Logging Errors to a File in Objective-C

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An even better error-hunting technique I use is to write the script s activity to a text file. Since doing this every second or third line of the script really will hamper performance, you will want the user to implement it only when there s a persistent error. Find a way for the user to flag the script that should log errors. You can also choose to log more of the script s activity, in which case the logical
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CHAPTER 17 USING ERRORS TO YOUR ADVANTAGE
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place in the script is between tasks the script performs. For instance, you can have the following comments logged in the proper places throughout your script: Exporting Excel file <name> to tabdelimited file, Now reading tab-delimited file into AppleScript, Now inserting images into page template, and so on. If you have an interface, then you can add a little check box called Log Activity. Otherwise, if your script has a dialog box that appears at the start, you may want to add a button to it called Settings. Clicking this button will display another dialog box that will allow users to turn logging on for the following execution of the script. This will turn a log_error global variable to true, which will then tell all the handlers in the script to log their activity. The idea with logging activity is to create a text file on the desktop bearing the date and time and add some text to that file every few lines. The text should describe the location and include some of the actual values used in the script. This can provide clues for the error. I tell my clients to e-mail me that resulting text file, which is handy for revealing the cause of the error. Since you will be utilizing the error logging feature again and again, it is a perfect candidate for a handler with a couple of parameters such as the line number, the part of the script it s called from, and maybe the values of a few variables coerced into a string. A handler call is much easier to embed multiple times in your script.
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Generating Errors
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Having spent the last portion of the chapter trying to contain and control errors, why in the world would you want to go around creating them yourself Well, in some situations during the script s execution, you will realize that the best way to proceed is to throw an error. In many cases, this error will be handled by the same error trapping mechanism you set up to trap other possible errors. To throw an error yourself, you use AppleScript s error statement. The most basic form of this statement is simply this: error Running this statement causes AppleScript to raise an unknown error with the generic message An error has occurred and with an error number of 2700. However, in most situations you ll want to supply a more meaningful error message and error number. Here s the full definition of the error statement: error [error_message] [number error_number] [from offending_object] [to expected_type] [partial result result_list] As you can see, the error statement has the same parameters as the try statement s on error line. The error message and error number are the two parameters you ll usually use for throwing errors. For example: error "E.T. could not phone home: wrong number" number 411 Figure 17-4 shows the error dialog box that this error, unhandled, will display.
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CHAPTER 17 USING ERRORS TO YOUR ADVANTAGE
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Figure 17-4. The error message shown in an AppleScript application error dialog box Throwing your own errors is a good idea when part of your script hits a problem it can t deal with itself. Not only does this let other parts of the script know there s a problem, but it also gives them a chance to deal with it if they want by trapping and handling that error in a try block. As an example, let s say you re writing a script where one part involves looking up a phone number based on a person s name. The names and phone numbers are stored in a list of lists like this: set contact_table to {{"Joe", "5553712"}, {"Pam", "5550232"}, {"Sam", "5556795"}} This list works as a simple lookup table: to get a phone number, you loop through each item in the main list, checking to see whether the name matches the one you want. When the loop finds a matching name, it returns the corresponding phone number. For example, if the name is Sam, then the value returned will be 5556795. You ll encounter one other possibility, though: what if none of the entries in the table matches the desired name Well, let s take a page out of AppleScript s book here. Just as saying get item 10 of {1, 2, 3, 4, 5} will cause AppleScript to raise a Can t get item . . . error, I ll have my tablesearching subroutine generate a similar error when it can t find the entry that was requested. Here s the error statement I ll use: error "The item was not found." number 1728 Since the type of error being reported is similar to an existing AppleScript error, I ve used the same error code as AppleScript, 1728. But I could make up my own error code for it if I preferred, such as error number 6000, Can t find an entry with the given key in a lookup table. Here s the full table lookup subroutine: on get_value_for_key(key_value_list, key_to_find) repeat with item_ref in key_value_list set {this_key, this_value} to item_ref if this_key = key_to_find then return this_value end repeat error "The item wasn't found." number 1728 end get_value_for_key And here are some examples of its use: get_value_for_key(contact_table, "Pam") --result: "555-0232" get_value_for_key(contact_table, "Frank") --error: The item wasn't found.
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