CODEWITHSUNDEEP
c#winformsuser-controls
Dumbo
Dumbo

Reputation: 14102

Getting update from UserControl

Can someone tell me how can I have a feature in my UserControl, that can let the host windowsform know what is the control is doing?

For example my usercontrol has a filebrowser, and if user uses this file browser to open a file I want in the statusstrip bar of my form to write "Loading file(s)".

Will this require using events? if so, how can I have a single event inside usercontrol to report anything it does (then I guess I have to call this event in all methods in the usercontrol).

Upvotes: 1

Views: 168

Answers (2)

Tergiver
Tergiver

Reputation: 14507

Simple

Yes, expose an event on the user control that the Form can subscribe to. You should use the standard event pattern:

class MyUserControl : UserControl
{
    public event EventHandler<EventArgs> FileOpened;

    protected virtual void OnFileOpened(EventArgs e)
    {
        EventHandler<EventArgs> handler = FileOpened;
        if (handler != null)
            handler(this, e);
    }
}

Then when the file is opened you call OnFileOpened(EventArgs.Empty) which fires the event.

With custom EventArgs

Now the Form probably needs to know what file was opened. You could expose a property on the user control that the Form can use to find out, or you can provide that information in your event like so:

public class FileOpenedEventArgs : EventArgs
{
    private string filename;
    public FileOpenedEventArgs(string filename)
    {
        this.filename = filename;
    }
    public string Filename { get { return filename; } }
}

class MyUserControl : UserControl
{
    public event EventHandler<FileOpenedEventArgs> FileOpened;

    protected virtual void OnFileOpened(FileOpenedEventArgs e)
    {
        EventHandler<FileOpenedEventArgs> handler = FileOpened;
        if (handler != null)
            handler(this, e);
    }
}

Then you fire the event with OnFileOpened(new FileOpenedEventArgs(filename)).

Optimal

When you create an event handler public event delegate Name;, you are allocating storage for the delegate on your object. Objects (especially Controls) often have a huge number of events that are never subscribed to. That's a whole lot of allocated storage not being used. There's an optimization built into the framework in the form of a EventHandlerList. This handy object stores event handlers only when they are actually used. All System.Windows.Forms.Control objects derive from System.ComponentModel.Component and it already provides an (protected) EventHandlerList that you can access in your derived Control.

To use it, you first create a static object that uniquely identifies your event, and then you provide the add {} and remove {} methods manually. Like so:

class MyUserControl : UserControl
{
    private static readonly object FileOpenedKey = new Object();
    public event EventHandler<FileOpenedEventArgs> FileOpened
    {
        add { Events.AddHandler(FileOpenedKey, value); }
        remove { Events.RemoveHandler(FileOpenedKey, value); }
    }

    protected virtual void OnFileOpened(FileOpenedEventArgs e)
    {
        var handler = (EventHandler<FileOpenedEventArgs>)Events[FileOpenedKey];
        if (handler != null)
            handler(this, e);
    }
}

Upvotes: 2

lc.
lc.

Reputation: 116438

Yes, you will need to create an event and subscribe to it. One suggestion following the standard pattern for events:

enum ControlStatus {Idle, LoadingFile, ...}

class StatusChangedEventArgs : EventArgs
{
    public ControlStatus Status {get; private set;}

    public StatusChangedEventArgs(ControlStatus status)
        : base()
    {
        this.Status = status;
    }
}

partial class MyControl : UserControl
{
    public ControlStatus Status {get; private set;}

    public event EventHandler<StatusChangedEventArgs> StatusChanged;

    protected virtual void OnStatusChanged(StatusChangedEventArgs e)
    {
        var hand = StatusChanged;
        if(hand != null) hand(this, e);
    }

    void LoadFiles()
    {
        ...

        Status = ControlStatus.LoadingFiles; 
        OnStatusChanged(new StatusChangedEventArgs(this.Status));

        ...

        Status = ControlStatus.Idle; 
        OnStatusChanged(new StatusChangedEventArgs(this.Status));
    }
}

partial class MyHostWindowsForm : Form
{
    public MyHostWindowsForm()
    {
        var ctl = new MyControl();
        ...
        ctl.StatusChanged += ctl_StatusChanged;
    }

    void ctl_StatusChanged(object sender, StatusChangedEventArgs e)
    {
        switch(e.Status)
        {
            case ControlStatus.Idle:
                statusStripBar.Text = null;
                break;
            case ControlStatus.LoadingFiles:
                statusStripBar.Text = "Loading file(s)";
                break;
            ...
        }
    }
}

Upvotes: 1

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