ko.computed property to determine visibility not w

问题:

In my KncokoutJS ViewModel, I have the follow computed property:

self.SelectedUserHasRoles = ko.computed(function () {
  if (self.isLoaded()) {
    return self.selectedUser().roles().length > 0;
  }
  return false;
});

And in my HTML, I have the following:

<!-- ko if: isLoaded() -->
  <!-- ko if: !SelectedUserHasRoles -->
  <div>
    <p>User has no Roles.</p>
  </div>
  <!-- /ko -->
  <!-- ko if: SelectedUserHasRoles -->
  <div class="roles-wrapper" data-bind="foreach: $root.selectedUser().roles()">
    <div class="role-token" data-bind="text: Name"></div>
  </div>
  <!-- /ko -->
<!-- /ko -->

In my code, I was to say this:

If data from AJAX call has finished loading (isLoaded is true), then for the currently selected user, check and see if he/she has any roles. If yes, then loop through them and show them, if not, show a bit of text saying 'User has no Roles.'

All seems to work, except for the showing User has no Roles text snippet. I've no idea why that isn't showing! I'm putting breakpoints into my computed property and can see that when I select a user with no roles, the expression (in console window) is false, and I'm negating that, so I should see that text snippet!

What am I doing wrong? I've created a screencast to make things easier to understand.

回答1:

Because you are not binding to a variable but to an expression, you need to add parenthesis here:

<!-- ko if: !SelectedUserHasRoles() -->
                               //^^ here

See the following snippet

function CreateVM() {
    var self = this;
    this.isTrue = ko.observable(false);
    this.selectedUser = ko.observable();
    this.isLoaded = ko.observable();
    self.SelectedUserHasRoles = ko.computed(function () {
       if (self.isLoaded()) {
          return self.selectedUser().roles().length > 0;
       }
       return false;
    });
}

var vm = new CreateVM();
ko.applyBindings(vm);

var userWithRoles = { roles: ko.observableArray([1,2]) }; 
var userWithoutRoles = { roles: ko.observableArray([]) }; 
vm.selectedUser(userWithoutRoles);
vm.isLoaded(true);

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>
<!-- ko if: isLoaded() -->
  <!-- ko if: !SelectedUserHasRoles() -->
  <div>
    <p>User has no Roles.</p>
  </div>
  <!-- /ko -->
  <!-- ko if: SelectedUserHasRoles -->
  <div class="roles-wrapper" data-bind="foreach: $root.selectedUser().roles()">
    <div class="role-token" data-bind="text: $data"></div>
  </div>
  <!-- /ko -->
  SelectedUserHasRoles: <span class="role-token" data-bind="text: SelectedUserHasRoles"></span>
<!-- /ko -->

See user3297291's answer for more details.

回答2:

When you want to negate an observable or computed value in a binding, you have to call it explicitly:

<!-- ko if: !SelectedUserHasRoles() -->

In the case of the if binding, there's also the ifnot counterpart:

<!-- ko ifnot: SelectedUserHasRoles -->

---

I think it's useful to understand why this is needed, since I see it happening a lot.

You could see the data-bind attribute as a comma separated string of key value pairs. Knockout wraps each of the values in a function, which it calls the valueAccessor.

Essentially, you'll go from:

 data-bind="if: SelectedUserHasRoles"

to

{ 
   "if": function() { return SelectedUserHasRoles }
}

SelectedUserHasRoles is an observable instance, which evaluates as truthy. When you negate this value using an !, it will always be false.

var myObs = ko.observable("anything");

var valueAccessor =    function() { return  myObs; };
var valueAccessorNeg = function() { return !myObs; };

console.log(valueAccessor());    // Returns the observable
console.log(valueAccessorNeg()); // Always prints false

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>

The valueAccessor function is passed to the init method of a binding. Usually, it is retrieved by calling it, and then unwrapped. Because the unwrap utility doesn't care about whether you pass it an observable or a plain value, you'll not see any errors when you make this mistake.

var myObs = ko.observable(false);

var va1 = function() { return myObs; };
var va2 = function() { return !myObs; };
var va3 = function() { return !myObs(); };

console.log(ko.unwrap(va1()));    // false
console.log(ko.unwrap(va2()));    // always false
console.log(ko.unwrap(va3()));    // true

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>

I hope this small peek under the hood might help you (and others that have made this mistake) to be able to determine when the () are needed in the future.

回答3:

You have the same check for isLoaded() twice, actually

<!-- ko if: isLoaded() -->
  <!-- ko if: !SelectedUserHasRoles -->

if isLoaded() evalutes to false, your SelectedUserHasRoles() won't even be evaluated.