Part 5. From data binding to MVVM - Xamarin (2023)

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The Model-View-ViewModel (MVVM) architectural pattern was invented with XAML in mind. The pattern enforces a separation between three layers of software: the XAML user interface, called the View, the underlying data, called the Model, and the intermediate between the view and the model, called the ViewModel. Views and ViewModels are usually connected through data bindings defined in XAML files. The View's BindingContext is usually an instance of the ViewModel.

a simple viewing model

As an introduction to ViewModels, let's first look at a program without ViewModels. Earlier you saw how to define new XML namespace declarations to allow XAML files to reference classes in other assemblies. Below is a program that does itSystemNamespaces:

xmlns:sys="clr-namespace:System;assembly=netstandaard"

The program can usex: staticGet the current date and time from staticdate Time. Nowproperty and set ittime of appointmentits valuebinding squadin astapel lay-out:

binding squadis a special feature: when you setbinding squadIn an element, it is inherited by all child elements of that element. that means all childrenstapel lay-outthey have the samebinding squad, can contain simple bindings to object properties.

insidedate/time of a downloadprogram, with two subkeys containing bindings to program propertiestime of appointmentvalue, but the other two subkeys contain bindings that seem to lack binding paths. this meanstime of appointmentThe value itself is used forstring format:

The problem is that the date and time are set once when the page is first created and never change:

An XAML file can display a clock that always shows the current time, but it takes some code to help you. Looking at MVVM, Model and ViewModel are classes written entirely in code. A view is usually an XAML file that references properties defined in the ViewModel via data binding.

A good model is ignorant of the ViewModel and a good ViewModel is ignorant of the View. However, developers often customize the data types displayed by ViewModels to match that of a particular user interface. For example, if the model accesses a database containing 8-bit ASCII strings, the ViewModel must convert those strings to Unicode strings to allow the exclusive use of Unicode in the user interface.

In simple MVVM examples like the one shown here, there is usually no model at all and the pattern contains only views and view models bound by data binding.

This is a ViewModel for a watch with only one nametime of appointment, with which thetime of appointmentProperties per second:

use system? Use System.ComponentModel; Using Xamarin.Forms? namespace XamlSamples{ class ClockViewModel: INotifyPropertyChanged { DateTime dateTime; public event PropertyChangedEventHandler PropertyChanged; public ClockViewModel() { this.Date.TimeTimeN; FromSeconds( 1 ), () => { this.DateTime = DateTime.Now; return true; }); } public DateTime DateTime { set { if (dateTime != value) { dateTime = value; if (PropertyChanged!= null) { PropertyChanged(this, new PropertyChangedEventArgs("DateTime")); } } } get { return DateTime; } } }}

ViewModels typically implementINotifyPropertyChangedinterface, meaning the class achanged ownershipAppears when one of its properties changes. The data binding mechanism in Xamarin.Forms adds a handler for thischanged ownershipevent so that it is notified when a property changes and updates the target with the new value.

A watch based on this ViewModel can be as simple as this:

notice howClock display modelDefined asbinding squadvanInscriptionUse attribute element tags. Alternatively, you can instantiateClock display modelin abroncollection and set it upbinding squada methodstatic resourcesMarkup extensions. Alternatively, the code-behind file can contain the ViewModel.

Ittie upmarker extension enabledtextits propertyInscriptionformtime of appointmentproperty. This is displayed:

Individual properties that you also have access totime of appointmentView ViewModel properties by separating them with dots:

Interactive MVVM

MVVM is often used with two-way data binding for interactive views based on the underlying data model.

This is called a programHslViewModelconversion onecolourvalue totuna,saturation, Into shinevalue and vice versa:

Use System, Use System.ComponentModel, Use Xamarin.Forms, namespace XamlSamples{ public class HslViewModel: INotifyPropertyChanged { duotone, saturation, brightness, colorColor, public event PropertyChangedEventHandler PropertyChanged; public double Hue { set { if (hue) hue = value, OnPropertyChanged("hue"); set newColor(); } } get { return hue; } } public double saturation { set { if (saturation != value) { saturation = value; OnPropertyChanged("Saturation"); set newColor(); } } get { return Saturation; } } public double brightness { set { if (brightness != value) { brightness = value; OnPropertyChanged("Brightness"); SetNewColor(); } } get { return Brightness; } } public Color Color { set { if (color != value) { color = value; OnPropertyChanged("color"); hue = value.hue; saturation = value.saturation; } } get { returncolor; } } void SetNewColor() { Color = Color.FromHsla(Hue, Saturation, Luminosity); } protected virtual void OnPropertyChanged(string propertyName) { PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)); } }}

The changetuna,saturation, Into shinelead propertiescolourThe property to change and changecolourIt causes changes in the other three properties. This looks like an infinite loop, the class just doesn't callchanged ownershipevent unless the property has changed. This ends an otherwise uncontrolled feedback loop.

