Simple .NET/ASP.NET PDF document editor web control SDK

Although the built-in exceptions cover a lot of ground and are sufficient for many purposes, there are times when you might want to create your own. For example, in the hyperdrive overload example, wouldn t it be more natural to have a specific HyperDriveError class representing error conditions in the hyperdrive It might seem that the error message is sufficient, but as you will see in the next section ( Catching Exceptions ), you can selectively handle certain types of exceptions based on their class. Thus, if you want to handle hyperdrive errors with special error-handling code, you would need a separate class for the exceptions. So, how do you create exception classes Just like any other class but be sure to subclass Exception (either directly or indirectly, which means that subclassing any other built-in exception is okay). Thus, writing a custom exception basically amounts to something like this: class SomeCustomException(Exception): pass Really not much work, is it

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This code works like a normal Ruby loop. The for loop iterates through the results of Entry.content_columns (an ActiveRecord method that returns each column as an object). In your application s case, the column names are Title, Content, and Created at (humanized versions of the actual column names: title, content, and created_at), so the preceding loop, which uses each column s human_name method, results in the following HTML being generated and returned to the visiting Web browser:

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The core part of the list view contains this code:

<% for entry in @entries %> <tr> <% for column in Entry.content_columns %> <td><%=h entry.send(column.name) %></td> <% end %> <td><%= link_to 'Show', :action => 'show', :id => entry %></td> <td><%= link_to 'Edit', :action => 'edit', :id => entry %></td> <td><%= link_to 'Destroy', { :action => 'destroy', :id => entry }, :confirm => 'Are you sure ', :post => true %></td> </tr> <% end %>

channel thus text- lled slides essentially ignore the capacity of the visual channel to ef ciently process information in sync with the verbal channel.

As mentioned earlier, the interesting thing about exceptions is that you can handle them (often called trapping or catching the exceptions). You do this with the try/except statement. Let s say you have created a program that lets the user enter two numbers and then divides one by the other, like this: x = input('Enter the first number: ') y = input('Enter the second number: ') print x/y This would work nicely until the user enters zero as the second number: Enter the first number: 10 Enter the second number: 0 Traceback (most recent call last): File "exceptions.py", line 3, in print x/y ZeroDivisionError: integer division or modulo by zero To catch the exception and perform some error handling (in this case simply printing a more user-friendly error message), you could rewrite the program like this: try: x = input('Enter the first number: ') y = input('Enter the second number: ') print x/y except ZeroDivisionError: print "The second number can't be zero!" It might seem that a simple if statement checking the value of y would be easier to use, and in this case it might indeed be a better solution. But if you added more divisions to your program, you would need one if statement per division; by using try/except, you need only one error handler.

This view code results in the primary part of the page being rendered: the actual list of entries. I won t go into this code line by line, but there a few key things to note. This whole section is a loop over each element of @entries (where for entry in @entries is an alternative way of saying @entries.each do |entry|). You should recall that your controller code placed Entry objects from the database into the @entries array, so the view code iterates over each element (or each entry). Next, within the main loop is another loop that iterates through each of the columns associated with each of these entries. After the data for the entry has been shown, you reach this:

<td><%= link_to 'Show', :action => 'show', :id => entry %></td> <td><%= link_to 'Edit', :action => 'edit', :id => entry %></td> <td><%= link_to 'Destroy', { :action => 'destroy', :id => entry }, :confirm => 'Are you sure ', :post => true %></td>

Myth: Graphics are nice to have, but they re not essential. Truth: Research shows that people learn better from words and pictures than from words alone. This applies when the pictures illustrate what the words say, not when pictures are added for decorative effect.

The important parts to look at are the calls to the link_to method. link_to is a special method provided by Rails that generates an HTML link to another controller and/or action within the application. Let s look at the first line: