Building an MVP Framework for .NET. Part 3: Designing a Windows Forms Views Engine

of the MVP Framework. This article describes the development of the real- life Windows Forms views manager and its attendant
classes for our MVP Framework.

The only responsibility of a views manager is switching between views. This might seem easy at first sight, however it
becomes more tricky as we delve deeper into the presentation specifics and take into account peculiarities of the views
mechanism. For example in the previous part we have already created a simple Windows Forms views manager, however it is
not able to treat user controls as views, neither can it handle dialogs or MDI forms.

That is why for creating a fully functional views manager we need to thoroughly analyz e the corresponding presentation
mechanism and construct requirements to that views manager. These requirements will typically include the description of
possible view types, their interrelation and so on. Thus our first step is building requirements to the constructed views manager.
We will assume that the basic requirements of working with simple forms are already implemented (see the end of the previous
article where we constructed a simple forms views manager) and proceed to the more advanced demands.

Requirements

User Control Views

The starting point for building the first requirement will be the fact that a user might want to have more than one interaction point
on his screen at a moment. Although separate, these views may be logically coupled, which discourages us from placing them
onto different forms. A more plausible (and popular in modern GUIs) solution is putting views into different parts of a single form.
For instance an orders view and an order lines view can be displayed as parts (e.g. upper and lower halves) of a common
window.

In .NET Windows Forms technology, such views are implemented as user controls. They are designed separately, but are
finally placed together and arranged on a single form. Thus in general our requirement sounds like this:

UserControl

class

descendants may be used as views.

To express this requirement in a more precise form, let us decide how a developer would mark a particular user control to be
used as a view. Here two alternatives are possible:

1. Let the Framework create an instance of the user control
2. Create the user control instance manually and place it on some view.

If a developer chooses 2 (to create an instance himself) then how will the Framework distinguish the user control- view from an
ordinary user control? The answer is quite obvious here: a user control class should implement the

IView

interface to make

the MVP Framework treat it as a view. So here is how the first use case looks:

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User control views
    User: Create a UserControl subclass which implements the IView interface.
          Place its instance on some view. Assign a ViewName property to it.
    System: Find this user control and initialize it (tie to the controller,
            register in the system and so forth)

Here we have included the view name assignment to the user's actions. That is because a view initializ ation requires the

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knowledge of that view's name.

However the view registration and initializ ation are not the only necessary activities. We should also consider how a user
control view should be activated. Unlike forms user controls cannot be activated, instead they have a

Focus()

method which

moves in the input focus. However focusing a user control is of little use if the parent form is inactive, therefore we should also
assure the parent form activation:

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Activating a user control view
    User: Trigger a user control view activation (via Navigator.Naviagate(.).
    System: Activate the parent form and Call the Focus() method on the
            user control view.

A manual user control view activation is possible too when a user clicks somewhere inside that user control. As a response the
system should perform navigation to this view:

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Manual user control view activation
    End user: Click on a user control view (or somehow else move the focus
              inside it).
    System: Perform the navigation to this view.

MDI Form Views

Another kind of view we might want to use is the MDI form. Although slightly out of fashion nowadays, MDI forms may prove
useful in various applications. That is why the next requirement will concern the usage of MDI forms as views.

Applying MDI forms in .NET is simple: the only thing needed is to specify the parent form by setting its

isMdiContainer

property to

true

and to set

MdiParent

property for all child forms. We could link child forms to the parent form instances

by ourselves, however it is not as easy since the MVP Framework and particularly the views manager itself creates forms and
holds their instances. The better approach is to somehow tell the Framework which views should act as MDI parents, and which
ones should be their children. A good way of doing so is applying a .NET attribute with necessary parameters to the view type,
like this:

[WinformsView("ChildView", typeof(MainTask), MdiParent = "ParentView")]

. So here is

the next use case:

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MDI form views
    User: Equip the form type with a WinformsView attribute with MdiParent
          or isMdiParent named parameters. Then at some point navigate to
          the corresponding view.
    System: Initialize the view as an MDI parent or child as specified.

