knockout.js input box event change - passes old va

问题:

When adding a change event binding to an input box using knockout.js the old value is passed to the change function when the event is fired. I can work around this by using blur. Is this the intended behavior? Is the idea to use the change event to have the old value and then use a normal selector to get the value from the dom? It seems counter intuitive.

jsFiddle Example

JavaScript
----------
var data = {
    saved_value:"1",
    value_changed: function(data){
        alert(data.saved_value());
    }
};
var viewModel = ko.mapping.fromJS(data);
ko.applyBindings(viewModel);

HTML
----
Current Value:<span data-bind="text:saved_value"></span><br/>
<input data-bind="event:{change:value_changed},value:saved_value"></input>

回答1:

Try using the text and value bindings:

Current Value:<span data-bind="text: saved_value"></span><br/>
<input data-bind="value: saved_value"></input>

And change the JavaScript to this:

var data = {
    saved_value: "1"    
};

var viewModel = ko.mapping.fromJS(data);
ko.applyBindings(viewModel);​

Here's an associated jsFiddle: http://jsfiddle.net/6zmJs/

If you want to alert() the value of saved_value when it is updated you could use ko.computed or viewModel.saved_value.subscribe(function(value) { alert(value); }); -- although those aren't the only ways to do this.

回答2:

Update: For newer versions of knockout, you can replace the value binding with textInput, which handles many edge cases not covered by valueUpdate.

---

No, this is not the right way to do things.

You're getting the old value because saved_value doesn't get updated until the textbox loses focus.

If you want to push the new value in as the user types, using the valueUpdate option of the input binding:

<input data-bind="event: { change: value_changed }, value: saved_value, valueUpdate: 'afterkeydown'" />

The valueUpdate option takes an event name (e.g. 'keyup'). When that event fires, your saved_value will be updated.

---

Now let me propose an alternate solution.

Still do the valueUpdate binding like I showed above, but instead of listening for the changed event, just subscribe to the observable:

<input data-bind="textInput: saved_value" />

Then in JS:

var viewModel = {
    saved_value: ko.observable("1"),
};
viewModel.saved_value.subscribe(function (newValue) {
   alert(data.saved_value());
});
ko.applyBindings(viewModel);

回答3:

If you put the 'event' option at the end you don't need the 'valueUpdate' option. Like this:

<input data-bind="value: saved_value, event: { change: value_changed }" />

Also note that when you make use of subscribe to an observable, it gets triggered every time your value changes. (either by user interaction or programmatically).

回答4:

Try this in the data bind write event handlers, after the value handlers and not before:

<input data-bind="value: property, event:{ change: doSomething}" />