Expression binding in FAB

SAPUI5 allows for the use of Expression binding 'in-line' to execute code (for example, to do certain calculations or comparisons) in places where one would normally use a variable, or call a custom function.

For example, on the screen below we want to display/hide the contents of the text control, depending on the value in the input field:

You could:

Define a Boolean variable in the data model (called 'visible', or something similar).
Assign this variable to the 'Visible' property in the Properties of the text field.
Write JavaScript to set the 'visible' field to true or false, depending on the value in the 'Value' input field.

OR, you could:

Use expression binding, accessing the value directly to set the "visible" property of the text control.

Example:

visible="{=${/value} > 10000 }"

(where 'value' is the data model field bound to the Value input field)

Notes on syntax:

The whole expression needs to be surrounded by curly brackets, and the first character within the curly brackets must the the equals sign ‘=’, i.e.
```
{= ..expression ....}
```
The reference to the data model field is made with the ${field} syntax. The '/' is not required, so both of the following options will work:
```
{$/field}
OR
{$field}
```
You can combine expressions using the JavaScript logical OR and/or the logical AND operators. Note that the logical OR is '||' and the logical AND is '&&'. However, for use in Expression binding, the logical AND must be escaped as '&&'. See examples below for the exact syntax in an expression.
Comparisons between fields must be done with the ‘is exactly equal to’ variations of the equality operations (3 characters not 2). That is, you must use '===' (but not '==') for equality and '!==' (but not '!=') for inequality.

Note - use in FAB:

When using the 'visible' or 'enabled' properties, or the Advanced property in FAB, you will need to include the surrounding curly brackets yourself.
For some of our properties - like Bound Field for inputs for example - FAB already inserts the surrounding curly brackets because it is a binding, so the expression should just be:
```
=...expression.    
```
Tip: Use the "Show XML" menu option on the control in FAB to see the final outcome - this way you can see how the expression will be presented to the browser.

Further notes / examples:

Determining a text value based on a field (like the Debit/Credit indicator in SAP), which also shows us an IF/ELSE type construct:

{=${SHKZG} === 'H’ ? 'Debit' : 'Credit' ) }

This translates to (in pseudo-code):

IF SHKZG = 'H' THEN
use/display 'Debit'
ELSE
use/display 'Credit'
ENDIF.

The 'state’ property of an element (Object Status), which is usually, ‘None’, ‘Success’, ‘Warning’ or ‘Error’, can be dynamically determined based on another model variable, for example:

{=${field} === 0 ? 'Success' : ( ${field} === 1 ? 'Warning' : 'Error' ) }

Note the chained / nested IF in the example above.

This translates to (in pseudo-code):

IF field = 0 THEN
use/display 'Success'
ELSE
IF field = 1 THEN
use/display 'Warning'
ELSE
use/display 'Error'
ENDIF.
ENDIF.

The following examples were sourced from the SAP documentation at: Expression Binding:

<!--Set to visible if the status is critical and the amount is above the threshold (note escaping of &&)-->
visible="{= ${status} === 'critical' &amp;&amp; ${amount} > 10000 }"

<!--Text for amount level using language-dependent texts from the resource model.-->
text="{= ${/amount} > 10000 ? ${i18n>/high} : ${i18n>/normal} }"

<!--Set to visible if the rating is VIP, ignoring case or if the order amount is greater than 10,000.-->
visible="{= ${/rating}.toUpperCase() === 'VIP' || ${/orderAmount} > 10000 }"

<!--Set to visible if the rating contains VIP, ignoring the case. -->
visible={= RegExp('vip', 'i').test(${/rating}) }

<!--Text is maximum of three values.-->
text="{= Math.max(${/value1}, ${/value2}, ${/value3}) }"

<!--Control is enabled only if the order status is set.--> 
enabled="{= ${/orderStatus} !== null }"

Credit:

Thanks to Adam Miles for his review, suggestions, examples and syntax additions.
Some examples taken from the SAP documentation at: https://help.sap.com/saphelp_uiaddon10/helpdata/en/da/f6852a04b44d118963968a1239d2c0/content.htm?no_cache=true