EventToCommand binding with parameters in portable view models - xaml

I am implementing a view model that is shared by applications on multiple platforms. I am using MvvmCross v3 that has its own MvxEventToCommand class, but I believe the challenge is the same for other frameworks like MVVM Light. As long as the event is used without parameters, the implementation is straightworward, and this is the case for simple interactions like tapping the control.
But when the command needs to handle event arguments things become more complicated. For example, the view model needs to act on certain scroll bar changes (and load more items in the associated list view). Here is the example of XAML:
<cmd:EventToCommand
Command="{Binding ScrollChanged}"
CommandParameter="{Binding EventArgs}" />
(MvvmCross uses MvxEventToCommand, but the principle is the same).
Then in my model I can have the following command handler:
public ICommand ScrollChanged
{
get
{
return new RelayCommand<ScrollChangedEventArgs>(e =>
{
MessageBox.Show("Change!");
});
}
}
(MvxCommand in MvvmCross).
The problem is that ScrollChangedEventArgs is platform specific and this code simply will not compile in a portable class library. This is a general problem with any command that needs not only a push when an event was fired but requires more specific event details. Moving this code in platform-specific part is silly because it more or less kills the concept of portable view models and code-behind-free views. I tried to search for projects that share view models between different platforms, but they all use simple events like "Tap" with no attached event details.
UPDATE 1 I agree with Stuart's remark that view models should only deal with higher level abstractions, so I will rephrase the original concern: how to map results of low-level interactions to a platform-neutral event that triggers a business logic command? Consider the example above: the business logic command is "load more items in a list", i.e. we deal with a list virtualization where a limited number of items from a large collection are loaded initially, and scrolling down to a bottom of a list should cause additional items to be loaded.
WinRT can take care of list virtualization by using observable collections that support ISupportIncrementalLoading interface. The runtime detects this capability and automatically requests extra items from a respective service when the user scrolls down the list. On other platforms this feature should be implemented manually and I can't find any other way than reacting on ScrollViewer ScrollChanged event. I can see then two further options:
Place OnScrollChanged handler in a code-behind file and call the portable view model higher level event (such as "OnItemsRequested");
Avoid code-behind stuff and struggle to wire the ScrollChanged event directly to a view model, then we will need to remap the platform-specific event first.
As long as there is no support for second option, putting event handler in code-behind file is OK as long as it is done for the sole purpose of event mapping. But I would like to investigate what can be done using the second option. MvvmCross has MapCommandParameter class which seems to be able to help, so I wonder if I should exploit that one.
UPDATE 2 I tried MapCommandParameter approach, and it worked allowing me to insert a platform-specific adapter that would map low-level events to view model-specific commands. So the second option worked without any struggle. Stuart also suggested listview-subclassing so there is no need to care about scrolling events. I plan to play with it later.

I agree that viewmodel commands should normally be expressed in terms of viewmodel concepts - so it would be 'strange' to send the viewmodel a command about the scrollbar value changing, but it might be ok to send the viewmodel a command about the user selecting certain list elements to be visible (which she does via scrolling)
One example where I've done this type of thing previously is in list selection.
I originally did this across multiple platforms using a cross-platform eventargs object -
https://github.com/slodge/MvvmCross/blob/vnext/Cirrious/Cirrious.MvvmCross/Commands/MvxSimpleSelectionChangedEventArgs.cs
this was then used on WindowsPhone (for example) via an EventToCommand class like https://github.com/slodge/MvvmCross/blob/vnext/Cirrious/Cirrious.MvvmCross.WindowsPhone/Commands/MvxSelectionChangedEventToCommand.cs
However... I have to admit that this code hasn't been used much... For list selection we have instead mainly used selecteditem binding, and there simply haven't been any apps that have needed more complex parameterized commands (so far) - you might even need to go back to very old v1 mvvmcross code to find any samples that use it.

Related

How can I refresh my composable when new data is being inserted?

