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• MVVM load data during or after ViewModel construction?

  - by mkmurray

  My generic question is as the title states, is it best to load data during ViewModel construction or afterward through some Loaded event handling? I'm guessing the answer is after construction via some Loaded event handling, but I'm wondering how that is most cleanly coordinated between ViewModel and View? Here's more details about my situation and the particular problem I'm trying to solve: I am using the MVVM Light framework as well as Unity for DI. I have some nested Views, each bound to a corresponding ViewModel. The ViewModels are bound to each View's root control DataContext via the ViewModelLocator idea that Laurent Bugnion has put into MVVM Light. This allows for finding ViewModels via a static resource and for controlling the lifetime of ViewModels via a Dependency Injection framework, in this case Unity. It also allows for Expression Blend to see everything in regard to ViewModels and how to bind them. So anyway, I've got a parent View that has a ComboBox databound to an ObservableCollection in its ViewModel. The ComboBox's SelectedItem is also bound (two-way) to a property on the ViewModel. When the selection of the ComboBox changes, this is to trigger updates in other views and subviews. Currently I am accomplishing this via the Messaging system that is found in MVVM Light. This is all working great and as expected when you choose different items in the ComboBox. However, the ViewModel is getting its data during construction time via a series of initializing method calls. This seems to only be a problem if I want to control what the initial SelectedItem of the ComboBox is. Using MVVM Light's messaging system, I currently have it set up where the setter of the ViewModel's SelectedItem property is the one broadcasting the update and the other interested ViewModels register for the message in their constructors. It appears I am currently trying to set the SelectedItem via the ViewModel at construction time, which hasn't allowed sub-ViewModels to be constructed and register yet. What would be the cleanest way to coordinate the data load and initial setting of SelectedItem within the ViewModel? I really want to stick with putting as little in the View's code-behind as is reasonable. I think I just need a way for the ViewModel to know when stuff has Loaded and that it can then continue to load the data and finalize the setup phase. Thanks in advance for your responses.

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• Pass or Get a value from Parent ViewModel down to Sub-ViewModel?

  - by mkmurray

  I am using the MVVM Light framework as well as Unity for DI. I have some nested Views, each bound to a corresponding ViewModel. The ViewModels are bound to each View's root control DataContext via the ViewModelLocator idea that Laurent Bugnion has put into MVVM Light. This allows for finding ViewModels via a static resource and for controlling the lifetime of ViewModels via a Dependency Injection framework, in this case Unity. It also allows for Expression Blend to see everything in regard to ViewModels and how to bind them. As I stated the Views have a healthy dose of nesting, but the ViewModels don't really know anything about each other. A parent view binds to its corresponding ViewModel via the static resource ViewModelLocator (which uses Unity to control the construction and lifetime of the ViewModel object). That parent view contains a user control in it that is another sub-view, which then goes and gets its corresponding ViewModel via the ViewModelLocator as well. The ViewModels don't have references to each other or know any hierarchy in regard to each other. So here's an example of how the ViewModels do interact via messaging. I've got a parent View that has a ComboBox databound to an ObservableCollection in its ViewModel. The ComboBox's SelectedItem is also bound (two-way) to a property on the ViewModel. When the selection of the ComboBox changes, this is to trigger updates in other Views and sub-Views. Currently I am accomplishing this via the Messaging system that is found in MVVM Light. So I'm wondering what the best practice would be to get information from one ViewModel to another? In this case, what I need to pass down to sub-ViewModels is basically a user Guid representing the currently logged in user. The top-most parent View (well, ViewModel) will know this information, but I'm not sure how to get it down into the sub-ViewModels. Some possible approaches I can think of: Should the sub-ViewModel ask the static resource ViewModelLocator for a reference to the same object the parent View is using and access the property that way? It seems like ViewModels going through each other's properties is not very clean and couples them together unnecessarily. I'm already using messaging to notify the sub-Views that the user selected a new item in the ComboBox and to update accordingly. But the object type that is being selected in the ComboBox is not really directly related to this data value that the sub-Views need.

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• Special thanks to everyone that helped me in 2010.

