var binding = new EBinding
{
    () => view.Text == vm.Text,
    () => view.Text == vm.Description.Title.Text,
    () => view.Text == (vm.Text ?? vm.FallbackText),
    () => view.Visible == !vm.TextVisible,
    () => view.Visible == (vm.TextVisible == vm.ImageVisible),
    () => view.Visible == (vm.TextVisible && vm.ImageVisible),
    () => view.Visible == (vm.TextVisible || vm.ImageVisible),
    () => view.FullName == $"{vm.FirstName} {vm.LastName}",
    () => view.FullName == vm.FirstName + " " + vm.LastName,
    () => view.Timestamp == Converter.DateTimeToEpoch(vm.DateTime),

    BindFlag.TwoWay,
    () => view.Text == vm.Text,
    () => view.SliderValueFloat == vm.AgeInt,

    BindFlag.OneTime | BindFlag.NoInitialTrigger,
    () => view.ShowImage(vm.ImageUri),
    () => Dispatcher.RunOnUiThread(() => view.ShowImage(vm.ImageUri)),

    BindFlag.OneTime,
    (view, nameof(view.Click), vm.OnViewClicked),
    (view, nameof(view.TextEditedEventHandler), () => vm.OnViewTextEdited(view.Text)),
    (view, "CustomEventForGesture", vm.OnViewClicked),

    (view, nameof(view.Click), vm.ViewClickCommand),
    (view, nameof(view.Click), () => vm.ViewClickCommand.TryExecute(view.Text)),

    new UserExtensions.CustomEBinding(),
};

binding.Dispose();

^{snippet source | anchor}

Three types of bindings here

• EQUALITY – bind a property or an expression of them to another property.
  Every change of every property on the right (vm.Text) leads to an assignment of the property on the left (view.Text).
  Binding between two properties may work in a Two-Way mode.

  () => view.Text == vm.Text,

• ACTION – bind a method to its target and parameters.
  Every time vm.ImageUri is changed view.ShowImage is invoked with a new value.

  () => view.ShowImage(vm.ImageUri),

• EVENT – bind a method or command to an event.
  Every time view.Click is raised vm.OnViewClicked is called.

  (view, nameof(View.Click), vm.OnViewClicked),

Key points

• The main binding update trigger is INotifyPropertyChanged.PropertyChanged.
  Boilerplate code may be avoided with Fody.PropertyChanged.
  There are some platform-specific pre-configured triggers. Additional ones are easy to configure.

• Bindings invoke immediately after the construction, except for the event binding.
  To skip initial invocation prepend BindFlag.NoInitialTrigger.

• Each property and field in a binding expression triggers the binding update
  

• Default binding mode is One-Way, Source-to-Target (right to left).
  This is the most common use-case. Explicit declaration of a two-way mode will help to avoid unreasonable performance loss.
  May be overridden in EBinding.DefaultConfiguration.DefaultFlag.

• For Target-to-Source binding simply swap operands.
  It seems more natural than bringing in an additional BindFlag.OneWayToSource.

• Expressions of any complexity are supported.
  Even MONSTERS like that:

  () => view.Text == new Func<string?>(() => new Dictionary<string, string?>((int)10.0){ { "Key", vm.Text } }["Key"])(),

  However, the more complex the expression, the longer it takes to create and invoke the binding from it. See benchmarks for the reference.

• BindFlag applies to all subsequent bindings till the next flag.

Configuration and Extensibility

Pre-Configured Triggers

Xamarin.Android

View/Control	Event	Property
Android.Views.View	Click
	LongClick
	FocusChange
Android.Widget.AdapterView	ItemSelected	SelectedItemPosition
	ItemClick
	ItemLongClick
Android.Widget.CalendarView	DateChange	Date
Android.Widget.CompoundButton	CheckedChange	Checked
Android.Widget.DatePicker	DateChanged	DateTime
Android.Widget.NumberPicker	ValueChanged	Value
Android.Widget.RatingBar	RatingBarChange	Rating
Android.Widget.SearchView	QueryTextChange	Query
Android.Widget.SeekBar	ProgressChanged	Progress
Android.Widget.TextView	TextChanged	Text
Android.Widget.TimePicker	TimeChanged	Hour (API 23+) Minute (API 23+) CurrentHour CurrentMinute