With respect to the view activation mechanism, MDI forms behave the same way as simple forms do. So let us turn to the next
requirement.

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Modal Form Views

Modal forms (dialogs) are very useful if we want a user to interact with only one form until he closes it. In .NET a form modality
depends on the way in which it is shown.

Form.Show()

displays form in an ordinary (not modal) state, while

Form.ShowModal()

displays it as modal. Anyway it is a job of the views manager to show forms, and we should somehow

indicate which forms to display as modal. As in the previous requirement we may use a .NET attribute with a named parameter

ShowModal: [WinformsView("ChildView", typeof(MainTask), ShowModal = true)]

. This is the use

case:

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Modal form views
    User: Equip the form type with a WinformsView attribute with a ShowModal
          named parameter. Then navigate to the corresponding view.
    System: Show the form as modal by calling Form.ShowDialog().

Dif f erent Views with Same Type

Applying the

[View("ViewName", ...)]

[WinformsView("ViewName", ...)]

attribute to a view type we

specify the concrete view type for the interaction point with

ViewName

view name. But what if another view should be of the

same type? The answer is straightforward: allow users to specify several

[(Winforms)View]

attributes with different view

names for a single view type:

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Different views with same type
    User: Equip a view type with several (Winforms)View attributes.
    System: Treat this as descriptions of different views with the
            same view type.

Notif ications to Views

It may be important for a view to react in a particular way when it gets (de)activated. For example a view may disable some
controls on it when it loses focus. A view may also need to perform some initializ ation steps when it is activated for the first time.
For this we need notifications to be sent to views whenever they are (de)activated or initializ ed (activated for the first time).

For handling a view (de)activation we might watch the change of the

Task.CurrentViewName

property. The view

initializ ation can be done in the

IView.Controller

setter method. Much more straightforward, however, is to have explicit

Activate(bool activate)

and

Initialize()

operations, invoked by the views manager. These operations may

be placed in a separate

INotifiedView

interface.

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Notifications to views
    User: Invoke navigation to/from a view that implements INotifiedView
          interface.
    System: Call Activate(true/false) on that view. Call Initialize() on

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it if activated for a first time.

Base Views Implementations

Each form or user control we want to be a view should implement the

IView

interface. It would be convenient if the

Framework provides developers with base form and user control classes which already implement

IView

and even

INotifiedView

interfaces. For example these could be

WinFormView

and

WinUserControlView

classes.

Now we have finished with building requirements and will proceed to the implementation phase.

Implementation

General Classes

First we will implement the

WinformsView

attribute with optional parameters as defined in the "MDI form views" and "Modal

form views" use cases:

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[AttributeUsage(AttributeTargets.Class, AllowMultiple =

true

)]

public

class

WinformsViewAttribute : ViewAttribute

{
...

public

WinformsViewAttribute(Type taskType,

string

viewName)

base

(taskType, viewName) { }

public

WinformsViewAttribute() { }

public

bool

ShowModal

...

public

bool

IsMdiParent

...

public

string

MdiParent

...
}

In the previous article, we have introduced the

DefaultViewInfosProvider

class for processing

[View]

attributes.

For each met

[View]

attribute it created a

ViewInfo

instance. Similarly we should treat

[WinformsViewAttribute]

attributes, except for that

WinformsViewInfo

objects should be created instead of simple

ViewInfo

objects.