My team and I have to make a license plate scanning app as a school project. With that, we have comments and pictures which can be added to cargo. Whenever the user scans a plate they also get the chance to change the checked info in case of a mistake. The problem is that whenever we delete data from the scanned plate it doesn't show on the screen that it has been deleted until we go to another screen. The same goes for the lazy column which we use for inserting new instances of comments and pictures. The data doesn't show on screen until we turn our screen or go back to another screen. private val pictureList = mutableListOf() is what we use for the pictures and for the text we use var countryCode by remember { mutableStateOf(CountryCodeText) }
var licenseNumber by remember { mutableStateOf(LicenseNumberText) }. pictureList is a global variable and the other ones are local variables which use global variables in the mutableStateOf. How can we make sure that the UI updates whenever the data changes? In advance I want to say thanks for the help! (Code is written in Kotlin and jetpack compose)
Just replace the mutableListOf() with a mutableStateListOf(...), and prefer to keep all the state logic confined to a ViewModel. The viewmodel should preferably be only one for the entire app, and the entire app should have only one activity.
The viewmodel should act as a single source of truth for the entire activity's UI state, while also handling all the updates to the UI efficiently.
The #Composables should only be incharge of displaying the state, and sending events to the viewmodel to update the state, for example, an onClick event may be sent up to the viewmodel by a button too trigger a state change in another part of the app.
Take this codelab to learn all about state in Compose (Well, not all, really, but good starter).
Also, changing screens destroys all the #Composables of the current screen, and so when you ce back there, all the #Composables are re-created, and the correct data is fetched. If you wish to trigger "recompositions" upon changing a variable, you must ensure that the concerned variable is a state-holder, i.e., initialized with one of the pre-built state initializers, like mutableStateOf(), or mutableStateMapOf, etc.
We usually have a mutableState*Of format for determining whether a pre-built state initializer is available. The most common ones are covered, obviously, but if not, you'll need to create a new type of initializer yourself, and if that is not something you know how to do, currently, you can just go about checking whether the type of data you wish to store is Immutable. If so, you can just wrap it in a mutableStateOf<DataType>() call, and it will trigger the recompositions. Know that this is not as efficient as pre-built initializers, but definitely gets the job done.
Also, I suggest you take the compose-pathway to get the basics down. It covers everything ranging from creating a simple UI using basic pre-built Layouts to creating a complex animation-driven application using custom Layouts.
So, definitely a lot to take it, but I hope you get there.
Happy composing,

How to fake keyboard event to widget in Qt5

I have made a custom virtual keyboard widget for my kiosk application, and now comes the time when I want it to produce fake keyboard events and feed them to an QLineEdit of choice.
I do the following:
// target is the QWidget to receive the events
// k is the Qt::Key (keycode) I want to send (Testing with an 'A')
Qt::Key k=Qt::Key_A;
if(0!=target){
//According to docs this will be freed once posted
QKeyEvent * press=new QKeyEvent(QKeyEvent::KeyPress, (int )k,0);
QKeyEvent * release=new QKeyEvent(QKeyEvent::KeyRelease, (int )k,0);
//Give the target focus just to be sure it is available for input
target->setFocus();
//Post the events (queue up and let the target consume them when the eventloop gets around to the target)
QCoreApplication::postEvent ( target, press) ;
QCoreApplication::postEvent ( target, release) ;
}
I see the target widget receive focus, but there are no letters typed into the input field like I would expect. What am I doing wrong? Which assumptions are wrong?
PS: I know that this could be solved by using existing virtual keyboards or at least using the platform interface as is done in this post. In our approach we have decided to build the kayboard into the application to obtain full control over the UX and keyboard design.
Thanks!
Since no-one stepped up, I will try to provide some closure.
It turns out that Qt5 comes with a library of testing facilities called testlib. It has all sorts of goodies to facilitate easy creation, management and running of unit tests for Qt application. Among these facilities there is a set of functions for sending fake events such as fake typing of text, mouse clicks etc. It is quite comprehensive and covers many use-cases. Since this is used internally by Qt developers to test Qt itself it is also production proven code.
I simply copied what I needed from there.