Xamarin.iOS

View/Control	Event	Property
UIBarButtonItem	Clicked
UIControl	TouchUpInside
	ValueChanged
UIDatePicker	ValueChanged	Date
UIPageControl	ValueChanged	CurrentPage
UISearchBar	TextChanged	Text
UISegmentedControl	ValueChanged	SelectedSegment
UISlider	ValueChanged	Value
UIStepper	ValueChanged	Value
UISwitch	ValueChanged	On
UITabBarController	ViewControllerSelected	SelectedIndex
UITextField	EditingChanged	Text
	EditingDidBegin
	EditingDidEnd
UITextView	Changed	Text

Xamarin.Forms

All views implement INotifyPropertyChanged so the main trigger is invoked for every bindable property change.

Member Triggers

In Configuration you may specify how to subscribe and unsubscribe for signals of property and field updates that are not tracked out of the box.
There are overloads for the property/field update handler as System.EventHandler, System.EventHandler<TEventArgs> or any other class:

EBinding.DefaultConfiguration.ConfigureTrigger<View, string>(
    v => v.Text,
    (v, h) => v.TextEditedEventHandler += h,
    (v, h) => v.TextEditedEventHandler -= h);

EBinding.DefaultConfiguration.ConfigureTrigger<View, View.TextEditedEventArgs, string>(
    v => v.Text,
    (v, h) => v.TextEditedGenericEventHandler += h,
    (v, h) => v.TextEditedGenericEventHandler -= h);

EBinding.DefaultConfiguration.ConfigureTrigger<View, Action<string>, string>(
    v => v.Text,
    trigger => _ => trigger(),
    (v, h) => v.TextEditedCustomEventHandler += h,
    (v, h) => v.TextEditedCustomEventHandler -= h);

^{snippet source | anchor}

Event Triggers

You may configure your own triggers for event bindings under custom identifiers even if they represent a Pub-Sub pattern differently from C# events (IObservable, Add/RemoveListener methods).

EBinding.DefaultConfiguration.ConfigureTrigger<View, View.GestureRecognizer>(
    "CustomEventForGesture",
    trigger => new View.GestureRecognizer(trigger),
    (v, h) => v.AddGestureRecognizer(h),
    (v, h) => v.RemoveGestureRecognizer(h));

^{snippet source | anchor}

The same identifier (event name) can be used with multiple classes.
For example on Xamarin.iOS a pre-defined Tap event can be used with both UIControl and UIView:

using EBind;
using static EBind.Platform.Configuration.ExtraEventNames;

var binding = new EBinding
{
    (uiButton, Tap, OnButtonClick),
    (uiImageView, Tap, OnImageClick),
};

Configuration of C# events is not required – they can be found by the name: nameof(obj.Event).
But it's recommended to specify subscription and unsubscription delegates to improve cold-start performance and avoid linker errors.

EBinding.DefaultConfiguration.ConfigureTrigger<View>(
    nameof(View.TextEditedEventHandler),
    (v, h) => v.TextEditedEventHandler += h,
    (v, h) => v.TextEditedEventHandler -= h);

EBinding.DefaultConfiguration.ConfigureTrigger<View, View.TextEditedEventArgs>(
    nameof(View.TextEditedGenericEventHandler),
    (v, h) => v.TextEditedGenericEventHandler += h,
    (v, h) => v.TextEditedGenericEventHandler -= h);

EBinding.DefaultConfiguration.ConfigureTrigger<View, Action<string>>(
    nameof(View.TextEditedCustomEventHandler),
    trigger => _ => trigger(),
    (v, h) => v.TextEditedCustomEventHandler += h,
    (v, h) => v.TextEditedCustomEventHandler -= h);

^{snippet source | anchor}

Event triggers configured for a class are available to all its children unless they have their own variation defined.