  - by mbcrump

  2010 has been a very good year for me and I wanted to create a list and thank everyone for what they have done for me.  I also wanted to thank everyone for reading and subscribing to my blog. It is hard to believe that people actually want to read what I write. I feel like I owe a huge thanks to everyone listed below. Looking back upon 2010, I feel that I’ve grown as a developer and you are part of that reason. Sometimes we get caught up in day to day work and forget to give thanks to those that helped us along the way. The list below is mine, it includes people and companies. This list is obviously not going to include everyone that has helped, just those that have stood out in my mind. When I think back upon 2010, their names keep popping up in my head. So here goes, in no particular order.  People Dave Campbell – For everything he has done for the Silverlight Community with his Silverlight Cream blog. I can’t think of a better person to get recognition at the Silverlight FireStarter event. I also wanted to thank him for spending several hours of his time helping me track down a bug in my feedburner account. Victor Gaudioso – For his large collection of video tutorials on his blog and the passion and enthusiasm he has for Silverlight. We have talked on the phone and I’ve never met anyone so fired up for Silverlight. Kunal Chowdhury – Kunal has always been available for me to bounce ideas off of. Kunal has also answered a lot of questions that stumped me. His blog and CodeProject article have green a great help to me and the Silverlight Community. Glen Gordon – I was looking frantically for a Windows Phone 7 several months before release and Glen found one for me. This allowed me to start a blog series on the Windows Phone 7 hardware and developing an application from start to finish that Scott Guthrie retweeted.  Jeff Blankenburg – For listening to my complaints in the early stages of Windows Phone 7. Jeff was always very polite and gave me his cell phone number to talk it over. He also walked me through several problems that I was having early on. Pete Brown – For writing Silverlight 4 in Action. This book is definitely a labor of love. I followed Pete on Twitter as he was writing it and he spent a lot of late nights and weekends working on it. I felt a lot smarter after reading it the first time. The second time was even better. John Papa – For all of his work on the Silverlight Firestarter and the Silverlight community in general. He has also helped me on a personal level with several things. Daniel Heisler – For putting up with me the past year while we worked on many .NET projects together in 2010. Alvin Ashcraft – For publishing a daily blog post on the best of .NET links. He has linked to my site many times and I really appreciate what he does for the community. Chris Alcock – For publishing the Morning Brew every weekday. I remember when I first appeared on his site, I started getting hundreds of hits on my site and wondered if I was getting a DOS attack or something. It was great to find out that Chris had linked to one of my articles. Joel Cochran – For spending a week teaching “Blend-O-Rama”. This was my one of my favorite sessions of this year. I learned a lot about Expression Blend from it and the best part was that it was free and during lunchtime. Jeremy Likness – Jeremy is smart – very smart. I have learned a lot from Jeremy over the past year. He is also involved in the Silverlight community in every way possible, from forums to blog post to screencast to open source. It goes on and on. The people that I met at VSLive Orlando 2010. I had a great time chatting with Walt Ritscher, Wallace McClure, Tim Huckabee and David Platt. Also a special thanks to all of my friends on Twitter like @wilhil, @DBVaughan, @DataArtist, @wbm, @DirkStrauss and @rsringeri and many many more. Software Companies / Events / May of gave me FREE stuff. =) Microsoft (3) – I was sent a free coupon code by Microsoft to take the Silverlight 4 Beta Exam. I jumped on the offer and took the exam. It was great being selected to try out the exam before it goes public even though Microsoft eventually published a universal coupon code for everyone. I am still waiting to find out if I passed the exam. My fingers are crossed. Microsoft reaching out to me with some questions regarding the .NET Community. I’ve never had a company contact me with such interest in the community. Having a contest where 75 people could win a $100 gift certificate and a T-Shirt for submitting a Windows Phone 7 app. I submitted my app and won. All of the free launch events this year (Windows Phone 7, Visual Studio 2010, ASP.NET MVC). Wintellect – For providing an awesome day of free technical training called T.E.N. Where else can you get free training from some of the best programmers in the world? I also won a contest from them that included a NETAdvantage Ultimate License from Infragistics. VSLive – I attended the Orlando 2010 Conference and it was the best developer’s conference that I have ever attended. I got to know a lot of people at this conference and hang out with many wonderful speakers. I live tweeted the event and while it may have annoyed some, the organizers of VSLive loved it. I won the contest on Twitter and they invited me back to the 2011 session of my choice. This is a very nice gift and I really appreciate the generosity. BarcodeLib.com – For providing free barcode generating tools for a Non-Profit ASP.NET project that I was working on. Their third party controls really made this a breeze compared to my existing solution. NDepend – It is absolutely the best tool to improve code quality. The product is extremely large and I would recommend heading over to their site to check it out. Silverlight Spy – I was writing a blog post on Silverlight Spy and Koen Zwikstra provided a FREE license to me. If you ever wanted to peek inside of a Silverlight Application then this is the tool for you. He is also working on a version that will support OOB and Windows Phone 7. I would recommend checking out his site. Birmingham .NET Users Group / Silverlight Nights User Group – It takes a lot of time to put together a user group meeting every month yet it always seems to happen. I don’t want to name names for fear of leaving someone out but both of these User Groups are excellent if you live in the Birmingham, Alabama area. Publishing Companies Manning Publishing – For giving me early access to Silverlight 4 in Action by Pete Brown. It was really nice to be able to read this awesome book while Pete was writing it. I was also one of the first people to publish a review of the book. Sams Publishing and DZone – For providing a copy of Silverlight 4 Unleashed by Laurent Bugnion for me to review for their site. The review is coming in January 2011. Special Shoutout to the following 3rd Party Silverlight Controls It has been a great pleasure to work with the following companies on 3rd Party Control Giveaways every month. It always amazes me how every 3rd Party Control company is so eager to help out the community. I’ve never been turned down by any of these companies! These giveaways have sparked a lot of interest in Silverlight and hopefully I can continue giving away a new set every month. If you are a 3rd Party Control company and are interested in participating in these giveaways then please email me at mbcrump29[at]gmail[d0t].com. The companies below have already participated in my giveaways: Infragistics (December 2010) - Win a set of Infragistics Silverlight Controls with Data Visualization!  Mindscape (November 2010) - Mindscape Silverlight Controls + Free Mega Pack Contest Telerik (October 2010) - Win Telerik RadControls for Silverlight! ($799 Value) Again, I just wanted to say Thanks to everyone for helping me grow as a developer.  Subscribe to my feed

