关于PB中英文翻译的文章（2天类结帖）（前天的帖子刚结帖了）
需要pb 相关技术的文章，最好原著是英文，且有中文的翻译。
2 天内结帖，人多了，另外开帖!
问题点数：100、回复次数：6Top
1 楼njupt_zhb（攒钱买飞机）回复于 2005-06-05 15:26:50 得分 100
http://www.tiantiansoft.com/bbs/dispbbs.asp?boardID=79&ID=68347&page=2
http://www.tiantiansoft.com/bbs/dispbbs.asp?boardID=79&ID=82482&page=2
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2 楼windyman518（风一样的男子）回复于 2005-06-06 02:25:44 得分 0
译自pb的联机帮助。
原文在：pb安装目录\PowerBuilder 9.0\Help\pbman90.chm:
Application Techniques -> Language Techniques -> Chapter3 -> Exception handling in PowerBuilder
因为我没有装PB9，能把Exception handling in PowerBuilder这篇英文帖出来吗？
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3 楼njupt_zhb（攒钱买飞机）回复于 2005-06-06 10:33:17 得分 0
Exception handling in PowerBuilder
When a runtime error occurs in a PowerBuilder application, unless that error is trapped, a single application event (SystemError) fires to handle the error no matter where in the application the error happened. Although some errors can be handled in the system error event, catching the error closer to its source increases the likelihood of recovery from the error condition.
You can use exception-handling classes and syntax to handle context-sensitive errors in PowerBuilder applications. This means that you can deal with errors close to their source by embedding error-handling code anywhere in your application. Well-designed exception-handling code can give application users a better chance to recover from error conditions and run the application without interruption.
Exception handling allows you to design an application that can recover from exceptional conditions and continue execution. Any exceptions that you do not catch are handled by the runtime system and can result in the termination of the application.
Machine code If you need to build machine code DLLs, you cannot use exception handling. The process of building machine code does not support the exception handling mechanism. Attempts to compile machine code DLLs from code that uses exception handling fail and display an error message. For more information about choosing between Pcode (PBDs) and machine code (DLLs), see "Compiler basics".
PowerBuilder clients can catch exceptions thrown from EAServer components and recover from them. Components developed with PowerBuilder can also define and throw their own exception types, making them more consistent with other EAServer component types like Java.
Exception handling can be found in such object-oriented languages as Java and C++. The implementation for PowerBuilder is similar to the implementation of exception handling in Java. In PowerBuilder, the TRY, CATCH, FINALLY, THROW, and THROWS reserved words are used for exception handling. There are also several PowerBuilder objects that descend from the Throwable object.
Basics of exception handling
Exceptions are objects that are thrown in the event of some exceptional (or unexpected) condition or error and are used to describe the condition or error encountered. Standard errors, such as null object references and division by zero, are typically thrown by the runtime system. These types of errors could occur anywhere in an application and you can include catch clauses in any executable script to try to recover from these errors.
User-defined exceptions
There are also exceptional conditions that do not immediately result in runtime errors. These exceptions typically occur during execution of a function or a user-event script. To signal these exceptions, you create user objects that inherit from the PowerScript Exception class. You can associate a user-defined exception with a function or user event in the prototype for the method.
For example, a user-defined exception might be created to indicate that a file cannot be found. You could declare this exception in the prototype for a function that is supposed to open the file. To catch this condition, you must instantiate the user-defined exception object and then throw the exception instance in the method script.
Objects for exception handling support
Several system objects support exception handling within PowerBuilder.
Throwable object type
The object type Throwable is the root datatype for all user-defined exception and system error types. Two other system object types, RuntimeError and Exception, derive from Throwable.
RuntimeError and its descendants
PowerBuilder runtime errors are represented in the RuntimeError object type. For more robust error-handling capabilities, the RuntimeError type has its own system-defined descendants; but the RuntimeError type contains all information required for dealing with PowerBuilder runtime errors.
One of the descendants of RuntimeError is the NullObjectError type that is thrown by the system whenever a null object reference is encountered. This allows you to handle null-object-reference errors explicitly without having to differentiate them from other runtime errors that might occur.
Error types that derive from RuntimeError are typically used by the system to indicate runtime errors. RuntimeErrors can be caught in a try-catch block, but it is not necessary to declare where such an error condition might occur. (PowerBuilder does that for you, since a system error can happen anywhere anytime the application is running.) It is also not a requirement to catch these types of errors.
