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    Knockout Projections

    Knockout.js observable arrays get smarter
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    knockout-projections

    Knockout.js observable arrays get smarter.

    This plugin adds observable map and filter features to observable arrays, so you can transform collections in arbitrary ways and have the results automatically update whenever the underlying source data changes.

    Installation

    Download a copy of knockout-projections-x.y.z.js from the dist directory and reference it in your web application:

    <script src='knockout-x.y.z.js'></script>              <!-- First reference KO itself -->
    <script src='knockout-projections-x.y.z.js'></script>  <!-- Then reference knockout-projections -->
    

    Be sure to reference it after you reference Knockout itself, and of course replace x.y.z with the version number of the file you downloaded.

    Usage

    Mapping

    More info to follow. For now, here's a simple example:

    var sourceItems = ko.observableArray([1, 2, 3, 4, 5]);
    

    There's a plain observable array. Now let's say we want to keep track of the squares of these values:

    var squares = sourceItems.map(function(x) { return x*x; });
    

    Now squares is an observable array containing [1, 4, 9, 16, 25]. Let's modify the source data:

    sourceItems.push(6);
    // 'squares' has automatically updated and now contains [1, 4, 9, 16, 25, 36]
    

    This works with any transformation of the source data, e.g.:

    sourceItems.reverse();
    // 'squares' now contains [36, 25, 16, 9, 4, 1]
    

    The key point of this library is that these transformations are done efficiently. Specifically, your callback function that performs the mapping is only called when strictly necessary (usually, that's only for newly-added items). When you add new items to the source data, we don't need to re-map the existing ones. When you reorder the source data, the output order is correspondingly changed without remapping anything.

    This efficiency might not matter much if you're just squaring numbers, but when you are mapping complex nested graphs of custom objects, it can be important to perform each mapping update with the minumum of work.

    Filtering

    As well as map, this plugin also provides filter:

    var evenSquares = squares.filter(function(x) { return x % 2 === 0; });
    // evenSquares is now an observable containing [36, 16, 4]
    
    sourceItems.push(9);
    // This has no effect on evenSquares, because 9*9=81 is odd
    
    sourceItems.push(10);
    // evenSquares now contains [36, 16, 4, 100]
    

    Again, your filter callbacks are only called when strictly necessary. Re-ordering or deleting source items don't require any refiltering - the output is simply updated to match. Only newly-added source items must be subjected to your filter callback.

    Chaining

    The above code also demonstrates that you can chain together successive map and filter transformations.

    When the underlying data changes, the effects will ripple out through the chain of computed arrays with the minimum necessary invocation of your map and filter callbacks.

    How to build from source

    First, install NPM if you don't already have it. It comes with Node.js.

    Second, install Grunt globally, if you don't already have it:

    npm install -g grunt-cli
    

    Third, use NPM to download all the dependencies for this module:

    cd wherever_you_cloned_this_repo
    npm install
    

    Now you can build the package (linting and running tests along the way):

    grunt
    

    Or you can just run the linting tool and tests:

    grunt test
    

    Or you can make Grunt watch for changes to the sources/specs and auto-rebuild after each change:

    grunt watch
    

    The browser-ready output files will be dumped at the following locations:

    • dist/knockout-projections.js
    • dist/knockout-projections.min.js

    License - Apache 2.0

    Copyright (c) Microsoft Corporation

    All rights reserved.

    Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

    THIS CODE IS PROVIDED AS IS BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OR CONDITIONS OF TITLE, FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABLITY OR NON-INFRINGEMENT.

    See the Apache Version 2.0 License for specific language governing permissions and limitations under the License.

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