(change) event hook in angular2

问题

I know about the (change) event Binding in angular2 but i am surprised why my code is not working as expected?

My code is here..

http://plnkr.co/edit/9pSWSeqBc5oaSAtsfwNY?p=preview

When the change event is called not both condition work as expected.

(change)="holiday= !holiday && employee= !employee"

When change event is called the first time it works fine but the second time it will work only for first condition i.e holiday. In my example what I expected is both the value to be either true or false but not as expected.

Surely there is some mistake. Is there anyone who can explain the life cycle of the (change) event properly?

回答1:

You should separate them with ; instead of && operator.

If first expression goes wrong, it would not evaluate the next expression.

(change)="holiday= !holiday;employee= !employee"

How && operator works?

1. Suppose a && b, if both are true then only it returns true(in short there shouldn't be any false value otherwise it will return false).
2. When evaluating a && b code will first check a value is true then only the interpreter will goes b value to evaluate it. If a value it self is false then it don't evaluate(check) b's value, and a && b expression will return false.

Before you were having holiday= !holiday && employee= !employee. On initial load both holiday & employee has false value. When you click on the checkbox it evaluates holiday= !holiday && employee= !employee, both holiday & employee value becomes true.

Basically behind the scene when 1st holiday= !holiday get evaluated, holiday becomes true to false & holiday= !holidayexpression return latest value(returns true), whether 2nd expression does the same thing & returns true.

Now holiday = true & employee = true. When you click on the check box again. It calls change event & again try to evaluate holiday= !holiday && employee= !employee. Where holiday= !holiday return false, so then as I mention above How && operator works?. It doesn't care about the next part of expression, and returns false value.

Now holiday = false & employee = true. If you click on checkbox again, then holiday turns out to true & proceed to evaluate other part of expression by which employee turns out to true from false.

来源：https://stackoverflow.com/questions/35101550/change-event-hook-in-angular2