| > The V and M never communicate directly in either direction. There’s nothing wrong in dropping a layer there. That intermediate VM layer makes sense when the data being presented is not precisely what’s stored in the model. Good place to implement filters or pagination. Good place to validate user input. However, in other cases one doesn’t need any of that, in which case there’s nothing wrong with data binding directly to models. > the rendering code might depend on both state layers, accessing both application state directly and derived or view-specific data from the presentation state. In .NET with MVVM that’s easily doable, expose a property on VM that returns the model, and this will work. > In practice, it’s unusual for a single UI event to affect both In the apps I work on this happens all the time. User clicks “do something” button, the model needs to start doing that, in the meantime the view needs to disable that button and show “doing something, please wait…” with a progress bar. For this reason, I normally handle such higher-level events in VMs and call models from there. > The presentation data layer is pure computation and can be easily unit tested. I think strongly typed languages handle 90% of what people usually unit test for, with better developer’s UX. If you change something that broke 33 places of other code, a compiler will tell you precisely which 33 places you gonna need to fix. > switching out one UI rendering library for another or updating to integrate with some new-style platform API probably shouldn’t affect any of the data I think that only works when switching to something extremely similar. I did when porting iPhone code to iPad, or WPF XAML to UWP XAML, but these were very similar platforms. UI libraries are complicated; replacing them with something else is huge amount of work. > including presentation data like diagram layouts or animation logic I don’t believe these pieces are portable in practice. Both coupled with UI libraries too much. For the diagram, half of the 2D rendering libraries are immediate mode other half are retained mode. Animations are totally different as well, CSS animations in HTML don’t have much in common with visual state manager in XAML. |
In the apps I work on this happens all the time. User clicks “do something” button, the model needs to start doing that, in the meantime the view needs to disable that button and show “doing something, please wait…” with a progress bar. For this reason, I normally handle such higher-level events in VMs and call models from there.
I used too strong a word before. As you point out, situations where you’d want to update both the application and presentation data aren’t really unusual. You gave one good example. Another might be controlling a modal UI, like closing a dialog box or moving to the next step of a wizard, when maybe you also want to commit data user has entered in a temporary form to the permanent application state. In these cases, handling a UI event would indeed need to get the relevant information to both of the other layers one way or another. This still easily fits within the architecture pattern I was describing, though.
I think strongly typed languages handle 90% of what people usually unit test for, with better developer’s UX.
I am a big fan of using expressive type systems to prevent defects, particularly as an alternative to huge unit test suites full of boilerplate that still don’t do as good a job.
However, no mainstream type system will verify that for a given list in your application data and a given sorting option chosen in the UI, a function in your presentation data layer has generated the correct top 10 items in order. (No unit test can ever verify that property in general either, of course, but at least we can test some representative example cases.)
I think that only works when switching to something extremely similar. I did when porting iPhone code to iPad, or WPF XAML to UWP XAML, but these were very similar platforms. UI libraries are complicated; replacing them with something else is huge amount of work.
I suppose that depends on what you consider extremely similar. Around 10–15 years ago, when what we call “single page applications” today were starting to gain serious traction, if you wanted an interactive visual representation of your data, you probably used one of the proprietary embedded technologies like Flash, Java or ActiveX. But if you were drawing, say, an org chart, you were still ultimately just putting boxes and lines and text in certain places within the allocated area on the page. Today we have tools like SVG, canvas and WebGL available handle the drawing part, but everything we’d want to display in that drawing might still be the same.
I don’t believe these pieces are portable in practice. Both coupled with UI libraries too much. For the diagram, half of the 2D rendering libraries are immediate mode other half are retained mode. Animations are totally different as well, CSS animations in HTML don’t have much in common with visual state manager in XAML.
I think perhaps we have very different types of animation in mind. From the above, I’m guessing you’re thinking of things like pulsing a button when it’s pressed or a little progress spinner while downloading some data? I’m thinking of things like smoothly moving all of the boxes on that org chart to their new positions, including animating the shapes of the paths between them, when the user does something that moves the chart around. Another example might be animating weather data overlaid over a map graphic through the next few hours when the user presses a “play” button. That is, I’m talking about animations where the data to be shown at each step needs to be determined according to complex requirements, not just playing a simple, predetermined effect that a UI library or platform API might handle for you almost for free anyway.