Action

Result

1

In the PhotoAlbum.cs file,

/// <summary>

add a private integer

/// Tracks the current index position

_currentPos.

/// when displaying the album.

/// </summary>

private int _currentPos = 0;

2

Add the CurrentPosition

public int CurrentPosition

property to get or set this

{

position.

get { return _currentPos; }

How-to

set

For the get access

{

if (value <= 0)

method, simply return the

{

current value.

_currentPos = 0;

For the set access

}

else if (value >= this.Count)

method, make sure the

{

given value is in range.

_currentPos = this.Count - 1;

}

else

{

_currentPos = value;

}

}

}

3

Ensure that this value is

protected override void OnClear()

reset when the album is

{

cleared.

_currentPos = 0;

base.OnClear();

How-to

}

Override the OnClear

Note: The base keyword used here is provided by C# as

method.

a convenient way to reference the base class of the cur-

Note: This protected

rent object.

method is provided by the

CollectionBase class to

permit collection-specific

code to be executed

before the Clear method

is invoked.

4

Implement the

public Photograph CurrentPhoto

CurrentPhoto property.

{

get

{

if (this.Count == 0)

return null;

return this[CurrentPosition];

}

}

INTERFACES REVISITED

149

Action

Result

5

Implement the

public bool CurrentNext()

CurrentNext method.

{

How-to

if (CurrentPosition+1 < this.Count)

{

Use the CurrentPosition

CurrentPosition ++;

property to set and get the

return true;

}

current index.

return false;

}

6

Implement the

public bool CurrentPrev()

CurrentPrev method.

{

if (CurrentPosition > 0)

{

CurrentPosition --;

return true;

}

return false;

}

Action

Result

7

In the PhotoAlbum.cs file,

protected override void OnRemoveComplete

override the

(int index, object val)

OnRemoveComplete

{

CurrentPosition = _currentPos;

method.

base.OnRemoveComplete(index, val);

Note: This protected

}

method is called after an

object is removed from

the contained collection.

150

CHAPTER 5 REUSABLE LIBRARIES