The following XAML file contains aview boxWHEREcolourproperties pledgedcolourproperties of the ViewModel and threesliderand threeInscriptionThe view is bound totuna,saturation, Into shinefeature:

every bindingInscriptionis the default settingA way.It just need to display the value. But the limitations for eachsliderAndtwo-way street.this allowssliderInitialized by ViewModel. Keep in mind that,colourproperty is set towater racewhen the ViewModel is installed. just a changesliderYou must also set new values ​​for properties in the ViewModel and then calculate new colors.

Use the ViewModel command

In many cases, the MVVM pattern is limited to operations on data elements: UI objects in the View and parallel data objects in the ViewModel.

However, sometimes the view needs to contain buttons that trigger different actions on the ViewModel. But ViewModel cannot containClickThe handler for the button as it binds the ViewModel to a specific UI instance.

To make the ViewModel more independent of specific UI objects, but allow methods to be called on the ViewModel,Seriethere is an interface. The following components in Xamarin.Forms support this command interface:

• knob
• Menu item
• toolbar item
• search bar
• the text(and so tooImage Cell)
• display list
• Tap Gesture ID

Exceptsearch barIndisplay listelements, which define two characteristics:

• SerietypeSystem.Windows.Input.ICCommand
• command parameterstypeGoal

Itsearch bardefinitionsearch commandInFind command parametersfunction whiledisplay listdefines oneextension ordertype attributeSerie.

ItSerieThe interface defines two methods and one event:

• execute invalid (object parameter)
• boolean CanExecute (objectparameter)
• Use EventHandler CanExecuteChanged

ViewModel can set type propertiesSerie.You can then link these propertiesSerieeveryone's propertyknobor other components, or possibly a custom view that implements this interface. You can choose to setcommand parametersDetermine someone's propertyknobThe object (or other element) to bind to this ViewModel property. Internal,knobphone callimplement, executeEvery time a user clicksknob, I pass it on to youimplement, executehis methodcommand parameters.

Itcan be performedMethods andChanges can be madeEvents are used in the following casesknobThe tap may not be valid at this time, soknobIt must be turned off. ThisknobTo callcan be performednever. . . neverSerieproperty is set first, wheneverChanges can be madeevent is fired. ifcan be performedYIELDInaccurate, Thisknobdisabled and unproducedimplement, executeTelephone.

To help you add commands to your ViewModel, Xamarin.Forms defines two classes that implementSerie:SerieInSerieWhereTunais the type of the parameterimplement, executeIncan be performed.These two classes define different constructors and aactionable changeA method that the ViewModel can call for enforcementSerieagainst fireChanges can be madeEvent.

Below is the ViewModel for a simple keyboard to enter a phone number. Keep in mind that,implement, executeIncan be performedMethods are defined as lambda functions in the constructor:

Use System, Use System.ComponentModel, Use System.Windows.Input, Use Xamarin.Forms, Namespace XamlSamples { class KeypadViewModel : INotifyPropertyChanged { string inputString = ""; *', '#' }; public event PropertyChangedEventHandler;//Changed public KeypadViewModel() { AddCharCommand = new command((key) => { // Add a key to the input string. InputString += key; }); DeleteCharCommand = new Command(() => { // Delete a character from the input string. InputString = InputString. Substring(0, InputString.Length - 1); }, () => { // Return true if deletion is needed Return InputString.Length > 0;}); } // public property public string InputString { protected set { if (inputString != value) { inputString = value; OnPropertyChanged("InputString"); DisplayText = FormatText(InputString); // Maybe the delete button should be enabled/disabled. ((command)DeleteCharCommand).ChangeCanExecute(); } } get { return string input; } } public string DisplayText { protected set { if (displayText != value) { displayText = value; OnPropertyChanged("Display Text"); } } get { return displayText; } } // public implementation ICommand ICommand AddCharCommand { secure set; to get? } public ICommand DeleteCharCommand { secure set; to get? } string FormatText(string str) { bool hasNonNumbers = str.IndexOfAny(specialChars!=1) ; String format = str; if (hasNonNumbers || str.Length < 4 || str.Length > 10) { } else if (str.Length < 8) { formatted = String.Format("{0}- {1} ", str.Substring( 0, 3), str.Substring(3)); } else { formatted = String.Format("({0}) {1}-{2}", str.Substring(0, 3 ), str.Substring(3, 3), str.Substring(6)); } return format; } protected void OnPropertyChanged(string propertyName) { PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)) ; } }}

This ViewModel assumesAdd drawing commandproperties pledgedSerieproperties of various buttons (or anything else with a command interface), each represented bycommand parameters.These buttons add charactersinput stringattribute, which is then formatted as a phone numberdisplay textproperty.