Triggers may be overwritten by onward setups including the pre-defined ones.

Binding Dispatcher

Sometimes ViewModel properties are set from the background thread that leads to data-binding view update triggering in the non-ui thread.

Establishing a proper thread switching in place is not possible for some code. Also, the risk of missing a thread is not acceptable or not worth caring about for some projects, and delegation of this problem is a good deal.
In this case, data-binding as a point of integration may be a good place to switch threads between the ui-threaded View and the thread-insensitive ViewModel layers.

Dispatchers which force the UI thread are set up in Configuration:

EBinding.DefaultConfiguration.AssignmentDispatchDelegate = Dispatcher.RunOnUiThread;
EBinding.DefaultConfiguration.ActionDispatchDelegate = Dispatcher.RunOnUiThread;

For Xamarin platform Xamarin.Essentials.MainThread.BeginInvokeOnMainThread will be the best option most of the time.

Custom Bindings

EBinding collection initializer accepts any form of IEBinding.
By implementing this simple interface you can create your own binding types and adapters for data-binding components of other systems (e.g. Rx.Net), use them in the same collection initializer, and keep all bindings in one place.

Custom binding creation can be encapsulated inside an extension method Add(this EBinding, ...) so that the binding inputs are accepted in the collection initializer (supported since C# 6).

After a custom binding is created, it must be added to the collection via EBinding.Add(IEBinding) so that it can be disposed along with other bindings.

public class CustomEBinding : IEBinding
{
    public void Dispose()
    {
    }
}

public static void Add(this EBinding bindingHolder, CustomEBinding binding,
    [System.Runtime.CompilerServices.CallerLineNumber] int sourceLineNumber = 0)
{
    try
    {
        if (binding == null)
            throw new ArgumentNullException(nameof(binding));

        if (bindingHolder.CurrentFlag == BindFlag.OneTime)
            throw new NotSupportedException();

        bindingHolder.Add(binding);
    }
    catch (Exception ex)
    {
        throw new Exception(
            $"Error in entry {bindingHolder.Count} at line #{sourceLineNumber}", ex);
    }
}

^{snippet source | anchor}

It's recommended to decorate exceptions during binding creation with additional information about the binding position (EBinding.Count) and its line number (CallerLineNumberAttribute) as debuggers do not highlight that.

Exception screenshot

Benchmarks

Environment

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19041.572 (2004/?/20H1)
AMD Ryzen 5 1600, 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=5.0.202
  [Host]     : .NET Core 5.0.5 (CoreCLR 5.0.521.16609, CoreFX 5.0.521.16609), X64 RyuJIT
  DefaultJob : .NET Core 5.0.5 (CoreCLR 5.0.521.16609, CoreFX 5.0.521.16609), X64 RyuJIT

Comparison: Trigger ^🔗

Method	Mean	Error	StdDev	Ratio	Gen 0	Allocated
EBind	68.71 ns	0.422 ns	0.395 ns	1.00	0.0305	128 B
Mugen	206.29 ns	2.305 ns	2.156 ns	3.00	0.0172	72 B
XamarinFormsCompiled	351.32 ns	1.516 ns	1.418 ns	5.11	0.0267	112 B
MvvmLight	1,070.28 ns	3.529 ns	3.128 ns	15.58	0.1259	528 B
MvvmCross	1,368.35 ns	8.320 ns	7.376 ns	19.92	0.1678	704 B
ReactiveUI	3,054.42 ns	30.750 ns	28.763 ns	44.45	0.1831	777 B
PraeclarumBind	150,506.37 ns	372.197 ns	329.943 ns	2,190.78	0.9766	4415 B

