Restricting Rule-triggers

A general rule based mechanism, as illustrated above, is clearly not the right choice in HTML forms where user action can trigger event handlers. We elaborate this further with the help of an example. Consider again the online purchase order form as shown in Figure 1. The relationships amongst the form fields can be summarised as:

• total cost is the sum of individual costs;
• total quantity is sum of individual quantities; and
• cost is a product of price and quantity.
• total cost is always deduced from individual costs; and
• total quantities is always deduced from individual quantities;

To make the total quantity reflect the change in any of the item quantities we make the field apparent i.e. change its types from hidden to text. Notice that we have not introduced any new fields in this process. If our approach was to mention the above relationships as suggested in the previous section we will get aberrant behaviour especially in following scenarios:

• A user might try to change the total quantity field;

Clearly, we need to disable user action for such fields a field like total quantity is always deduced from other fields. So, it should be read-only. This scenario is interesting because even though we disable user action we can still use DOM to assign values to this field. The effect of such a restriction is that it reduces the possible number of rule triggers. We will incorporate this provision to disable user actions on certain fields and still retain the flexibility offered by the rule based mechanism.