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Per my test, due to the action property is one of the few properties that does not support if expressions. So, it is not possible to make the plus button unclickable when the Registration is full.
As a workaround, you could make the plus button invisible when the Registration is full. Here are codes.
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "div",
"style": {
"flex-directon": "row",
"justify-content": "left",
"align-items": "center",
"flex-wrap": "nowrap"
},
"children": [
{
"elmType": "div",
"style": {
"display": "=if(indexOf([$PeopleWhoRegistered.email], @me) == -1 && length([$PeopleWhoRegistered]) < [$NumberOfPlaces] && @now <= [$RegistrationEndDate], '','none')",
"flex-directon": "row",
"justify-content": "left",
"align-items": "center",
"flex-wrap": "wrap"
},
"children": [
{
"elmType": "button",
"customRowAction": {
"action": "setValue",
"actionInput": {
"PeopleWhoRegistered": "=appendTo([$PeopleWhoRegistered], @me)"
}
},
"attributes": {
"class": "ms-fontColor-themePrimary ms-fontColor-themeDarker--hover",
"title": "Register"
},
"style": {
"border": "none",
"background-color": "transparent",
"cursor": "pointer",
"display": "flex",
"flex-directon": "row",
"justify-content": "left",
"align-items": "center",
"flex-wrap": "wrap"
},
"children": [
{
"elmType": "span",
"attributes": {
"iconName": "AddTo"
},
"style": {
"padding": "0px"
}
}
]
}
]
},
{
"elmType": "div",
"style": {
"display": "=if(indexOf([$PeopleWhoRegistered.email], @me) > -1 && @now <= [$RegistrationEndDate], '','none')",
"flex-directon": "row",
"justify-content": "left",
"align-items": "center",
"flex-wrap": "wrap"
},
"children": [
{
"elmType": "button",
"customRowAction": {
"action": "setValue",
"actionInput": {
"PeopleWhoRegistered": "=removeFrom([$PeopleWhoRegistered], @me)"
}
},
"attributes": {
"class": "ms-fontColor-themePrimary ms-fontColor-themeDarker--hover",
"title": "Unregister"
},
"style": {
"border": "none",
"background-color": "transparent",
"cursor": "pointer",
"display": "flex",
"flex-directon": "row",
"justify-content": "left",
"align-items": "center",
"flex-wrap": "wrap"
},
"children": [
{
"elmType": "span",
"attributes": {
"iconName": "Blocked12"
},
"style": {
"padding": "0px"
}
}
]
}
]
},
{
"elmType": "div",
"children": [
{
"elmType": "span",
"txtContent": "Registered",
"style": {
"display": "=if([$RegistrationEndDate] > @now && indexOf([$PeopleWhoRegistered.email], @me) > -1, '','none')",
"padding-left": "5px",
"word-break": "keep-all"
}
}
]
},
{
"elmType": "div",
"children": [
{
"elmType": "span",
"txtContent": "- Registration is full",
"style": {
"display": "=if([$RegistrationEndDate] > @now && length([$PeopleWhoRegistered]) >= [$NumberOfPlaces], '','none')",
"padding-left": "5px",
"word-break": "keep-all"
}
}
]
},
{
"elmType": "div",
"children": [
{
"elmType": "span",
"txtContent": "Registration has ended",
"style": {
"display": "=if([$RegistrationEndDate] > @now, 'none','inherit')",
"padding-left": "5px",
"word-break": "keep-all"
}
}
]
}
]
}
Result:
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