There is also a second type of houseSerieWas calledcommand to delete characters.This is tied to a Return button, but should disable the button if there are no characters to delete.

The keyboard below is not that visually appealing. Instead, the formatting has been reduced to a minimum to more clearly demonstrate the use of the command interface:

ItSeriefirst propertyknobThe display of this format is limited bycommand to delete characters, the rest is mandatoryAdd drawing commandIncommand parameterswith radiance in itknobSight. Here's the program in action:

asynchronous call method

Commands can also call asynchronous methods. This is done by usingasynchroonInTo waitWhen specifying keywordsimplement, executemethod:

DownloadCommand=newCommand(async() => wait for DownloadAsync());

This showsdownload asynchronouslymethod is oneWorkhave to wait:

asynchronous task DownloadAsync(){ waiting for Task.Run(() => Download()); } invalid download(){ ...}

Implement the navigation menu

ItExample XMLThe program that contains all the source code for this article series uses a ViewModel as its home page. This ViewModel is the definition of a short class with three namestype,title, Into describeIt contains the type, title, and short description of each sample page. In addition, the ViewModel defines a nameatHere is a collection of all program pages:

public class PageDataViewModel{ public PageDataViewModel(Type type, String title, String description) { Type = type; title = title; description = description; } public TypeType { private set; to get? } public string Title { private set; to get ; } public string description { private set; to get? } static PageDataViewModel() { All = new list1 Interaction with slider and button"), // Part 2. Basic XAML syntax new PageDataViewModel(typeof(GridDemoPage), "Grid Demo", "Explore XAML syntax with Grid"), new PageDataViewModel(typeof(AbsoluteDemoPage), "Absolute Demo", "Explore the XAML syntax with Absolute"), //Section 3 Part XAML Markup Extensions new PageDataViewModel(typeof(SharedResourcesPage), "Shared Resources", "Using resource dictionaries shared resources"), new PageDataViewModel(typeof(StaticConstantsPage) ), " Static Constants", "Using x:Static Markup"), "Using x:Static Markup"), new PageDataViewModel(typeof(RelativeLayoutPage), "RelativeLayout", "Explore XAML Markup Extensions") , // Section 4. Data binding basics new PageDataViewModel(typeof(SliderBindingsPage), "Slider Bindings", "Bind properties of two views onto the page"), new PageDataViewModel(typeof(SliderTransformsPage), "Slider Transforms", "Use Slider with reverse bindings" ), PageDataViewModel (typeof( ListViewDemoPage), "ListView Demo", "Use a ListView with data bindings"), // Part 5. From data binding to MVVM new PageDataViewModel(typeof(OneShotDateTimePage), "One-Shot DateTime", "Get the current DateTime and display it "), new PageDataViewModel(typeof(ClockPage), "Clock", "Dynamic display the current time" ), new PageDataViewModel(typeof(HslColorScrollPage), "HSL Color Scroll", "Use a display model to display an HSL color" ), new PageDataViewModel(typeof(KeypadPage), "Keypad", "Use a display model for numeric keypad logic") }; } public static list ILAll { private set; to download? }}

xaml files forAt homedefines onelist boxWHEREelement bronproperty is set toatreal estate, exclthe textto displaytitleInto describeProperties of each page:

These pages appear in a scrollable list:

When the user selects an item, the handler in the code-behind file is triggered. Operator settingsselected itemits propertylist boxback toinvalidThen create the selected page and go to it:

private asynchrone leegte OnListViewItemSelected(object afzender, SelectedItemChangedEventArgs args){ (afzender als ListView).SelectedItem = null; if (args.SelectedItem != null) { PageDataViewModel pageData = args.SelectedItem as PageDataViewModel; page page = (Page)Activator.CreateInstance(pageData.Type); 等面 Navigation.PushAsyncＢ}

video

Xamarin Evolve 2016: MVVM Made Easy με Xamarin.Forms και Prism

sum up

XAML is a powerful tool for defining user interfaces in Xamarin.Forms applications, especially when using data binding and MVVM. The result is a clean, elegant and potentially organized view of the user interface, with all backend support in code.

• Example XML
• Part 1 Getting started with XAML
• deel 2 Basis XAML-syntax
• Part 3 XAML Markup Extensions
• Part 4. Basics of data connection

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