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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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Table 3-4. Object Status Properties
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Property
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IsNew IsDirty IsValid IsSavable IsDeleted
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Indicates whether the object s primary identifying value in memory corresponds to a primary key in a database if not, the object is new Indicates whether the object s data in memory is known to be different from data in the database if different, the object is dirty Indicates whether the object currently has any broken validation rules if so, the object is not valid Combines IsValid and IsDirty only a valid and dirty object is savable Indicates whether the object is marked for deletion
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I will now discuss the concepts behind an object being new, dirty, valid, and marked for deletion.
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IsNew
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When an object is new, it means that the object exists in memory, but not in the database or other persistent store. If the object s data resides in the database, then the object is considered to be old. I typically think of it this way: if the primary key value in the object corresponds to an existing primary key value in the database, then the object is old; otherwise it is new. The value behind the IsNew property is stored in an _isNew field. When an object is first created, this value defaults to the object being new: private bool _isNew = true; If the object is then loaded with data from the database, the _isNew field is set to false, through a protected MarkOld() method: protected virtual void MarkOld() { _isNew = false; MarkClean(); } Notice that this process also sets the object to a clean status a concept discussed later for the IsDirty property. When an object s data has just been loaded from the database, it is safe to assume that the object s data matches the data in the database and so has not been changed and thus is clean. There s also a corresponding MarkNew() method: protected virtual void MarkNew() { _isNew = true; _isDeleted = false; MarkDirty(); } Typically, this method is called upon deletion of an existing object, but it can be used any time the business developer knows that the object does not correspond to data in the database. In such a case, not only is the object new, but it must also be dirty, because the data in the object does not match data in the database. The concept of being marked for deletion will be discussed later with the IsDeleted property, but a new object shouldn t be marked for deletion, and so this flag is set to false.
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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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Knowing whether an object is new or old will allow for implementation of the data access code in 4. The IsNew property will control the choice of whether to insert or update data into the database. Sometimes, the IsNew property can be useful to the UI developer as well. Some UI behaviors may be different for a new object than for an existing object. The ability to edit the object s primary key data is a good example this is often editable only up to the point that the data has been stored in the database. When the object becomes old, the primary key is fixed.
IsDirty
An object is considered to be dirty, or changed, when the values in the object s fields do not match the values in the database. If the values in the object s fields do match the values in the database, then the object is not dirty. It is virtually impossible to always know whether the object s values match those in the database, so the implementation shown here acts on a best guess. The implementation relies on the business developer to indicate when an object has been changed and thus has become dirty. The current status of the value is maintained in a field: private bool _isDirty = true; The value is then exposed as a property: [Browsable(false)] public virtual bool IsDirty { get { return _isDirty; } } Notice that this property is marked as virtual. This is important because sometimes a business object isn t simply dirty because its data has changed. For instance, consider a business object that contains a collection of child objects even if the business object s data hasn t changed, it will be dirty if any of its child objects have changed. In this case, the business developer will need to override the IsDirty property to provide a more sophisticated implementation. This will be clearly illustrated in 7, in the implementation of the example business objects. Also notice that the property is adorned with the [Browsable()] attribute from the System. ComponentModel namespace. This attribute tells data binding not to automatically bind this property. Without this attribute, data binding would automatically display this property in grids and on forms and typically, this property shouldn t be displayed. This attribute is used on other properties in BusinessBase as well. The IsDirty property defaults to true, since a new object s field values won t correspond to values in the database. If the object s values are subsequently loaded from the database, this value will be changed to false when MarkOld() is called. Remember that MarkOld() calls a MarkClean() method: protected void MarkClean() { _isDirty = false; OnUnknownPropertyChanged(); } This method not only sets the value to false, but calls the OnUnknownPropertyChanged() method implemented in Csla.Core.BindableBase to raise the PropertyChanged event for all object properties. This notifies data binding that the object has changed, so Windows Forms can refresh the display for the user. There s a corresponding MarkDirty() method as well. This method will be called from various points in an object s lifetime, including any time a property value is changed, or when the MarkNew() method is called.
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