敲除包装值绑定 [英] Knockout wrap value binding

问题描述

我正在使用 mathias bynen的占位符代码，如果要删除，我想将其与我做一个简单的自定义绑定，像这样:

I am using mathias bynen's placeholder code and I wanted to use it along with knockout, if I do a simple custom binding like this:

ko.bindingHandlers.placeholder = {
    init: function (element) {
        $(element).placeholder();
    }
};

和html

<input placeholder = "Line 1" data-bind="placeholder: {}, value: addressLine1">

它可以工作，但是我想将它们合并"到一个自定义绑定中，以便像这样使用它

It works, but I would like to "merge" them into one custom binding, for using it like

<input placeholder = "First Name" data-bind="placeholderValue: firstName">

所以我尝试了这段代码:

So I tried this code:

ko.bindingHandlers.placeholderValue = {
    init: function (element, valueAccessor) {
        $(element).placeholder();
        ko.bindingHandlers.value.init(element, valueAccessor);
    },
    update: function (element, valueAccessor) {
        ko.bindingHandlers.value.update(element, valueAccessor);
    }
};

但是它给我带来了一个

Uncaught TypeError: undefined is not a function 

我还没有真正掌握ko

I'm not really getting the grip of ko yet

推荐答案

在创建委派自定义绑定作为最佳实践时，应始终通过

When you are creating a delegating custom binding as a best practice you should always pass all the arguments of the init and update to the inner bindings, because you can never know what parameters the inner binding uses:

ko.bindingHandlers.placeholderValue = {
    init: function (element, valueAccessor, allBindingsAccessor, 
                    viewModel, bindingContext) {
        $(element).placeholder();
        ko.bindingHandlers.value.init(element, valueAccessor, 
             allBindingsAccessor, viewModel, bindingContext);
    },
    update: function (element, valueAccessor, allBindingsAccessor, 
                     viewModel, bindingContext) {
        ko.bindingHandlers.value.update(element, valueAccessor, 
             allBindingsAccessor, viewModel, bindingContext);
    }
};

您有例外，因为value出价的init使用了allBindingsAccessor参数，但是由于您没有在其中传递参数，因此引发了例外.

You have got the exception because the init of the value biding uses the allBindingsAccessor parameter but because you haven't passed that in it raises the exception.

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