(() => {
    // A template source represents a read/write way of accessing a template. This is to eliminate the need for template loading/saving
    // logic to be duplicated in every template engine (and means they can all work with anonymous templates, etc.)
    //
    // Two are provided by default:
    //  1. ko.templateSources.domElement       - reads/writes the text content of an arbitrary DOM element
    //  2. ko.templateSources.anonymousElement - uses ko.utils.domData to read/write text *associated* with the DOM element, but
    //                                           without reading/writing the actual element text content, since it will be overwritten
    //                                           with the rendered template output.
    // You can implement your own template source if you want to fetch/store templates somewhere other than in DOM elements.
    // Template sources need to have the following functions:
    //   text() 			- returns the template text from your storage location
    //   text(value)		- writes the supplied template text to your storage location
    //   data(key)			- reads values stored using data(key, value) - see below
    //   data(key, value)	- associates "value" with this template and the key "key". Is used to store information like "isRewritten".
    //
    // Optionally, template sources can also have the following functions:
    //   nodes()            - returns a DOM element containing the nodes of this template, where available
    //   nodes(value)       - writes the given DOM element to your storage location
    // If a DOM element is available for a given template source, template engines are encouraged to use it in preference over text()
    // for improved speed. However, all templateSources must supply text() even if they don't supply nodes().
    //
    // Once you've implemented a templateSource, make your template engine use it by subclassing whatever template engine you were
    // using and overriding "makeTemplateSource" to return an instance of your custom template source.

    ko.templateSources = {};

    // ---- ko.templateSources.domElement -----

    // template types
    var templateScript = 1,
        templateTextArea = 2,
        templateTemplate = 3,
        templateElement = 4;

    ko.templateSources.domElement = function(element) {
        this.domElement = element;

        if (element) {
            var tagNameLower = ko.utils.tagNameLower(element);
            this.templateType =
                tagNameLower === "script" ? templateScript :
                tagNameLower === "textarea" ? templateTextArea :
                    // For browsers with proper <template> element support, where the .content property gives a document fragment
                tagNameLower == "template" && element.content && element.content.nodeType === 11 ? templateTemplate :
                templateElement;
        }
    }

    ko.templateSources.domElement.prototype['text'] = function(/* valueToWrite */) {
        var elemContentsProperty = this.templateType === templateScript ? "text"
                                 : this.templateType === templateTextArea ? "value"
                                 : "innerHTML";

        if (arguments.length == 0) {
            return this.domElement[elemContentsProperty];
        }
        var valueToWrite = arguments[0];
        if (elemContentsProperty === "innerHTML")
            ko.utils.setHtml(this.domElement, valueToWrite);
        else
            this.domElement[elemContentsProperty] = valueToWrite;
    };

    var dataDomDataPrefix = ko.utils.domData.nextKey() + "_";
    ko.templateSources.domElement.prototype['data'] = function(key /*, valueToWrite */) {
        if (arguments.length === 1) {
            return ko.utils.domData.get(this.domElement, dataDomDataPrefix + key);
        }
        ko.utils.domData.set(this.domElement, dataDomDataPrefix + key, arguments[1]);
    };

    var templatesDomDataKey = ko.utils.domData.nextKey();
    function getTemplateDomData(element) {
        return ko.utils.domData.get(element, templatesDomDataKey) || {};
    }
    function setTemplateDomData(element, data) {
        ko.utils.domData.set(element, templatesDomDataKey, data);
    }

    ko.templateSources.domElement.prototype['nodes'] = function(/* valueToWrite */) {
        var element = this.domElement;
        if (arguments.length == 0) {
            var templateData = getTemplateDomData(element),
                nodes = templateData.containerData || (
                    this.templateType === templateTemplate ? element.content :
                    this.templateType === templateElement ? element :
                    undefined);
            if (!nodes || templateData.alwaysCheckText) {
                // If the template is associated with an element that stores the template as text,
                // parse and cache the nodes whenever there's new text content available. This allows
                // the user to update the template content by updating the text of template node.
                var text = this['text']();
                if (text && text !== templateData.textData) {
                    nodes = ko.utils.parseHtmlForTemplateNodes(text, element.ownerDocument);
                    setTemplateDomData(element, {containerData: nodes, textData: text, alwaysCheckText: true});
                }
            }
            return nodes;
        }
        var valueToWrite = arguments[0];
        if (this.templateType !== undefined) {
            this['text']("");   // clear the text from the node
        }
        setTemplateDomData(element, {containerData: valueToWrite});
    };

    // ---- ko.templateSources.anonymousTemplate -----
    // Anonymous templates are normally saved/retrieved as DOM nodes through "nodes".
    // For compatibility, you can also read "text"; it will be serialized from the nodes on demand.
    // Writing to "text" is still supported, but then the template data will not be available as DOM nodes.

    ko.templateSources.anonymousTemplate = function(element) {
        this.domElement = element;
    };
    ko.templateSources.anonymousTemplate.prototype = new ko.templateSources.domElement();
    ko.templateSources.anonymousTemplate.prototype.constructor = ko.templateSources.anonymousTemplate;
    ko.templateSources.anonymousTemplate.prototype['text'] = function(/* valueToWrite */) {
        if (arguments.length == 0) {
            var templateData = getTemplateDomData(this.domElement);
            if (templateData.textData === undefined && templateData.containerData)
                templateData.textData = templateData.containerData.innerHTML;
            return templateData.textData;
        }
        var valueToWrite = arguments[0];
        setTemplateDomData(this.domElement, {textData: valueToWrite});
    };

    ko.exportSymbol('templateSources', ko.templateSources);
    ko.exportSymbol('templateSources.domElement', ko.templateSources.domElement);
    ko.exportSymbol('templateSources.anonymousTemplate', ko.templateSources.anonymousTemplate);
})();