Monday, 24 June 2013

Knockout Observable Arrays



If you want to detect and respond to changes on one object, you’d use observables. If you want to detect and respond to changes of a collection of things, use an observableArray. This is useful in many scenarios where you’re displaying or editing multiple values and need repeated sections of UI to appear and disappear as items are added and removed.
Var myObservableArray = ko.observableArray();    // Initially an empty arraymyObservableArray.push('Some value');      // Adds the value and notifies observers

Example:

<form data-bind="submit: addItem">
    New item:
    <input data-bind='value: itemToAdd, valueUpdate: "afterkeydown"' />
    <button type="submit" data-bind="enable: itemToAdd().length > 0">Add</button>
    <p>Your items:</p>
    <select width="50" multiple="multiple" data-bind="options: items"> </select>
</form>

var SimpleListModel = function(items) {
    this.items = ko.observableArray(items);
    this.itemToAdd = ko.observable("");
    this.addItem = function() {
        if (this.itemToAdd() != "") {
            this.items.push(this.itemToAdd()); // Adds the item. Writing to the "items" observableArray causes any associated UI to update.
            this.itemToAdd(""); // Clears the text box, because it's bound to the "itemToAdd" observable
        }
    }.bind(this);  // Ensure that "this" is always this view model
};

ko.applyBindings(new SimpleListModel(["Alpha", "Beta", "Gamma"]));



Key point: An observableArray tracks which objects are in the array, not the state of those objects:
Simply putting an object into an observableArray doesn’t make all of that object’s properties themselves observable. Of course, you can make those properties observable if you wish, but that’s an independent choice. An observableArray just tracks which objects it holds, and notifies listeners when objects are added or removed.


Prepopulating an observableArray
If you want your observable array not to start empty, but to contain some initial items, pass those items as an array to the constructor. For example,
// This observable array initially contains three objects
var anotherObservableArray = ko.observableArray([
    { name: "Bungle", type: "Bear" },
    { name: "George", type: "Hippo" },
    { name: "Zippy", type: "Unknown" }
]);


Reading information from an observableArray
Behind the scenes, an observableArray is actually an observable whose value is an array. So, you can get the underlying JavaScript array by invoking the observableArray as a function with no parameters, just like any other observable. Then you can read information from that underlying array. For example,
alert('The length of the array is ' + myObservableArray().length);
alert('The first element is ' + myObservableArray()[0]);
Technically you can use any of the native JavaScript array functions to operate on that underlying array, but normally there’s a better alternative. KO’s observableArray has equivalent functions of its own, and they’re more useful because:
1)    They work on all targeted browsers. (For example, the native JavaScript indexOf function doesn’t work on IE 8 or earlier, but KO’s indexOf works everywhere.)
2)    For functions that modify the contents of the array, such as push and splice, KO’s methods automatically trigger the dependency tracking mechanism so that all registered listeners are notified of the change, and your UI is automatically updated.
3)    The syntax is more convenient. To call KO’s push method, just write myObservableArray.push(...). This is slightly nicer than calling the underlying array’s push method by writing myObservableArray().push(...).
The rest of this page describes observableArray’s functions for reading and writing array information.
indexOf - The indexOf function returns the index of the first array item that equals your parameter. For example, myObservableArray.indexOf('Blah') will return the zero-based index of the first array entry that equals Blah, or the value -1 if no matching value was found.
Slice - The slice function is the observableArray equivalent of the native JavaScript slice function (i.e., it returns the entries of your array from a given start index up to a given end index). Calling myObservableArray.slice(...) is equivalent to calling the same method on the underlying array (i.e., myObservableArray().slice(...)).


Manipulating an observableArray
observableArray exposes a familiar set of functions for modifying the contents of the array and notifying listeners.

pop, push, shift, unshift, reverse, sort, splice
All of these functions are equivalent to running the native JavaScript array functions on the underlying array, and then notifying listeners about the change:
  • myObservableArray.push('Some new value') -  adds a new item to the end of array.
  • myObservableArray.pop() -  removes the last value from the array and returns it.
  • myObservableArray.unshift('Some new value') - inserts a new item at the beginning of the array.
  • myObservableArray.shift() - removes the first value from the array and returns it
  • myObservableArray.reverse() -  reverses the order of the array
  • myObservableArray.sort() - sorts the array contents.
    • By default, it sorts alphabetically (for strings) or numerically (for numbers).
    • Optionally, you can pass a function to control how the array should be sorted. Your function should accept any two objects from the array and return a negative value if the first argument is smaller, a positive value is the second is smaller, or zero to treat them as equal. For example, to sort an array of ‘person’ objects by last name, you could write myObservableArray.sort(function(left, right) { return left.lastName == right.lastName ? 0 : (left.lastName < right.lastName ? -1 : 1) })
  • myObservableArray.splice() - removes and returns a given number of elements starting from a given index. For example, myObservableArray.splice(1, 3) removes three elements starting from index position 1 (i.e., the 2nd, 3rd, and 4th elements) and returns them as an array.

remove and removeAll
observableArray adds some more useful methods that aren’t found on JavaScript arrays by default:
  • myObservableArray.remove(someItem) - removes all values that equal someItem and returns them as an array.
  • myObservableArray.remove(function(item) { return item.age < 18 }) - removes all values whose age property is less than 18, and returns them as an array.
  • myObservableArray.removeAll(['Chad', 132, undefined]) - removes all values that equal 'Chad', 123, or undefined and returns them as an array.
  • myObservableArray.removeAll() - removes all values and returns them as an array.

destroy and destroyAll
The destroy and destroyAll functions are mainly intended as a convenience for developers using Ruby on Rails:
  • myObservableArray.destroy(someItem) - finds any objects in the array that equal someItem and gives them a special property called _destroy with value true.
  • myObservableArray.destroy(function(someItem) { return someItem.age < 18 }) - finds any objects in the array whose age property is less than 18, and gives those objects a special property called _destroy with value true.
  • myObservableArray.destroyAll(['Chad', 132, undefined]) - finds any objects in the array that equal 'Chad', 123, or undefined and gives them a special property called _destroy with value true.
  • myObservableArray.destroyAll() - gives a special property called _destroy with value true to all objects in the array.
So, what’s this _destroy thing all about? It’s only really interesting to Rails developers. The convention in Rails is that, when you pass into an action a JSON object graph, the framework can automatically convert it to an ActiveRecord object graph and then save it to your database. It knows which of the objects are already in your database, and issues the correct INSERT or UPDATE statements. To tell the framework to DELETE a record, you just mark it with _destroy set to true.
Note that when KO renders a foreach binding, it automatically hides any objects marked with _destroy equal to true. So, you can have some kind of “delete” button that invokes the destroy (someItem) method on the array, and this will immediately cause the specified item to vanish from the visible UI. Later, when you submit the JSON object graph to Rails, that item will also be deleted from the database (while the other array items will be inserted or updated as usual).



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