titanium how do i add a view to the single context

So i have a titanium app, and i just read about single contexts. (Incidentally, somebody here should write a book about programming in titanium... the only one out there doesn't really mention single contexts or any of that new-fangled stuff. Heck, make it an eBook. I'd buy it)
The titanium documentation stresses their use (http://docs.appcelerator.com/titanium/latest/#!/guide/Coding_Strategies-section-29004891_CodingStrategies-Executioncontexts) and then politely forgets how to implement a single context!!
So, question:
Let's say i have the awesomeWidget page - this just shows a button, and when you click on a button a new screen appears.
The aswesomeWidget page is accessed through another page - it is not from the root of the titanium app.
Keeping to single contexts, how do i add the view that the button creates to the current window?
Do I:
keep a global pointer to the current (and only) window?
pass the variable holding the current window down to all the following pages that use it
something else?
First off, Titanium keeps a reference to your current window anyway for you, so this use case is easy. For example:
awesomeWidgetButton.addEventListener('click' function(e) {
var yourView = Ti.UI.createView({...});
Titanium.UI.currentWindow.add(yourView);
});
If you want to dig further, the concept of a single context is closely tied to the use of CommonJS modules and the require keyword. It is very simple to keep a single context, just never open a window with the url component filled out, and liberally use the require() keyword. Other than that, its up to your imagination to keep track of who points to what and vice versa, there are standard patterns and best practices that apply here (MVC, Singletons, just keep it simple) just as in coding in any other language.

Page Navigation in Windows 8 XAML (without using code behind)

For my windows 8 application i am trying to navigate between pages with out using code behind.
For example, i have one image in my UI without creating tapped event for that image i need to navigate to another page,
<Image Source="ms-appx:///Assets/Logo.png" Width="155" Height="110" Tapped="{ // Navigation method here }"/>
Is it possible to navigate between pages like this...? If possible, how can i get this to work??
XAML is just a declarative language without action part so code behind is an essential part of it.
All interactions work via events and event can be handled in a code behind only. So what you want is not possible with XAML(at least with WinRT XAML).
If you are asking if you can specify the code inside the .xaml file, then no, that is not possible.
If you are asking if you can avoid adding code to the .xaml.cs file, then yes, that is possible. You will still need to specify a method but it can even be done as a simple lambda. You will need to use the Command hooks rather than the Event Hooks, e.g.
<Button Command="{Binding GoConnectionCommand}" ... />
The code for this command is usually defined in the ViewModel as part of the MVVM pattern, and Josh Smith explains it far better than I will.
AlSki mentioned using a ViewModel. Although technically the ViewModel is not part of the "code behind" for the XAML file, it's still code and I believe you were asking for a no code solution.
ixSci is correct that there is no way to do this out of the box without code behind in WinRT XAML.
In full WPF it's possible to do this using a behavior called NavigateToScreenAction. You can read about it here. Unfortunately behaviors don't ship out of the box with WinRT, but they can be added back in by an open source project called WinRtBehaviors.
There is no NavigateToScreenAction behavior for WinRT, but one could be created. There is a good article on creating behaviors with the library here. It will obviously require code to create the behavior, but after it's created you could use it in XAML without any code.
Really, the short answer is it's not possible to navigate without code on WinRT.
Dev support, design support and more awesome goodness on the way: http://bit.ly/winappsupport

Methods best practice - VB.NET

I have a program that is getting pretty big and it is a pain to find everything through all the functions and classes.
I am trying to break it up into other files based on their method.
Some of these functions have calls to others in the main class. I changed most my functions from private to public to access this. I had problems calling certain code created windows so importing mainwindow helped that.
My last problem is editing the mainwindow ui from one of the module files. I want to make sure im on the right page before i continue breaking it up. My only guess is that anything they updates the ui should be left on the main class.
Thanks
The only code in your form class should be code that talks to other classes and updates the UI based on data from other classes.
Depending on your application, the form class might handle change events from other classes to update the UI or pass user input to other classes in Change or Click events.
A couple options:
Use callbacks into the your main window.
Create events for when you need the form updated. Your program logic raises the events, and your main window class can consume them.