^sources

Comparison: Creation, One-Way ^🔗

Method	Mean	Error	StdDev	Ratio	Gen 0	Allocated
XamarinFormsCompiled	2.955 us	0.0117 us	0.0110 us	0.74	0.1831	768 B
EBind	3.994 us	0.0240 us	0.0213 us	1.00	0.5264	2232 B
MvvmLight	7.118 us	0.0506 us	0.0474 us	1.78	0.5951	2504 B
Mugen	8.075 us	0.1554 us	0.1790 us	2.01	0.4349	2014 B
MvvmCross	9.263 us	0.0583 us	0.0546 us	2.32	0.9155	3873 B
ReactiveUI	49.217 us	0.8883 us	0.8309 us	12.34	3.5400	14953 B
PraeclarumBind	300.919 us	0.6047 us	0.5657 us	75.32	2.4414	10634 B

^sources

Comparison: Creation, Two-Way ^🔗

Method	Mean	Error	StdDev	Ratio	Gen 0	Allocated
XamarinFormsCompiled	3.058 us	0.0167 us	0.0156 us	0.62	0.1831	768 B
EBind	4.961 us	0.0309 us	0.0289 us	1.00	0.7553	3184 B
Mugen	7.669 us	0.0444 us	0.0370 us	1.55	0.4883	2064 B
MvvmLight	8.812 us	0.0465 us	0.0435 us	1.78	0.7782	3288 B
MvvmCross	13.041 us	0.0972 us	0.0909 us	2.63	1.0529	4449 B
ReactiveUI	80.447 us	0.4630 us	0.4331 us	16.22	6.4697	27237 B
PraeclarumBind	572.390 us	3.0060 us	2.8118 us	115.39	3.9063	19551 B

^sources

Comparison: Cold Start ^🔗

IterationCount=1 LaunchCount=100 RunStrategy=ColdStart

Type	Method	Mean	Error	StdDev	Ratio	Allocated
Creation	EBind	13,948.6 us	411.78 us	1,214.14 us	1.00	3504 B
Creation	MvvmLight	14,719.1 us	365.87 us	1,078.77 us	1.06	3216 B
Creation	XamarinFormsCompiled	18,116.8 us	93.64 us	276.11 us	1.30	848 B
Creation	MvvmCross	19,323.5 us	46.63 us	137.50 us	1.39	6144 B
Creation	PraeclarumBind	21,130.5 us	70.75 us	208.60 us	1.52	12312 B
Creation	Mugen	74,633.6 us	117.36 us	346.03 us	5.37	7472 B
Creation	ReactiveUI	151,959.5 us	181.95 us	536.48 us	10.94	16304 B
Trigger	EBind	705.5 us	3.10 us	9.15 us	0.05	224 B
Trigger	MvvmCross	1,144.8 us	9.13 us	26.91 us	0.08	704 B
Trigger	XamarinFormsCompiled	1,307.2 us	6.60 us	19.46 us	0.09	152 B
Trigger	MvvmLight	2,010.1 us	13.96 us	41.15 us	0.14	608 B
Trigger	ReactiveUI	3,354.7 us	9.44 us	27.84 us	0.24	792 B
Trigger	PraeclarumBind	4,118.7 us	15.92 us	46.93 us	0.30	5000 B
Trigger	Mugen	4,326.1 us	18.50 us	54.54 us	0.31	152 B

