mvvmfx-validation/src/main/java/de/saxsys/mvvmfx/utils/validation/ObservableRuleBasedValidator.java
public void addRule(ObservableValue<Boolean> rule, ValidationMessage message) {
	    booleanRules.add(rule);

		rule.addListener((observable, oldValue, newValue) -> {
			validateBooleanRule(newValue, message);
		});

		validateBooleanRule(rule.getValue(), message);
	}
public void addRule(ObservableValue<Boolean> rule, ValidationMessage message) {
	    booleanRules.add(rule);

		rule.addListener((observable, oldValue, newValue) -> {
			validateBooleanRule(newValue, message);
		});

		validateBooleanRule(rule.getValue(), message);
	}
Add a rule for this validator.
<p>
The rule defines a condition that has to be fulfilled.
<p>
A rule is defined by an observable boolean value. If the rule has a value of <code>true</code> the rule is
"fulfilled". If the rule has a value of <code>false</code> the rule is violated. In this case the given message
object will be added to the status of this validator.
<p>
There are some predefined rules for common use cases in the {@link ObservableRules} class that can be used.

@param rule
@param message