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The Mutex and Monitor classes provide a locking mechanism that is useful for preventing a section of code from being executed simultaneously by multiple threads. However, there are times when a lock that can distinguish between a reader and a writer can increase performance. This is especially true when there are more readers than writers or if the writer is infrequent with its updates. Fortunately, the ReaderWriterLock class provides this functionality. Listing 14.10 contains the definition of the ReaderWriterLock class.
You can use protected declarations to define a component writer's interface to the class. Application units do not have access to the protected parts, but derived classes do. This means that component writers can change the way a class works without making the details visible to application developers.
A totally alternative approach for building large strings or writing to and reading from streams is to use the new reader and writer classes introduced in Delphi 2009, again mapped to their .NET counterparts. Each of the writers has two sets of overloaded operations without (Write) and with ( WriteLine) an end-of-line separator. Here is the first set For writing to an in-memory string you can use a specific stream class or use the TStringWriter class, which uses either a TStringBuilder object passed to its constructor or creates an internal one. At the end you can ask for the complete string. The TStringReader works on a string passed as parameter to its only constructor, but it has no easy way to detect the end of the string. The only ready-to-use solution I've found (without extending the class) has been to see if the Peek call returns any value. The following event handler (again from the ReaderWriter application) fills a string in memory with a writer and than reads it back...
LockCookie overload procedure DowngradeFromWriterLock(var lockCookie LockCookie) function ReleaseLock LockCookie procedure RestoreLock(var lockCookie LockCookie) function AnyWritersSince(seqNum Integer) Boolean property IsReaderLockHeld Boolean read property IsWriterLockHeld Boolean read property WriterSeqNum integer read end As expected, two types of locks are available. These locks are referred to as a reader lock and a writer lock. Locks are acquired by using the AcquireReaderLock() and AcquireWriterLock() methods, respectively. A reader lock is acquired only when no writer locks are being held. That way, multiple readers are allowed. However, all readers are blocked when a writer lock is obtained. Only one writer lock is allowed. Find the code on the CD Code Chapter 14 Ex07 TestRWLock.dpr for an example of how to use the ReaderWriterLock class.
Component writers that create their own custom descendants from TDataSet must override all appropriate IProviderSupport methods if their datasets are to supply data to a provider. If the provider need only provide data packets on a read-only basis (that is, if it does not need to apply updates), the IProviderSupport methods implemented in TDataSet may be sufficient.
Component writers can use the classes in the QStdActns and DBActns units as examples for deriving their own action classes to implement behaviors specific to certain controls or components. The base classes for these specialized actions (TEditAction, TWindowAction, and so on) generally override HandlesTarget, UpdateTarget, and other methods to limit the target for the action to a specific class of
In COM, inheriting via interface is the only option. In the .NET Framework, inheriting via interface or inheriting via implementation is a design decision. .NET component writers can choose to add a new method or property at any time. If changes are made to the .NET component, any COM client that depends on the layout of the interface (e.g. by caching dispids) will break. A .NET component writer must choose to expose type information in an exported type library it is not the default behavior. This is done through the use of the ClassInterfaceAttribute custom attribute. ClassInterfaceAttribute is found in the System.Reflection.InteropServices namespace. It can take on the values of the ClassInterfaceType enumeration, which are, AutoDispatch (the default), AutoDual, and None.
System and widget events, which are mainly of concern when writing components, are handled differently by the VCL and CLX. The most important difference is that VisualCLX controls do not respond directly to Windows messages, even when running on Windows (see Chapter 7, Handling messages and system notifications, in the Component Writer's Guide.) Instead, they respond to notifications from the underlying widget layer. Because the notifications use a different system, the order and timing of events can sometimes differ between corresponding the VCL and CLX objects. This difference occurs even if your CLX application is running on Windows rather than Linux. If you are porting a VCL application to Linux, you may need to change the way your event handlers respond to accommodate these differences. For information on writing components that respond to system and widget events (other than those that are reflected in the published events of CLX components), see Responding to system...
Before changing the message handling of your components, make sure that is what you really want to do. Delphi translates most Windows messages into events that both the component writer and the component user can handle. Rather than changing the message-handling behavior, you should probably change the event-handling behavior.
24 procedure AddAttributesToRender(Writer HTMLTextWriter) override 27 procedure Render(Writer HTMLTextWriter) override 28 103 procedure Self.UniquelD) 'Text') DisplayText) 112 procedure TPostBackInputWebControl.Render(Writer HTMLTextWriter) procedure Self.UniquelD) 'Text') DisplayText) end procedure TPostBackInputWebControl.Render(Writer HTMLTextWriter) begin 32 procedure Render(Writer HTMLTextWriter) override 188 procedure TNewUserInfoControl.Render(Writer HTMLTextWriter) 191 '1', False) 'right') 201 Writer.RenderEndTag Closes the TD tag 204 Writer.RenderEndTag Closes the TD tag 207 Writer.RenderEndTag Closes the TD tag 208 Writer.RenderEndTag Closes the TR tag 'right') 216 Writer.RenderEndTag Closes the TD tag 219 Writer.RenderEndTag Closes the TD tag 222 Writer.RenderEndTag Closes the TD tag 223 Writer.RenderEndTag Closes the TR tag 'right') 231 Writer.RenderEndTag Closes the TD tag 234 Writer.RenderEndTag Closes the TD tag 237 Writer.RenderEndTag Closes the TD tag 238...
