Lack of usage on validation

[geohuz]

I can only find "validate" in API doumetation, and some usage examples after searching through github issues, but there is no official validation usage example. So what is the suggested way of doing validation with custom error messages?

[jamonholmgren]

We should add some examples to the documentation, for sure.

Here's one example that might help:

https://mobx-state-tree.js.org/API/#custom

const DecimalPrimitive = types.custom<string, Decimal>({
    name: "Decimal",
    fromSnapshot(value: string) {
        return new Decimal(value)
    },
    toSnapshot(value: Decimal) {
        return value.toString()
    },
    isTargetType(value: string | Decimal): boolean {
        return value instanceof Decimal
    },
    getValidationMessage(value: string): string {
        if (/^-?\d+\.\d+$/.test(value)) return "" // OK
        return `'${value}' doesn't look like a valid decimal number`
    }
})

const Wallet = types.model({
    balance: DecimalPrimitive
})