重拾VB6（18）： Externally Creatable and Dependent Ojbects,

来自MSDN-2001-OCT: Visual Tools and Languages/Visual Studio 6.0 Documentation/Visual Basic Documentation/Using Visual Basic/Programmer’s Guide/Part 2: What Can You Do With Visual Basic/Programming With Components

1. Working with Externally Creatable and Dependent Objects

(1) Most large ActiveX-enabled applications and other ActiveX components provide a top-level externally creatable object in their object hierarchy that:

• a) Provides access to other objects in the hierarchy.
• b) Provides methods and properties that affect the entire application.

(2) In addition to these top-level externally creatable objects,ActiveX components can also provide externally creatable objects that are lower on the component's object hierarchy. You can access these objects either directly as an externally creatable object or indirectly as a dependent object of a higher-level externally creatable object.

(3) Some objects provide an Application object,but give it a different name. For example,the Microsoft Jet database engine in Microsoft Access calls its top-level object the DBEngine object.

(4) You can get a reference to a dependent object in only one way — by using a property or method of an externally creatable object to return a reference to the dependent object.

2. Handling Run-Time Errors in ActiveX Components

(1) The application that provides the object might pass back its own error. In some cases,an application might use the same error code that Visual Basic uses for a different error. In these cases,you should use On Error Resume Next and check for errors immediately after each line that might cause an error. This type of error checking is called inline error-handling.

(2) Is Nothing won't detect whether or not a valid object reference has become unavailable. For example,if you assign a Microsoft Word object reference to an object variable and Microsoft Word becomes unavailable,the variable will still hold a valid object reference. In this situation,use your error handler to trap the error that results when your code tries to use the variable.

3. Handling Requests Pending to an ActiveX Component

(1) A component may reject your request because it has a modal dialog box open,or because a user edit operation is in progress. For example,Microsoft Excel rejects requests from a client application while a spreadsheet cell is being edited.

Visual Basic assumes that the busy condition is temporary and keeps trying the request for a specified timeout interval. When that time is up,Visual Basic displays the Component Busy dialog box

(2) Once a component has accepted your application's request,your application must wait until the request has been completed. If the request takes a long time,the user may attempt to minimize your program or resize it to get it out of the way.

After a short timeout interval,Visual Basic responds to such attempts by displaying the Component Request Pending dialog.

4. Changing the Component Busy or Request Pending Messages

(1) 缺省的pending和busy对话框可能不适用，你也可以用MSGBOX来提示用户，MSGBOX的标题和内容可通过APP的如下属性定制：OLEServerBusyMsgText、OLEServerBusyMsgTitle、OLERequestPendingMsgText、OLERequestPendingMsgTitle。

(2) For the request pending condition,the message box has only an OK button. For the component busy condition,an OK button and a Cancel button are provided. If the user presses Cancel,error -2147418111 (&h80010001) will be raised in the procedure in which you made the request.

5. Controlling Timeout Intervals

You can set the timeout intervals that determine when Visual Basic displays the Component Busy and Component Request Pending dialog boxes,using two properties of the App object: OLEServerBusyTimeout,OLERequestPendingTimeout

6. Raising an Error on Component Busy Timeout

(1) For the component busy condition,you can bypass both the Component Busy dialog box and the replacement message by setting the Boolean OLEServerBusyRaiseError property of the App object to True. Visual Basic will retry your request for the length of time specified by the OLEServerBusyTimeout property,and then raise an error in the procedure that made the Automation request,just as if the user had pressed the Cancel button on the Component Busy dialog box.

(2) The error returned is –2147418111 (&h80010001). In the error handler for the procedure,you can then take whatever action is most appropriate. For example,you could display a complex dialog box that offered the user several retry options or alternatives.

(3) This property will be particularly useful for components designed to run on remote network computers,using the Remote Automation feature of Visual Basic,Enterprise Edition. Such a component may call on other components,and it must handle errors in those calls without displaying any forms.

(4) There is no corresponding property for the request pending condition. Once an Automation request has been accepted by the component,the client program must wait until the request is complete.

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