This chapter is a brief introduction of object-oriented concepts for programmers who are just starting out with the Delphi language. For more details on object-oriented programming for programmers who want to write components that can be installed on the Component palette, see Chapter 1, Overview of component creation, of the Component Writer's Guide.
All components are installed in the IDE as packages. If you've written your own components, create and compile a package that contains them. (See Creating and editing packages on page 16-7.) Your component source code must follow the model described in the Component Writer's Guide.
Kylix uses method pointers to implement events. A method pointer is a special pointer type that points to a specific method in an instance object. As a component writer, you can treat the method pointer as a placeholder When your code detects that an event occurs, you call the method (if any) specified by the user for that event.
The global variable Application, of type TApplication, is in every CLX-based application. Application encapsulates your application as well as providing many functions that occur in the background of the program. For instance, Application would handle how you would call a help file from the menu of your program. Understanding how TApplication works is more important to a component writer than to developers of stand-alone applications, but you should set the options that Application handles in the Project l Options Application page when you create a project.
When you look up TForm in online Help, you'll notice that TForm is called a component. Don't let this confuse you. All components and controls are also objects. The terminology used in the documentation comes from the inheritance hierarchy of the Delphi Visual Component Library. Figure 6.3 is a greatly simplified diagram of the hierarchy, omitting some intermediary objects. To see a more complete diagram, refer to the Delphi Component Writer's Guide
The client controls connected to actions are usually menu items and various types of buttons (push buttons, check boxes, radio buttons, speed buttons, toolbar buttons, and the like), but nothing prevents you from creating new components that hook into this architecture. Component writers can even define new actions, as we'll do in Chapter 11, and new link action objects.
For information on creating a component unit file, see Using the Component Expert in Chapter 2. You can find an example of unit file source code for a new component on page 72. The Delphi Component Writer's Guide provides complete information on creating new Delphi components.
This XML code is generated using as a helper the TTrivialXmlWriter class as a helper that I covered in Chapter 3, in the section The Trivial XML Writer Class under the heading XML Streaming . This is a class that let's you write to an XML stream or string, using the TTextWri ter interface introduced in Delphi 2009. It keeps track of the XML nodes you open so it can close them in the reverse order, without specifying which tag you are closing.
Component writers that create their own custom descendants from TDataSet must override all appropriate IProviderSupport methods if their datasets are to supply data to a provider. If the provider only provides data packets on a read-only basis (that is, if it does not apply updates), the IProviderSupport methods implemented in TDataSet may be sufficient.
NOTE In case the secondary forms were not owned by the main one, we could have used the FreeNotification method to get the secondary form to notify the main form when they are destroyed. FreeNotification receives as parameter the component to notify when the current component is destroyed. The effect is a call to the Notification method coming from a component other than the owned ones. FreeNotification is generally used by component writers to safely connect components on different forms or data modules.
Whereas the streaming classes encapsulate the block of data, one requires readers and writers to read from a stream and to write to a stream, respectively. For a text-based stream, these are the StreamReader and StreamWriter classes. For binary data, these are the BinaryReader and BinaryWriter classes. Their usage will be illustrated in the following sections.
CIL includes a volatile modifier that allows the compiler to specify that a memory access really needs to go to or come from physical memory. The static methods Thread. VolatileRead and Thread.VolatileWrite allow application writers to do volatile reads and writes on an as as-needed basis the volatile field modifier allows you to guarantee that all access to a field is done as a volatile access.
This event allows you to override the size constraints when an attempt is made to resize the control. The values of the constraints are passed as var parameters which can be changed inside the event handler. OnConstrainedResize is published for container objects (TForm, TScrollBox, TControlBar, and TPanel). In addition, component writers can use or publish this event for any descendant of TControl.
Never make an assignment to a property in a property's writer method. For example, examine the following property declaration Because you are accessing the property itself (not the internal storage field), you cause the SetSomeProp() method to be called again, which results in a infinitely recursive loop. Eventually, the application will throw a stack overflow exception. Likewise, the same applies for reading a property value in its reader method. Always access the internal storage field in the reader and writer methods of properties. necessary data and making it available to component users as event handler parameters. One of the nice things about the whole scheme is that it enables component writers to take information that might be somewhat complex to understand and make it available to component users in a much more understandable and easy-to-use format.
