrayCallBacks is supported. The retain field may be NULL, in which case the MDQuery will not add a retain to the created results for the query. The release field may be NULL, in which case the MDQuery will not remove the query's retain (such as the one it gets from the create function) on the result objects when the query is destroyed. If the copyDescription field is NULL, the query will create a simple description for the result objects. If the equal field is NULL, the query will use pointer equality to test for equality of results. This callbacks parameter itself may be NULL in which case it is treated as a valid version 0 structure with all fields NULL. Otherwise, if any of the fields are not valid pointers to functions of the correct type, or this parameter is not a valid pointer to a CFArrayCallBacks callbacks structure, the behavior is undefined. If any of the value values returned from the create function is not one understood by one or more of the callback functions, the behavior when those callback functions are used is undefined. For example, if the create function can return NULL, then NULL must be understood by the callback functions as a possible parameter. The retain and release callbacks must be a matched set, you should not assume that the retain function will be unused or that additional reference counts will not be taken on the created results.

Discussion

If no create function is specified for an MDQuery, the default result objects are MDItemRefs. Results created after the function MDQuerySetCreateResultFunction is called are created through the specified create function, but values created before the function was set, or after it is unset, are not modified. It is not advisable to change this function after the function MDQueryExecute has been called. The create-result function is called lazily as results are requested from a query, it is not called on all results, and may not be called at all. This avoids the cost of creating potentially hundreds of thousands of what might be temporary objects.

See Also

Setting Callback Functions