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• Silverlight 4 + MVVM + KeyDown event

  - by jturn

  I'm trying to build a sample game in Silverlight 4 using the MVVM design pattern to broaden my knowledge. I'm using Laurent Bugnion's MvvmLight toolkit as well (found here: http://mvvmlight.codeplex.com/ ). All I want to do right now is move a shape around within a Canvas by pressing specific keys. My solution contains a Player.xaml (just a rectangle; this will be moved around) and MainPage.xaml (the Canvas and an instance of the Player control). To my understanding, Silverlight doesn't support tunneling routed events, only bubbling. My big problem is that Player.xaml never recognizes the KeyDown event. It's always intercepted by MainPage.xaml first and it never reaches any child controls because it bubbles upward. I'd prefer that the logic to move the Player be in the PlayerViewModel class, but I don't think the Player can know about any KeyDown events firing without me explicitly passing them on down from the MainPage. I ended up adding the handler logic to the MainPageViewModel class. Now my problem is that the MainPageViewModel has no knowledge of Player.xaml so it cannot move this object when handling KeyDown events. I guess this is expected, as ViewModels should not have any knowledge of their associated Views. In not so many words...is there a way this Player user control within my MainPage.xaml can directly accept and handle KeyDown events? If not, what's the ideal method for my MainPageViewModel to communicate with its View's child controls? I'm trying to keep code out of the code-behind files as much as possible. Seems like it's best to put logic in the ViewModels for ease of testing and to decouple UI from logic. (MainPage.xaml) <UserControl x:Class="MvvmSampleGame.MainPage" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:game="clr-namespace:MvvmSampleGame" xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity" xmlns:cmd="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Extras.SL4" mc:Ignorable="d" Height="300" Width="300" DataContext="{Binding Main, Source={StaticResource Locator}}"> <i:Interaction.Triggers> <i:EventTrigger EventName="KeyDown"> <cmd:EventToCommand Command="{Binding KeyPressCommand}" PassEventArgsToCommand="True" /> </i:EventTrigger> </i:Interaction.Triggers> <Canvas x:Name="LayoutRoot"> <game:Player x:Name="Player1"></game:Player> </Canvas> (MainViewModel.cs) public MainViewModel() { KeyPressCommand = new RelayCommand<KeyEventArgs>(KeyPressed); } public RelayCommand<KeyEventArgs> KeyPressCommand { get; private set; } private void KeyPressed(KeyEventArgs e) { if (e.Key == Key.Up || e.Key == Key.W) { // move player up } else if (e.Key == Key.Left || e.Key == Key.A) { // move player left } else if (e.Key == Key.Down || e.Key == Key.S) { // move player down } else if (e.Key == Key.Right || e.Key == Key.D) { // move player right } } Thanks in advance, Jeremy

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