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public

class

WinformsViewInfosProvider : DefaultViewInfosProvider

{

protected

override

ViewInfo newViewInfo(Type viewType, ViewAttribute viewAttr)

{
WinformsViewInfo viewInfo =

new

WinformsViewInfo(viewType);

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(!(viewAttr

WinformsViewAttribute))

return

viewInfo;

viewInfo.IsMdiParent = (viewAttr

WinformsViewAttribute).IsMdiParent;

viewInfo.MdiParent = (viewAttr

WinformsViewAttribute).MdiParent;

viewInfo.ShowModal = (viewAttr

WinformsViewAttribute).ShowModal;

return

viewInfo;

}
}

public

class

WinformsViewInfo : ViewInfo

{

public

WinformsViewInfo(Type viewType) :

base

(viewType)

{ }

public

bool

ShowModal

...

public

bool

IsMdiParent

...

public

string

MdiParent

...
}

Notice that

AllowMultiple = true

is specified for the

WinformsViewAttribute

(as well as for the

ViewAttribute

). By doing so we meet the "Different views with same type" requirement.

One more simple requirement we will implement before proceeding to the more complicated ones is the "Base views
implementations" requirement. For that we will create

Form

and

UserControl

descendants and make them implement

IView

and

IWinformsView

interfaces in the simplest way: with the use of virtual properties with backing fields and empty

virtual methods:

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public

class

WinFormView : Form, IView, IWinformsView

{

// IView and IWinformsView implementations with virtual

// methods and virtual properties with backing fields

...
}

public

class

WinUserControlView : UserControl, IView, IWinformsView

{

// IView and IWinformsView implementations with virtual

// methods and virtual properties with backing fields

...
}

Winf ormsViewsManager Class

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WinformsViewsManager

class will inherit from

ViewsManagerBase

- the simplest

IViewsManager

implementation:

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public

class

WinformsViewsManager : ViewsManagerBase

...

public

override

void

ActivateView(

string

viewName)

    {
        IView view = FindOrCreateView(viewName);
        NotifyViewsOnActivation(view);

(view

Form)

ActivateFormView(view);

else

(view

UserControl)

ActivateUserControlView(view);
}

In the

ActivateView

method above, we do the following: get already created view from an internal hash or create a new

one and activate this view in a manner depending on the view kind. Between these two steps we notify views about their
(de)activation in the

NotifyViewsOnActivation(...)

method - this is required by the "Notifications to views" use

case:

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public

class

WinformsViewsManager : ViewsManagerBase

...

private

void

NotifyViewsOnActivation(IView activatedView)

{
IWinformsView prevActiveWFView = prevActiveView

IWinformsView;

(prevActiveWFView !=

null

) prevActiveWFView.Activate(

false

);

IWinformsView winformsView = activatedView

IWinformsView;

(winformsView !=

null

) winformsView.Activate(

true

);

prevActiveView = activatedView;
}

FindOrCreateView(...)

method instantiates views if they do not exist yet.

Type

information is taken from the

appropriate

WinformsViewInfo

object. Then, depending on the view type (form or user control), the corresponding view

initializ ation method is invoked:

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public

class

WinformsViewsManager : ViewsManagerBase

...

private

IView FindOrCreateView(

string

viewName)

{
IView result = views[viewName]

IView;

(result ==

null

)

{
WinformsViewInfo viewInf = ViewInfos[viewName]

WinformsViewInfo;

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result = CreateHelper.Create(ViewInfos[viewName].ViewType)

IView;

result.ViewName = viewName;

(result

UserControl)

InitializeUserControlView(result

UserControl);

else

(result

Form)

InitializeFormView(result

Form, viewInf);

}

return

result;

}

Below are the methods for user control and form initializ ation:

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public

class

WinformsViewsManager : ViewsManagerBase

...

protected

virtual

void

InitializeUserControlView(UserControl userControlView)

{
InitializeView(userControlView

IView);

userControlView.Enter +=

new

EventHandler(view_ActivatedManually);

NotifyInitialize(userControlView

IView);

InitializeChildViews(userControlView);
}

protected

virtual

void

InitializeFormView(Form form, WinformsViewInfo viewInf)

{
InitializeView(form

IView);

form.Activated +=

new

EventHandler(view_ActivatedManually);

form.IsMdiContainer = viewInf.IsMdiParent;

string

mdiParent = viewInf.MdiParent;

(mdiParent !=

null

)

form.MdiParent = views[mdiParent]

Form;

NotifyInitialize(form

IView);

InitializeChildViews(form);
}

Both these methods use the

InitializeView(...)

and

NotifyInitialize(...)

methods which contain common

initializ ation steps regardless of the view type.