Exception object type
The system object Exception also derives from Throwable and is typically used as an ancestor object for user-defined exception types. It is the root class for all checked exceptions. Checked exceptions are user-defined exceptions that must be caught in a try-catch block when thrown, or that must be declared in the prototype of a method when thrown outside of a try-catch block.
The PowerScript compiler checks the local syntax where you throw checked exceptions to make sure you either declare or catch these exception types. Descendants of RuntimeError are not checked by the compiler, even if they are user-defined or if they are thrown in a script rather than by the runtime system.
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4 楼njupt_zhb（攒钱买飞机）回复于 2005-06-06 10:33:40 得分 0
Handling exceptions
Whether an exception is thrown by the runtime system or by a THROW statement in an application script, you handle the exception by catching it. This is done by surrounding the set of application logic that throws the exception with code that indicates how the exception is to be dealt with.
TRY-CATCH-FINALLY block
To handle an exception in PowerScript, you must include some set of your application logic inside a try-catch block. A try-catch block begins with a TRY clause and ends with the END TRY statement. It must also contain either a CATCH clause or a FINALLY clause. A try-catch block normally contains a FINALLY clause for error condition cleanup. In between the TRY and FINALLY clauses you can add any number of CATCH clauses.
CATCH clauses are not obligatory, but if you do include them you must follow each CATCH statement with a variable declaration. In addition to following all of the usual rules for local variable declarations inside a script, the variable being defined must derive from the Throwable system type.
You can add a TRY-CATCH-FINALLY, TRY-CATCH, or TRY-FINALLY block using the Script view Paste Special feature for PowerScript statements. If you select the Statement Templates check box on the AutoScript tab of the Design Options dialog box, you can also use the AutoScript feature to insert these block structures.
Example
Example catching a system error This is an example of a TRY-CATCH-FINALLY block that catches a system error when an arccosine argument, entered by the application user (in a SingleLineEdit) is not in the required range. If you do not catch this error, the application goes to the system error event, and eventually terminates:
Double ld_num ld_num = Double (sle_1.text) TRY sle_2.text = string (acos (ld_num)) CATCH (runtimeerror er) MessageBox("Runtime Error", er.GetMessage()) FINALLY // Add cleanup code here of_cleanup() Return END TRY MessageBox("After", "We are finished.")
The system runtime error message might be confusing to the end user, so for production purposes, it would be better to catch a user-defined exception--see the example in "Creating user-defined exception types"--and set the message to something more understandable.
The TRY reserved word signals the start of a block of statements to be executed and can include more than one CATCH clause. If the execution of code in the TRY block causes an exception to be thrown, then the exception is handled by the first CATCH clause whose variable can be assigned the value of the exception thrown. The variable declaration after a CATCH statement indicates the type of exception being handled (a system runtime error, in this case).
CATCH order
It is important to order your CATCH clauses in such a way that one clause does not hide another. This would occur if the first CATCH clause catches an exception of type Exception and a subsequent CATCH clause catches a descendant of Exception. Since they are processed in order, any exception thrown that is a descendant of Exception would be handled by the first CATCH clause and never by the second. The PowerScript compiler can detect this condition and signals an error if found.
If an exception is not dealt with in any of the CATCH clauses, it is thrown up the call stack for handling by other exception handlers (nested try-catch blocks) or by the system error event. But before the exception is thrown up the stack, the FINALLY clause is executed.
FINALLY clause
The FINALLY clause is generally used to clean up after execution of a TRY or CATCH clause. The code in the FINALLY clause is guaranteed to execute if any portion of the try-catch block is executed, regardless of how the code in the try-catch block completes.
If no exceptions occur, the TRY clause completes, followed by the execution of the statements contained in the FINALLY clause. Then execution continues on the line following the END TRY statement.
In cases where there are no CATCH clauses but only a FINALLY clause, the code in the FINALLY clause is executed even if a return is encountered or an exception is thrown in the TRY clause.
If an exception occurs within the context of the TRY clause and an applicable CATCH clause exists, the CATCH clause is executed, followed by the FINALLY clause. But even if no CATCH clause is applicable to the exception thrown, the FINALLY clause still executes before the exception is thrown up the call stack.
If an exception or a return is encountered within a CATCH clause, the FINALLY clause is executed before execution is transferred to the new location.
