Incremental Update (Magic xpa 4.x)
Magic xpa enables you to incrementally update a variable, meaning that the update expression value is evaluated and added or subtracted from the updated value based on a set of rules (explained below).
When you use the Incremental Update, always place it in the Record Suffix section. This way, Magic xpa executes the Update operation once and only once after the record has been edited.
You can use the Incremental Update to accumulate totals in parent tasks or in variables of linked records. In these cases, set the Undo property to N to prevent end-user cancellation of the parent records after the subtask records have already been accepted.
The incremental update is relevant for Online tasks. For Rich Client tasks, incremental update is not supported, although it might work in some scenarios.
This property has the following values:
No – The updated variable is replaced by the evaluated value of the update expression. This is the normal way to update a variable.
Yes – For Incremental, Magic xpa evaluates the update expression and adds this value to or subtracts this value from the updated variable according to the following rules:
-
If the current mode of operation of the task is "Create", Magic xpa adds the value of the expression to the updated variable.
-
If the current mode of operation of the task is "Modify", Magic xpa subtracts the old (a) value of the update expression from the updated variable, then adds to it the new (b) value of the update expression.
Old value of the update expression means: the value resulting from the update expression evaluated when the values of its component variables are those present when the record is read.
New value of the update expression means: the value resulting from the update expression evaluated when the Update operation is executed.
-
If the current mode of operation of the task is "Delete", Magic xpa subtracts the value of the update expression from the updated variable.
-
If the updated variable is a variable selected within a link definition, and if the link criteria are changed during the record interaction, resulting in a new linked record being read, Magic xpa will still maintain the integrity of data as follows: Magic xpa will decrement the Old value of the update expression from the original variable of the original linked record, and Magic xpa will increment the new variable from the newly-linked records by the New value of the update expression.
For example, in an online Order Entry program running in Modify mode, the user may replace the customer number, which is used as a link expression to the customer's file in order to accumulate the orders' total value. In this case when Magic xpa executes the Update operation, Magic xpa subtracts the total for the current (modified) order from the original customer record and adds it to the new customer record, regardless of whether any other changes in the order's total value have been made.
-
As a rule, do not use constants in the update expression for an Incremental Update operation. When the task is in Modify mode, such constants are canceled by the process of decrementing and incrementing. However, such an expression will have full effect while the task is in the Create or Delete mode. For example, the constant 1 may be used as the expression of an Incremental Update operation, to keep count of the number of records in a subtask's data view, where the count is held in a variable selected in a parent task or from a linked table in the current task. As long as the subtask is in Modify mode, no change will occur to the count, but when a record is deleted from the subtask's data view or a new record is created in it, the count will immediately be updated correctly.
Assume you have two files: a Receive File, and a linked Item File. You want to update your stock count, which is kept in the Stock field in the linked Item File by the amount of stock you receive, which is kept in the Quantity variable in the Receive File. Stock is variable A and Quantity is variable B. The Operation is: Update A with Expression B in Incremental mode. If the contents of Stock (A1) is 30 when you start, then:
In Create Mode, if Quantity (B1)is entered as 5, Stock will end up (A2) as 35.
In Modify mode, if Quantity (B1) is currently 5 and you modify (B2) it to 7, Stock will be 35-5+7=37, that is (A2)-(B1)+(B2) = A3.
In Delete mode, if you delete the Receiving transaction record and it still has 7 (B2) in the Quantity variable, stock will be 37-7=30, that is (A3)-(B2)=(A4).
Assume that you did not yet delete the record with Quantity = 7 and that Stock = 37, and then you change the linked Item Record to another one whose Stock = 50. The result will be: Stock of the original record will be 37-7=30 and Stock of the new linked record will be 50+7=57.
This special behavior of Incremental update saves you having to implement additional procedures to preserve data integrity.
Update Expressions
How Can I Determine Magic xpa Behavior When Several Users Are Modifying the Same Row?
The Online and Rich Client Samples projects (program TS34, TS35, RTS34 and RTS35)