InitializeView(...)

binds together a view with its controller.

NotifyInitialize(...)

sends an

Initialize

message to the view accordingly to the "Notifications to views"

requirement:

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public

class

WinformsViewsManager : ViewsManagerBase

...

private

void

InitializeView(IView view)

    {
        views[view.ViewName] = view;
        view.Controller = Navigator.GetController(view.ViewName);

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view.Controller.View = view;
}

private

void

NotifyInitialize(IView view)

{
INotifiedView winformsView = view

INotifiedView;

(winformsView !=

null

)

winformsView.Initialize();
}

Note that the

InitializeFormView(...)

method contains code specific to views represented as forms: if needed, it

makes a form MDI child or parent, thus satisfying the "MDI form views" requirement.

InitializeChildViews(...)

is a method that searches for user control views inside a form or another user control

view. The search is done recursively, for found user control views, the user control- specific

InitializeUserControlView(...)

method is called. By doing so we implement the "User control views" use case:

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public

class

WinformsViewsManager : ViewsManagerBase

...

protected

void

InitializeChildViews(Control container)

{

foreach

(Control c

container.Controls)

{
IView childView = c

IView;

((childView !=

null

) && (!IsInitialized(childView)))

InitializeUserControlView(childView

UserControl);

else

                InitializeChildViews(c);
        }
    }

By handling the

Enter

event of user controls we meet the "Manual user control view activation" requirement:

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private

void

view_ActivatedManually(

object

sender, EventArgs e)

{
Navigator.TryNavigateToView((sender

IView).ViewName);

}

The last two methods left are the view activation methods

ActivateFormView

and

ActivateUserControlView

. The

former shows and makes active a form view, taking into account that it could be configured as modal (and thus meeting the
"Modal form views" requirement):

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public

class

WinformsViewsManager : ViewsManagerBase

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...

private

void

ActivateFormView(IView view)

{
Form form = view

Form;

WinformsViewInfo viewInf = ViewInfos[view.ViewName]

WinformsViewInfo;

(viewInf.ShowModal)

{

(!form.Visible) form.ShowDialog();

}

else

        {
            form.Show();
            form.Activate();
        }
    }

ActivateUserControlView(...)

method not only focuses the user control but it firstly activates the parent of this

control, thus implementing the "Activating a user control view" use case:

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public

class

WinformsViewsManager : ViewsManagerBase

...

private

void

ActivateUserControlView(IView view)

{
UserControl uc = view

UserControl;

        uc.Focus();
        uc.FindForm().Show();
        uc.FindForm().Activate();
    }

Summary

Throughout this article, we have been building a comprehensive Windows Forms views engine for the Model- View- Presenter
Framework. We have started with a list of requirements for the future views engine and then implemented these requirements in
the

WinformsViewsManager

class and other satellite classes. As a result these classes comprise a fully- functional views

engine suitable for various MVP applications with Windows Forms- based UI. However it is not restricted to further extend this
views engine tailoring it for specific needs.

Project Website

www.MVCSharp.org

License

This article, along with any associated source code and files, is licensed under

The Code Project Open License (CPOL)

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About the Author

Oleg Zhukov

Architect

Russian Federation

Member

Oleg Zhukov, born and living in Russia is a software development consultant in a company which
provides business solutions. He has graduated from Moscow Institute of Physics and Technology
(MIPT) (department of system programming) and has got a M.S. degree in applied physics and
mathematics. His research and development work concerns architectural patterns, domain- driven
development and systems analysis. Being the adherent of agile methods he applies them
extensively in the projects managed by him.

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