EBind Creation ^🔗

Method	Mean	Error	StdDev	Gen 0	Allocated
(a, "nameof(a.Event)", Method)	337.5 ns	2.31 ns	1.93 ns	0.0858	360 B
a.Method()	3,256.9 ns	20.82 ns	18.46 ns	0.4349	1824 B
a.Prop == b.Prop // INPC	4,089.8 ns	54.06 ns	50.57 ns	0.5264	2232 B
a.Method(b.Prop.Method())	4,406.1 ns	20.41 ns	17.05 ns	0.5493	2312 B
a.Prop == !b.Prop	4,611.5 ns	46.32 ns	43.33 ns	0.5646	2392 B
a.Prop == b.Prop // EventHandler	4,658.2 ns	46.37 ns	43.37 ns	0.5112	2152 B
a.Float == b.Int	4,785.8 ns	45.54 ns	38.03 ns	0.5798	2448 B
a.Enum == b.Enum	5,265.1 ns	22.89 ns	21.42 ns	0.5493	2328 B
a.Prop == Static.Method(b.Prop)	6,325.2 ns	67.12 ns	62.78 ns	0.6790	2864 B
a.Prop == (b.Prop && c.Prop)	6,423.7 ns	21.64 ns	18.07 ns	0.8087	3408 B
a.Prop == (b.Prop == c.Prop)	6,535.5 ns	49.44 ns	46.25 ns	0.8163	3432 B
a.Prop == (b.Prop || c.Prop)	6,653.2 ns	54.95 ns	51.40 ns	0.8163	3432 B
a.Prop == b.Method(c.Prop)	6,823.8 ns	60.00 ns	56.13 ns	0.7553	3184 B
a.Prop == (b.Prop ?? c.Prop)	7,271.6 ns	53.72 ns	50.25 ns	0.8392	3536 B
a.Prop == b.Method(c.Prop, d.Prop)	7,357.3 ns	50.22 ns	46.98 ns	0.8011	3360 B
a.Prop == b.Method(c.Prop, d.Prop, e.Prop)	7,552.6 ns	40.44 ns	37.83 ns	0.8392	3536 B
a.Prop == b.Prop + c.Prop	7,743.0 ns	77.14 ns	64.41 ns	0.8850	3736 B
a.Prop == $"{b.Prop}_{c.Prop}"	9,898.2 ns	20.60 ns	16.08 ns	0.9918	4176 B
a.Prop == (b.Prop + c.Prop).Method()	12,735.2 ns	71.11 ns	63.04 ns	0.9155	3880 B
Static.Method(() => Method(a.Prop))	16,007.6 ns	131.27 ns	116.37 ns	1.2512	5344 B

^sources

EBind Trigger ^🔗

Method	Mean	Error	StdDev	Gen 0	Allocated
a.Prop == !b.Prop	69.32 ns	0.879 ns	0.822 ns	0.0305	128 B
a.Prop == b.Prop // INPC	72.64 ns	0.810 ns	0.758 ns	0.0305	128 B
a.Method()	79.61 ns	0.653 ns	0.611 ns	0.0248	104 B
a.Enum == b.Enum	80.15 ns	1.008 ns	0.943 ns	0.0362	152 B
a.Prop == b.Prop // EventHandler	82.12 ns	0.523 ns	0.489 ns	0.0076	32 B
a.Prop == (b.Prop || c.Prop)	85.83 ns	0.672 ns	0.628 ns	0.0362	152 B
a.Prop == (b.Prop && c.Prop)	88.38 ns	1.761 ns	1.884 ns	0.0362	152 B
a.Prop == (b.Prop == c.Prop)	104.50 ns	1.162 ns	1.087 ns	0.0362	152 B
a.Prop == (b.Prop ?? c.Prop)	115.92 ns	0.883 ns	0.826 ns	0.0134	56 B
a.Prop == b.Prop + c.Prop	122.91 ns	1.117 ns	1.045 ns	0.0200	84 B
a.Float == b.Int	128.55 ns	0.769 ns	0.719 ns	0.0362	152 B
a.Method(b.Prop.Method())	128.60 ns	1.223 ns	1.144 ns	0.0324	136 B
a.Prop == Static.Method(b.Prop)	168.87 ns	2.003 ns	1.873 ns	0.0267	112 B
a.Prop == b.Method(c.Prop)	261.03 ns	1.913 ns	1.597 ns	0.0286	120 B
a.Prop == b.Method(c.Prop, d.Prop)	265.79 ns	3.130 ns	2.928 ns	0.0305	128 B
a.Prop == b.Method(c.Prop, d.Prop, e.Prop)	297.89 ns	2.282 ns	2.135 ns	0.0324	136 B
a.Prop == $"{b.Prop}_{c.Prop}"	351.95 ns	2.445 ns	2.042 ns	0.0362	152 B
a.Prop == (b.Prop + c.Prop).Method()	417.61 ns	3.072 ns	2.724 ns	0.0515	216 B

^sources

Linking

The library is linker-safe internally, but some exposed APIs rely on Linq Expression trees and therefore the reflection which have always been hard to process for the mono linker.