It's not often that you would want to override a method unless you are creating new components. You should be aware that you can do so, though, and that you won't receive any warning or error message from the compiler. You can read more about overriding methods in the Delphi Component Writer's Guide.
This is a truly opinionated and arguable statement, but Delphi's Object Pascal is a much better language than C C++ for two reasons It is not as arcane, and it is much stronger typed. While it is possible to write easily understandable code in C C++, it is just as easy to write code that is completely unintelligible and unmanageable. C C++ has a precompiler that allows one to redefine function names (among other things). While this can be quite useful in some instances, if used indiscriminately it can lead to a major headache when trying to decipher what a piece of code accomplishes. C C++ has many other language constructs that allow one to write very archaic instructions decipherable only by its author or a C C++ guru. The primary reason this is important is because, in today's development shops, a section of code may be written by one programmer but modified or updated by several others. It is an incredible waste of time and energy if a programmer tasked with the responsibility of...
Just like C++ functions, some methods take parameters and return values, and others do not. It depends entirely on how the method was written by the component writer. For example, the GetTextBuf() method retrieves the text of a TEdit component. This method could be used to get the text from an edit control as follows
Delphi encapsulates the Windows GDI (Qt in CLX applications) at several levels. The most important to you as a component writer is the way components display their images on the screen. When calling GDI functions directly, you need to have a handle to a device context, into which you have selected various drawing tools such as pens, brushes, and fonts. After rendering your graphic images, you must restore the device context to its original state before disposing of it.
I don't want to cover XSD schemas and Xsd.exe both because it would take too much room and because only some .NET programmers will need to deal with them. I'll say only that, for the most part, you should leave the XML attributes to Xsd. exe. If you need to read and write XML in a particular format and you don't have a formal schema, you are probably better off using the XML libraries (Chapter 18) than trying to get XmlSerializer to read and write the format. While it's very convenient to serialize a class straight to XML and to deserialize straight from XML to a class, it can require a lot of fighting with XmlSerializer and the XML attributes to get this to work it can be easier just to manually write code to copy each datum to and from an XML writer or XML reader, or to and from an XML DOM data tree.
Note that there isn't just a single help file in Delphi. Most of the time, you'll invoke Delphi Help, but this file is complemented by an Object Pascal Help file, the Windows API Help, the Component Writer's Help, and many others (depending on your version of Delphi). These and other Help files have a common outline and a common search engine you can activate by pressing the Help Topics button while in the Help system. The Windows help engine dialog box that appears allows you to browse the contents of all of the help files in the group, search for a keyword in the index, or start the Find engine. The three capabilities are available in separate pages of the Help Topics dialog box.
Property Attributes The properties of the property (sorry, I couldn't resist) are determined by the writer of the component. A property can be read-only. A read-only property can be read--its value can be retrieved--but not Some methods take parameters and return values, others don't. It depends entirely on how the method was written by the component writer. For example, the GetTextBuf method retrieves the text of a TEdit component. This method can be used to get the text from an edit control as follows
The Classes unit is at the heart of both VCL and CLX libraries, and though it sees many internal changes from the last version of Delphi, there is little new for the average users. (Most changes are related to modified IDE integration and are meant for expert component writers.)
The most stringent rule for packages is the following one used by component writers For long-term deployment and maintenance of code in packages, plan on having a major release with minor maintenance releases. A major release of your package will require all client programs to be recompiled from source the package file itself should be renamed with a new version number, and the interface sections of units can be modified. Maintenance releases of that package should be restricted to implementation changes to preserve full compatibility with existing executables and units.
A class can have any amount of data and any number of methods. However, for a good object-oriented approach, data should be hidden, or encapsulated, inside the class using it. When you access a date, for example, it makes no sense to change the value of the day by itself. In fact, changing the value of the day might result in an invalid date, such as February 30. Using methods to access the internal representation of an object limits the risk of generating erroneous situations, as the methods can check whether the date is valid and refuse to modify the new value if it is not. Encapsulation is important because it allows the class writer to modify the internal representation in a future version.
By itself, the FileStream only offers byte-by-byte access to a file. There are blocking and asynchronous methods to read and write individual bytes or byte arrays, but usually you will use a FileStream through a Reader or Writer object. The StreamReader and StreamWriter classes are character-oriented classes that read and write text files the BinaryReader and BinaryWriter read and write binary streams. When you create a Reader or Writer object, you pass it either an open FileStream or a filename and let it create the FileStream for you. When you are done reading and writing the file, you should Close the Reader or Writer object. This closes the underlying FileStream. The Reader and Writer classes implement IDisposable, and call Close in their Dispose method. Thus, creating a Reader and Writer object within the to-be-Dispose()d expression of a uses statement guarantees that the file will be closed at the end of the uses statement. The second, false parameter to the StreamWriter...
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