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Ensure that this value is reset when the album is cleared.
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Override the OnClear method. Note: This protected method is provided by the CollectionBase class to permit collection-specific code to be executed before the Clear method is invoked. 4 Implement the
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CurrentPhoto property.
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Note: The base keyword used here is provided by C# as a convenient way to reference the base class of the current object.
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public Photograph CurrentPhoto { get { if (this.Count == 0) return null; return this[CurrentPosition]; } }
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IMPLEMENT ALBUM POSITION OPERATIONS (continued) Action 5 Implement the
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public bool CurrentNext() { if (CurrentPosition+1 < this.Count) { CurrentPosition ++; return true; } return false; }
Use the CurrentPosition property to set and get the current index.
Implement the
CurrentPrev method.
public bool CurrentPrev() { if (CurrentPosition > 0) { CurrentPosition --; return true; } return false; }
We can now add photographs to and remove photographs from our album, and track the current position for display purposes. Since we expect the CurrentPosition property to return a valid index, we should also update this setting whenever a Photograph is removed from the album.
ENSURE ALBUM POSITION REMAINS VALID Action 7 In the PhotoAlbum.cs file, override the
OnRemoveComplete
Result
protected override void OnRemoveComplete (int index, object val) { CurrentPosition = _currentPos; base.OnRemoveComplete(index, val); }
method. Note: This protected method is called after an object is removed from the contained collection.
As you can see, this code ensures that the current position is updated whenever an object is removed from the collection. This includes both the Remove and RemoveAt methods. By resetting the property, we ensure that the _currentPos variable is reset as appropriate for the new bounds of the album. With the interfaces for our PhotoAlbum class fully implemented, let s head back to the Photograph class to deal with various issues related to the robustness of our new library.
CHA PTE R 5
REUSABLE LIBRARIES
ROBUSTNESS ISSUES
While our classes are basically ready, there are some additional issues that will affect the robustness of our application in future chapters. This section will address a number of these issues in order to make our library a bit more sturdy. These topics apply more generally to any class library, so are probably worth considering while developing your own libraries as well. This section will look at the following areas: Handling the potential exception when a bitmap is created. Ensuring that photographs are compared as expected. Cleaning up system resources used by our classes. Associating a file name with an album. We will examine each issue separately.
HANDLING AN INVALID BITMAP We discussed the concept of exceptions in chapter 2. Here, there is a potential exception when we create our Bitmap object for the Image property. Look back at our definition of this property.
public Bitmap Image { get { if (_bitmap == null) { _bitmap = new Bitmap(_fileName); } return _bitmap; } }
If the file is an invalid bitmap, or cannot be loaded for some reason, this presents a real problem. On the one hand, this is an error, so perhaps we should return null or allow the exception to be thrown. On the other hand, the caller is expecting to display a Bitmap, and checking for null or an exception every time seems a bit cumbersome, not to mention the issue of what the caller should then display in lieu of a Bitmap object. As an alternative approach, we will instead create a special bitmap to return whenever the file cannot be loaded. This provides a Bitmap that the caller can display in any situation, but still indicates that something is wrong. We will create a private static member of our Photograph class to hold this special image, and provide a new property to indicate if a valid image for the current Photograph exists. Let s see how this looks.
ROBUSTNESS ISSUES
Set the version number of the MyPhotoAlbum library to 5.4. HANDLE THE BITMAP EXCEPTION Action 1 In the Photograph.cs file, create a private static member to hold a Bitmap object. Add a public property to retrieve this bitmap.
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