Although linker can analyze expression trees and some reflection patterns pretty well, the following code units may not be mentioned in the code, appear unused and end up trimmed away:

• Property setters
• Events (which are not configured)

The most common solution for hinting the linker to keep a member is to imitate its usage with a dummy call and mark it with a [Preserve] attribute. Your project may already have a LinkerPleaseInclude.cs file for that purpose.

EBind.LinkerIncludeGenerator will generate such files for the mentioned members used in EBinding and there wont be any EBind-related linker issues in your project.
Adding its NuGet package is enough for the installation:

AOT Compilation

This library uses C# 9 function pointers to create fast delegates for property accessors. It's the safest solution for AOT compilation so far.
However, Xamarin.iOS AOT compiler used for device builds requires a direct indication of value-types as a generic type parameter for them.
All standard structs are pre-seeded.

If you came across an exception like that:

System.ExecutionEngineException:
  Attempting to JIT compile method 'object EBind.PropertyAccessors.PropertyAccessor`2<..., ...>:Get (object)' while running in aot-only mode.

please add a hint for the compiler:

EBind.Platform.AotCompilerHints.Include<MyStruct>(); // custom struct as a member
// or
EBind.Platform.AotCompilerHints.Include<MyStruct, PropertyType>(); // custom struct as a target

Contributions

If you've found an error, please file an issue.

Patches are encouraged and may be submitted by forking this project and submitting a pull request.
If your change is substantial, please raise an issue or start a discussion first.

Development

Requirements

• C# 9 and .NET 5 workload
• Xamarin.Android and Xamarin.iOS SDKs for device testing

Device Testing

Device tests may run manually from the test app UI or automatically with XHarness CLI.

Android:

xharness android test \
  --app="./EBind.Tests.Android/bin/Release/EBind.Tests.Android-Signed.apk" \
  --package-name="EBind.Tests.Android" \
  --instrumentation="ebind.tests.droid.xharness_instrumentation" \
  --device-arch="x86" \
  --output-directory="./EBind.Tests.Android/TestResults/xharness_android" \
  --verbosity

iOS:

xharness apple test \
  --app="./EBind.Tests.iOS/bin/iPhoneSimulator/Release/EBind.Tests.iOS.app" \
  --output-directory="./EBind.Tests.iOS/TestResults/xharness_apple" \
  --targets=ios-simulator-64 \
  --verbosity \
  -- -app-arg:xharness # to be passed in Application.Main(string[])

It's also a part of the github actions workflow. Check it out!

The Story

Once upon a time, there was a small but powerful library called Bind from the Praeclarum family.

No other library could be compared with it in terms of concision and beauty. Every developer was dreaming about using it in his project. But it had several problems and missed some optimizations. The bravest of us could dare to use it in production.

Beauty was stronger than fear and this is how another fork has begun.
Bugs were fixed, some optimizations were made but it was never enough. That library deserved to SHINE!

So...

• Factory method Create replaced with a collection initializer to make bindings a separate block and utilize more syntax variations.
• Binding chaining became redundant, operator && can be used in binding expressions now.
• Added flags that alter binding behavior, as a part of the collection initializer.
• Added support for binding properties/fields to functions (Action bindings).
• Added support for binding functions and commands to events (Event bindings).
• Binding triggers made configurable, all the basic ones for Xamarin are pre-defined.
• Manual invalidation doesn't seem necessary anymore – removed.
• Thread-safety improved by the means of dispatchers and concurrent collections.
• Performance brought to another level with expression interpretation, fast delegates (incl. C# 9 function pointers), and a different architecture.
• The library became more strict and informative in terms of exceptions and errors.
• Added a Source Generator that hints the mono linker about code usage.
• Unbind replaced by IDisposable, useful for aggregation (e.g. CompositeDisposable).

It ended up being completely rewritten with only some tests remained.

Credits

• @praeclarum & Praeclarum.Bind - ❤️ Code and Inspiration

License

This project is licensed under the Apache License, Version 2.0 - see the LICENSE and NOTICE files for details.