Mega Code Archive
How to write a callback function
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Title: How to write a callback function
Question: How can you split code from the user-interface from other code - for
example the 'business rules' ?
Answer:
The function CopyAllFiles copys files from A to B. This process could take
longer, therefore a feedback to the user-interface is necessary.
We don't want to just implement this - the aim is to write a code that is
independent from the user-interface and can be used in many application
areas.
To achieve this we need a procedure-parameter - this you can call callback.
We just have to define a type of a procedure - in this example TProcessEvent -
than we can define a parameter from this type in the function CopyAllFiles:
type
TProcessEvent = procedure(aAmount, aCurrent: integer;
aFilename: string;
var aCancel: boolean);
This is how we create an exit from our function that can start a procedure in
the main program.
procedure CopyAllFiles(aPathPlusMask: string; aSubFolders: boolean;
aDestPath: string; aOverwrite: boolean;
aProcessEvent: TProcessEvent);
begin
...
// a lot of cpu-cicles needed - propobly in a loop
if Assigned(aProcessEvent) then aProcessEvent(...);
...
end;
In the main program the 'event-handles' or the 'callback-function' can be
defined with the same parameterlist than the type TProcessEvent is forcing it:
procedure ProcessProc(aAmount, aCurrent: integer;
aFilename: string;
var aCancel: boolean);
begin
// User-Interface Code
end;
This function can than be passed to CopyAllFiles:
CopyAllFiles('*.*',false, 'c:\temp', true, ProcessProc);
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