ASP.NET Web PDF Document Viewer/Editor Control Library

When you first start using Ajax techniques, the way your application works may not be clear to your users We ve trained users over the years that Web applications work in a certain way, and Ajax adds an asynchronous component that may throw them Put simply, don t surprise your users If your pre-Ajax application didn t save the form when they tabbed off the last field, your post-Ajax application probably shouldn t either The most important issue to consider when implementing Ajax is simple: it s all about the user, stupid Always remember the user, and don t practice r sum -driven design The desire to pass your next employer s buzzword filter is not a good enough reason to add Ajax to your application; if using Ajax benefits your user by providing a richer experience, by all means, get it out there.

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Figure 2-14. Getting a Display Number block from the Custom palette Complete the rest of the program shown in Figure 2-15. The program first fills the screen with the Smile01 image and then keeps displaying the value of a random number that changes every second on Row 3.

F# provides an alternative way to define functions using the keyword fun; you can see this in the following example Ordinarily, you would use this notation when it is not necessary to give a name to a function, so these are referred to as anonymous functions and sometimes called lambda functions or even just lambdas The idea that a function does not need a name may seem a little strange, but if a function is to be passed as an argument to another function, then often you don t need to give it a name of its own I demonstrate this idea in the section Lists later in this chapter when you look at operations on lists.

The arguments defined for this style of function can follow the same rules as when defining a function with a name, meaning that you can define the arguments so they can be partially applied or defined in the form of a tuple (see the section Defining Types later in this chapter for an explanation of tuples) The following example shows two functions that are created and then immediately applied to arguments so that the identifier x holds the result of the function rather than the function itself: #light let x = (fun x y -> x + y) 1 2 You can create an anonymous function with the keyword function Creating functions this way differs from using the keyword fun since you can use pattern matching when using the keyword function (see the section Pattern Matching later in this chapter).

Figure 2-15. Display Number test program When you run the test program, the NXT screen should look something like Figure 2-16. Notice that the program erases the entire row every time without disturbing the rest of the display. No matter how large the value might have been, no part of it will remain from one time to the next.

But don t forget just because you can, doesn t mean you should Be smart, think of your users first, and you should be fine We ll talk more about security later, but we want to point out that Ajax has some security considerations Remember that source is viewable in a browser, meaning that anyone can figure out how you created your snappy widget Since you have to include uniform resource locators (URLs) when you set up your XHR objects, it is possible that someone with malicious intent could hack your site and run their own code Applying Ajax judiciously can mitigate this risk..

Consequently, it can be passed only one argument, but you can do a couple of things if the function needs to have multiple parameters The first line of the following example shows a function using the function keyword written so the function can be partially applied, meaning the arguments can be passed one at a time if needed The second line shows a function defined using the function keyword that takes a tuple argument let x1 = (function x -> function y -> x + y) 1 2 let x2 = (function (x, y) -> x + y) (1, 2) The keyword fun is generally preferred when defining anonymous functions because it is more compact; you can see this is the case when browsing the source for the libraries and examples distributed with F#..

Figure 2-16. Screen for Display Number test We made Display Number simple to illustrate how to create My Blocks. A more useful block would also allow you to write an identifying label before the number. Also you might want to erase the entire screen at some point. The content of a My Block called Display Value has these added features and is shown in Figure 2-17. The details of how to create this block will be left as an exercise.

Summary

The service is changed dynamically, as shown in Figure 10-4.