[SelectCheckbox] fix onChange behavior

[c3dr0x]

Fix issue #1412

Current behaviour

onChange is called with two parameters : key and newStatus

New behaviour

• onChange is now called by default with the newly selected values as an array of key
• A new option legacyOnChange is available to get the old behavior

Caution

The field-component-behaviour._wrappedOnChange method will truncate the second parameter. Therefore legacy onChange will not work when inside a fieldFor

[Hartorn]

Great comments, and code cleaning.

Overall, I don't like the fact we are using the selectedValues of the state, instead of the props even in the case of legacyOnChangeFalse.

Else, there are some minor corrections to do

[Hartorn]

Ugly, this is modifying the state directly.
This should be something like

const selectedValues = newStatus ? this.state.selectedValues.concat([key]) : this.state.selectedValues.filter(elt => elt !== key);

[c3dr0x]

Agreed, I didn't push enough the upgrade

[Hartorn]

Avoid nested if : it is not clear
I think in this case it is better with

if(this.props.onChange && this.props.legacyOnChange) {
    this.props.onChange(key, newStatus);
} else if(this.props.onChange) {
    this.props.onChange(selectedValues);
} else {
 this.setState({ value: selectedValues });
}

[Hartorn]

@c3dr0x Maybe one day we'll clean this up, so the state is not used if value is taken from props, and onChange too