Raising custom events from user controls Part 106

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ترتيب الدرس : 106
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Text version of the video
http://csharp-video-tutorials.blogspot.com/2013/01/raising-custom-events-from-user.html

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http://csharp-video-tutorials.blogspot.com/2013/08/part-106-raising-custom-events-from.html

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In this video we will discuss about
1. Adding events to UserControls
2. Events and delegates

Most people feel "events and delegates" are complex and difficult to understand. Events and delegates are not that complex to understand, if the basics are right. To get the most out of this video, I strongly recomend to watch parts 36, 37 , 38 and 39 from C# Video series, and parts 104 and 105 from asp.net video series, before proceeding with this video.

C# Video tutorial link
http://www.youtube.com/playlist?listPLAC325451207E3105

ASP.NET tutorial link
http://www.youtube.com/playlist?listPL6n9fhu94yhXQS_p1i-HLIftB9Y7Vnxlo

Very important points to keep in mind, when understanding "Events and Delegates"
1. Delegates are function pointers, and their syntax is very similar to that of a function.
2. Events are variables of type delegates with an event keyword.
If these points are not clear at the moment, don't worry, they will be much clear as we progress.

At the moment, the CalendarUserControl does not have any custom events. Let us say, we want to raise CalendarVisibilityChanged event every time the visibility of the calendar changes. The visibility of the calendar is toggled by clicking on the image button.

The following are the steps to raise CalendarVisibilityChanged event from the CalendarUserControl
Step 1: Create CalendarVisibilityChangedEventArgs class that will contain the event data.
public class CalendarVisibilityChangedEventArgs : EventArgs
{
private bool _isCalendarVisible;

// Constructor to initialize event data
public CalendarVisibilityChangedEventArgs(bool isCalendarVisible)
{
this._isCalendarVisibleisCalendarVisible;
}

// Returns true if the calendar is visible otherwise false
public bool IsCalendarVisible
{
get
{
return this._isCalendarVisible;
}
}
}

Step 2: Create CalendarVisibilityChangedEventHandler delegate. "sender" is the reference variable that points to the instance of the CalendarUserControl, that raises this event. "CalendarVisibilityChangedEventArgs" object will contain "CalendarVisibilityChanged" event data.
public delegate void CalendarVisibilityChangedEventHandler(object sender, CalendarVisibilityChangedEventArgs e);

Step 3: Create CalendarVisibilityChanged event. Remember that, an event is a variable of type delegate. In the line below, we are just creating a variable "CalendarVisibilityChanged" of type "CalendarVisibilityChangedEventHandler" with delegate keyword infornt of it.
public event CalendarVisibilityChangedEventHandler CalendarVisibilityChanged;

Step 4: Create a protected virtual method to raise the event. Since this method is protected and virtual, all classes deriving from the CalendarUserControl class can overridde this method, if they wish to do so. This method enables the derived classes to do some additional work before the event can be raised. Just before raising the event, we are checking if CalendarVisibilityChanged is null. If you are not sure about this, please don't worry. This will be much clear in the next video session, when we discuss about consuming CalendarVisibilityChanged event.
protected virtual void OnCalendarVisibilityChanged(CalendarVisibilityChangedEventArgs e)
{
if (CalendarVisibilityChanged ! null)
{
CalendarVisibilityChanged(this, e);
}
}

For example, if we have a class "DerivedCalendarUserControl" that derives from CalendarUserControl class. "DerivedCalendarUserControl" can override the virtual "OnCalendarVisibilityChanged()" method as shown below. "CalendarVisibilityChanged" will only be raised when "base.OnCalendarVisibilityChanged(e);" is invoked. So, using a "protected virtual" method to raise events is a very useful technique.

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Sep 23, 2020 07:32:26
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