birt barcode plugin HARDWARE EMULA TION in visual basic

Printer Quick Response Code in visual basic HARDWARE EMULA TION

Undo is conceptually the opposite of redo. Undo information is generated by the database as you make modifications to data so that the data can be put back the way it was before the modifications took place. This might be done in support of multi-versioning as we learned in 7 Concurrency and Multi-

assembly to reference the interface assembly. This is recursive, and thus can t be compiled. Hence both interface and implementation have to be defined in the same assembly. What is possible is the definition of IVisitor interface could be in one assembly, and the implementation of IVisitor could be in another. However, the types referenced in IVisitor then need to be defined with public scope, which defeats the purpose of the Bridge and Factory patterns. There is a way to have your cake and eat it too. The better solution is to use Generics and modify the IVisitor interface to the following: public interface IVisitor { void Process< type>( type parameter) where type : class ; } In the new definition of IVisitor, the method Process has been altered to use a Generic type. The Generic type is the parameter of the type being inspected. Using Generics in this way delegates the responsibility of identifying the type to the implementation of IVisitor. The advantage of this approach is that it s possible to move both IBase and IVisitor to an interfacesonly assembly for reference by an implementations assembly. To demonstrate these principles, I ll show you XML serialization to a stream via a Visitor pattern. The modified SampleClass from the XML serialization example is defined as follows: [XmlRoot("SampleClass")] public class SampleClass : IBase { private int _value; private string _buffer; public SampleClass() { } public SampleClass(int value, string buffer) { _value = value; _buffer = buffer; } [XmlElement( "Value")] public int Value { get { return _value; } set { _value = value; } } [XmlElement( "Buffer")] public string Buffer { get { return _buffer; }

#1: Method 1 of Class 1 #2: Method 2 of Class 1 #3: Method 1 of Class 2 #4: Method 2 of Class 2 #5: Method 1 of Class 3 #6: Method 2 of Class 3 #7: Method 3 of Class 2 #8: Method 3 of Class 1 #9: Method 4 of Class 1

The translation application has been dissected and restructured to use multiple patterns. If you consider the incrementing complexity from the initial application to the resulting architecture, the question that must be asked is, Why do this The answer is flexibility and testability. The initial code can t be easily tested, nor easily extended. Even though the translation application is simple, most applications aren t. That means being able to test and extend are important attributes that can make or break an application. The application used Generic types extensively to illustrate how powerful it is to write code that uses black boxes that are manipulated by an adapter. Because of the Generic types, no type information is lost at the developer level, as using the Object type requires the developer to think about what type is being passed around. As illustrated, the type could be System. Windows.Forms.TextBox, or a wx.TextCtrl, or NUnit PropertyKeeper. It doesn t matter, as the adapter knows what the type is and how to process the type. Moving on, I ll show you how to extend this translation application in the next section.

<NotUndoable()> _ Private mBindingEdit As Boolean Private mNeverCommitted As Boolean = True Notice that mBindingEdit is declared with the <NotUndoable()> attribute. This field controls interaction with the UI, not internal object state; and because of this, there s no reason to make it part of the object s snapshot data, as that would just waste memory. A second flag is also declared, and is used to track whether ApplyEdit() has been called on the object. This value was set to False in the ApplyEdit() implemented earlier, and will be used to control whether a child object should remove itself from its parent collection. The three interface methods are implemented as follows: Private Sub IEditableObject_BeginEdit() _ Implements System.ComponentModel.IEditableObject.BeginEdit If Not mBindingEdit Then BeginEdit() End If End Sub Private Sub IEditableObject_CancelEdit() _ Implements System.ComponentModel.IEditableObject.CancelEdit If mBindingEdit Then CancelEdit() If IsNew AndAlso mNeverCommitted AndAlso _ EditLevel <= EditLevelAdded Then If Not Parent Is Nothing Then Parent.RemoveChild(Me) End If End If End If End Sub Private Sub IEditableObject_EndEdit() _ Implements System.ComponentModel.IEditableObject.EndEdit If mBindingEdit Then ApplyEdit() End If End Sub Notice that the methods are declared using syntax to explicitly implement the IEditableObject interface. This is required because BeginEdit() and CancelEdit() are already public methods in the class, and this avoids any naming conflict. All three methods call the corresponding edit methods implemented earlier. The mBindingEdit field is used to determine whether the BeginEdit() method has been called already so any subsequent method calls can be ignored. The mBindingEdit field is set to True when an edit process is started, and to False when either CancelEdit() or ApplyEdit() is called. The mNeverCommitted field tracks whether the ApplyEdit() method has ever been called. If it hasn t ever been called, and data binding attempts to cancel the edit operation, this flag is used to control whether the object should remove itself from its parent collection. The mNeverCommitted field starts out True and is set to False if ApplyEdit() is called. With this mechanism in place, the implementation of IEditableObject.BeginEdit() calls only the real BeginEdit() method if no edit session is currently underway. With the implementation of the n-level undo methods and System.ComponentModel.IEditableObject, business objects now provide full control over editing and undo capabilities, both to the UI developer and to Windows Forms data binding.