NuGet package here
MAUI version here
MAUI NuGet here

Release notes for 1.2.2

Amended the example showing
... Status == 'Administrator' ...
to
Status == \'Administrator\' ...

Added notes about escaping strings below

Release notes for 1.2.1

All csharp value types supported including nullable types.
Casting is supported. Cast to any type found in ExpressionParserZero.Operands.OperandType, e.g. (NullableULong) 42

Overview

z:Bind is a xaml markup extension that allows you to bind directly to an expression rather than just a property

Put simply, it allows you to do things like this:

<StackLayout 
	IsVisible="{z:Bind '(Item.Count != 0) AND (Status == \'Administrator\' '}" > ...

Expressions are re-evaluated only if a property on which they depend changes

Features

• Full parsing and evaluation of ViewModel and/or view properties
• Alisaing of operators, so you can represent (for example) && in xaml as AND rather than &&
• Registration of methods that can be called directly from xaml

Quickstart

Use the package manager to add the NugetPackage FunctionZero.zBind

Add a namespace alias to your xaml, like this:

xmlns:z="clr-namespace:FunctionZero.zBind.z;assembly=FunctionZero.zBind"

And that's all there is to it, you can now z:Bind to any properties in your ViewModel

Usage

Just like the Xamarin.Forms Binding object, z:Bind binds to properties in your BindingContext by default, or you can specify any suitable alternative by setting the Source property:

Sample	Notes
{z:Bind Count}	Bind to Count
{z:Bind IsExpanded, Source={x:Reference MyExpander}}	Bind to a property on an element named MyExpander
{z:Bind Count * 2}	Bind to an expression that yields Count * 2
{z:Bind (Count * 2) LT 10}	True if (Count * 2) < 10
{z:Bind Sin(Count / 25.0)}	Calls a function (see below)

The Great Escape, and other cautionary tales

Similar to xaml, string literals can be enclosed within 'single quotes' or "double quotes" with appropriate use of xml escape-sequences.

Commas

If your expression string has commas in it, you must hide them from the xaml parser, otherwise z:Bind will be given an incomplete string and things won't work as expected.

You can do this by enclosing the string inside quotes, like this:

{z:Bind 'SomeFunction(param1, param2)'}

Strings

If your expression string has string literals in it, you must 'escape' them, otherwise z:Bind will be given an incorrect string and things won't work as expected.
For example:

{z:Bind Status == \'Administrator\'}

Long form

If your expression is getting bogged down in escape-sequences and commas and quotes, or if that's just the way you roll, you can use the long-form of expressing a z:Bind expression:

<Label>
    <Label.Text>
        <z:Bind>
            'Score: '+ Count + ' points'
        </z:Bind>
    </Label.Text>
</Label>

Casting

Casting of function parameters follows csharp rules, so for example Sin(someFloat) is fine even though Sin expects a double.
You can explicitly cast to any csharp type if you so wish, e.g. Sin((Double)someFloat)
See ExpressionParserZero.Operands.OperandType for all the types you can cast to.

Short-circuit

Just like c#, the underlying expression parser supports short-circuit, so expressions like (thing != null) AND (thing.part == 5) will work even if thing is null

Errors

Error reporting is quite good, so check the debug output if things aren't working as expected.

Aliases supported to simplify xaml

Alias	Operator
NOT	!
MOD	%
LT	<
GT	>
GTE	>=
LTE	<=
BAND	&
XOR	^
BOR	|
AND	&&
OR	||

Supported value types

All csharp value types are supported, see the enum ExpressionParserZero.Operands.OperandType for a complete list

Supported reference types

string, object

Advanced Usage - Functions, aliases and operator-overloads

z:Bind uses FunctionZero.ExpressionParserZero to do the heavy lifting, so take a look at the documentation if you want to take a deeper dive. Here is a taster ...

Functions

Sin, Cos and Tan and more are registered by default, as are the aliases listed above.

<Label TranslationX="{z:Bind Sin(Count / 25.0) * 100.0}" ...

Note: Lerp is also pre-registered; the following is just for example. Suppose you wanted a new function to to do a linear interpolation between two values, like this:

double Lerp(double a, double b, double t)
{
  return a + t * (b - a);
}

For use like this: (Note the expression is enclosed in quotes because it contains commas)

<Label Rotation="{z:Bind 'Lerp(0, 360, rotationPercent / 100.0)'}" ...