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5 楼njupt_zhb（攒钱买飞机）回复于 2005-06-06 10:34:22 得分 0
Creating user-defined exception types
You can create your own user-defined exception types from standard class user objects that inherit from Exception or RuntimeError or that inherit from an existing user object deriving from Exception or RuntimeError.
Inherit from Exception object type
Normally, user-defined exception types should inherit from the Exception type or a descendant, since the RuntimeError type is used to indicate system errors. These user-defined objects are no different from any other nonvisual user object in the system. They can contain events, functions, and instance variables.
This is useful, for example, in cases where a specific condition, such as the failure of a business rule, might cause application logic to fail. If you create a user-defined exception type to describe such a condition and then catch and handle the exception appropriately, you can prevent a runtime error.
Throwing exceptions
Exceptions can be thrown by the runtime engine to indicate an error condition. If you want to signal a potential exception condition manually, you must use the THROW statement.
Typically, the THROW statement is used in conjunction with some user-defined exception type. Here is a simple example of the use of the THROW statement:
Exception le_ex le_ex = create Exception Throw le_ex MessageBox ("Hmm", "We would never get here if" & + "the exception variable was not instantiated")
In this example, the code throws the instance of the exception le_ex. The variable following the THROW reserved word must point to a valid instance of the exception object that derives from Throwable. If you attempt to throw an uninstantiated Exception variable, a NullObjectError is thrown instead, indicating a null object reference in this routine. That could only complicate the error handling for your application.
Declaring exceptions thrown from functions
If you signal an exception with the THROW statement inside a method script--and do not surround the statement with a try-catch block that can deal with that type of exception--you must also declare the exception as an exception type (or as a descendant of an exception type) thrown by that method. However, you do not need to declare that a method can throw runtime errors, since PowerBuilder does that for you.
The prototype window in the Script view of most PowerBuilder painters allows you to declare what user-defined exceptions, if any, can be thrown by a function or a user-defined event. You can drag and drop exception types from the System Tree or a Library painter view to the Throws box in the prototype window, or you can type in a comma-separated list of the exception types that the method can throw.
Example
Example catching a user-defined exception This code displays a user-defined error when an arccosine argument, entered by the application user, is not in the required range. The try-catch block calls a method, wf_acos, that catches the system error and sets and throws the user-defined error:
TRY wf_acos() CATCH (uo_exception u_ex) MessageBox("Out of Range", u_ex.GetMessage()) END TRY
This code in the wf_acos method catches the system error and sets and throws the user-defined error:
uo_exception lu_error Double ld_num ld_num = Double (sle_1.text) TRY sle_2.text = string (acos (ld_num)) CATCH (runtimeerror er) lu_error = Create uo_exception lu_error.SetMessage("Value must be between -1" & + "and 1") Throw lu_error END TRY
Integration with EAServer
If you declare exceptions on a method of a user object and deploy the user object as a component to EAServer, the exceptions are translated to IDL (CORBA) as part of the method prototype. This means that PowerBuilder components in EAServer can be defined to throw exceptions that can be handled by any type of EAServer client application.
Other benefits for EAServer applications Another benefit for component development is that you can handle runtime errors in the component. If you do not handle an error, it is automatically translated into an exception and the component stops executing.
PowerBuilder client applications that use EAServer components can handle exceptions thrown by any type of EAServer component. If a Java EAServer component has a method on it that is defined to throw an exception and a PowerBuilder proxy is created to use that component, the method on the PowerBuilder proxy is also declared to throw a user-defined exception. The definition of the user-defined exception is created automatically at the time of the PowerBuilder proxy creation.
For more information about error handling in EAServer clients, see "Handling errors ".
IDL restrictions Deployment of components to EAServer imposes restrictions on the way you can use exception handling within PowerBuilder. Only the public instance variables defined on the exception type are actually translated to IDL. This is because IDL exceptions cannot have methods declared on them. Therefore if the exception type has methods defined on it, those methods can be called within the execution of the component but cannot be called by client applications that catch the exception thrown.
You must keep this restriction in mind when designing exception objects for distributed applications, exposing all exception information as public instance variables instead of through accessor methods on an exception object.
Two other interface restrictions also apply to exception types of a user object that is deployed as an EAServer component. Instance variables of exceptions on the user object methods cannot have object datatypes. Null data is supported only for instance variables with simple datatypes; if instance variables are structures or arrays, null values for individual elements are not maintained.
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6 楼windyman518（风一样的男子）回复于 2005-06-08 04:30:25 得分 0
一直忙,现在结贴.Top
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