First you will need a reference to the default ExpressionParser

var ep = ExpressionParserFactory.GetExpressionParser();

Then register a function that takes 3 parameters:

ep.RegisterFunction("Lerp", DoLerp, 3);

Finally write the DoLerp method referenced above, with the following signature:

private static void DoLerp(Stack<IOperand> operandStack, IBackingStore backingStore, long paramCount)
{
    // Pop the correct number of parameters from the operands stack, ** in reverse order **
    // If an operand is a variable, it is resolved from the backing store provided
    IOperand third = OperatorActions.PopAndResolve(operandStack, backingStore);
    IOperand second = OperatorActions.PopAndResolve(operandStack, backingStore);
    IOperand first = OperatorActions.PopAndResolve(operandStack, backingStore);

    double a = (double)first.GetValue();
    double b = (double)second.GetValue();
    double t = (double)third.GetValue();

    // The result is of type double
    double result = a + t * (b - a);

    // Push the result back onto the operand stack
    operandStack.Push(new Operand(-1, OperandType.Double, result));
}

Aliases

Get a reference to the default ExpressionParser:

var ep = ExpressionParserFactory.GetExpressionParser();

Then register a new operator and use the existing matrix for &&

(See the ExpressionParserZero source and documentation for more details)

ep.RegisterOperator("AND", 4, LogicalAndMatrix.Create());

Overloads

Suppose you want to add a long to a string, yielding a result of type string

Get a reference to the default ExpressionParser:

var ep = ExpressionParserFactory.GetExpressionParser();

Then simply register the overload like this

// Overload that will allow a long to be appended to a string
// To add a string to a long you'll need to add another overload
parser.RegisterOverload("+", OperandType.String, OperandType.Long, 
    (left, right) => new Operand(OperandType.String, (string)left.GetValue() + ((long)right.GetValue()).ToString()));

and to add a string to a long:

// Overload that will allow a string to be appended to a long
// To add a long to a string you'll need to add another overload
parser.RegisterOverload("+", OperandType.Long, OperandType.String, 
    (left, right) => new Operand(OperandType.String, (long)left.GetValue() + ((string)right.GetValue()).ToString()));

Putting the above into action, you can then start to really have some fun

<Label 
    Text="{z:Bind 'Player 1 score ' + playerOne.Score + 'points'}"
    Rotation="{z:Bind Lerp(0, 360, rotationPercent / 100.0)}"
/>

If that's not enough, you can entirely replace the ExpressionParser with your own by calling ExpressionParserFactory.ReplaceDefaultExpressionParser(..) This is how z:Bind created the default parser before these registrations were moved to ExpressionParserZero and can be used as a guide to creating your own:

var ep = new ExpressionParser();

ep.RegisterFunction("Sin", DoSin, 1, 1);
ep.RegisterFunction("Cos", DoCos, 1, 1);
ep.RegisterFunction("Tan", DoTan, 1, 1);
...

ep.RegisterOperator("AND", 4, LogicalAndMatrix.Create());
ep.RegisterOperator("OR", 4, LogicalOrMatrix.Create());
ep.RegisterOperator("LT", 4, LessThanMatrix.Create());
ep.RegisterOperator("LTE", 4, LessThanOrEqualMatrix.Create());
ep.RegisterOperator("GT", 4, GreaterThanMatrix.Create());
ep.RegisterOperator("GTE", 4, GreaterThanOrEqualMatrix.Create());
ep.RegisterOperator("BAND", 4, BitwiseAndMatrix.Create());
ep.RegisterOperator("BOR", 4, BitwiseOrMatrix.Create());

ReplaceDefaultExpressionParser(ep, false);

Here are the methods referenced above:

private static void DoSin(Stack<IOperand> stack, IBackingStore store, long paramCount)
{
    IOperand first = OperatorActions.PopAndResolve(stack, store);
    double val = (double)first.GetValue();
    var result = Math.Sin(val);
    stack.Push(new Operand(-1, OperandType.Double, result));
}

private static void DoCos(Stack<IOperand> stack, IBackingStore store, long paramCount)
{
    IOperand first = OperatorActions.PopAndResolve(stack, store);
    double val = (double)first.GetValue();
    var result = Math.Cos(val);
    stack.Push(new Operand(-1, OperandType.Double, result));
}

private static void DoTan(Stack<IOperand> stack, IBackingStore store, long paramCount)
{
    IOperand first = OperatorActions.PopAndResolve(stack, store);
    double val = (double)first.GetValue();
    var result = Math.Tan(val);
    stack.Push(new Operand(-1, OperandType.Double, result));
}

Even more ...

Assignment operator:

Set the property Sample.Value

<Label Text="{z:Bind 'Sample.Value = Interest * Complexity'}" />

Comma operator

Evaluate multiple expressions, provide the result of the last

<Label Text="{z:Bind 'Test.Value = Count, Count * 2'}" />

Caution

<!--    This will (of course) lock up the main thread, 
        because it is re-evaluated every time `a` changes -->
<Label Text={z:Bind a = a + 1} />

Notes

Expressions are reevaluated only when one of the properties they reference change.