JavaScript/Notes/PrivateProxy: Difference between revisions
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<code>Program.getOrCreate</code> returns a <code>ProgramProxy</code>. | |||
<code>ProgramProxy</code> has two methods, both of which delegate to <code>Program</code> instances. | |||
The body of buildHTML | The body of buildHTML: | ||
<source lang="javascript"> | <source lang="javascript"> | ||
setPhases: function(phases) { | setPhases: function(phases) { | ||
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</source> | </source> | ||
Because <code>ProgramProxy</code> and each <code>Program</code> have the same id, any <code>Program</code> or <code>ProgramProxy</code> instance counterpart can access its corresponding counterpart from within any of its instance methods. | |||
This is implemented by both <code>Program</code> and <code>ProgramProxy</code> instances being stored in a corresponding object for, keyed by id. | |||
See also: [http://sourcemaking.com/design_patterns/proxy Proxy Design Pattern]. |
Revision as of 19:12, 17 May 2014
Proxy
A proxy is as an interface to something else.
Private Proxy
The private proxy exposes a public interface object. This public interface object delegates some of its responsibility to a private instance.
Structural Pattern: The private and public instances are tightly coupled. They share data with each other only within the scope where they are defined.
Usage: Details and complexities of the underlying object must be hidden. By exposing a very limited set of methods, the public interface offers little opportunity to modify and break the objects it produces, while protecting the integrity of the underlying, corresponding private objects.
Example: User wants to be able to interact with "program" objects on a calendar. The program can be then modified by the user.
The application handles modification of program objects through a ProgramProxy, by calling setPhases, setCampaigns. The client of the API may call buildHTML to generate new HTML for the program or toJSON to get a JSON representation of it.
Alternatives
The popular alternative to this approach is MVC with libraries such as Backbone or YUI app framework. Prefabricated solutions often entail a much larger codebase with less specificity to the design of your particular situation.
<source lang="javascript"> var Program = function(A) {
"use strict";
var phaseListClassName = "program-phase-list", phaseClassName = "program-phase";
function Program(id, config) { var start = config.startDate, end = config.endDate; this.id = id; this.dateRange = new A.DateRange(start, end); this.phases = config.phases.slice(); }
var programs = {}, programProxies = {};
var phaseListElement = document.createElement("ul"), phaseElement = document.createElement("li"); phaseListElement.className = phaseListClassName; phaseElement.className = phaseClassName;
function ProgramProxy(id, config) { this.id = id; this.start = config.startDate; this.displayName = config.name; this.end = config.endDate; programs[id] = new Program(id, config); }
ProgramProxy.prototype = { setPhases: function(phases) { if(phases && phases.slice) { programs[this.id].phases = phases.slice(); } },
buildHTML: function() { var program = programs[this.id], html = ""; html += buildPhaseHTML(program); html += buildCampaignHTML(program); return html; } };
var div = document.createElement("div"); function buildPhaseHTML(program) { div.innerHTML = ""; var i, li, phases = program.phases, len = phases.length, phaseRationalValue, ul = phaseListElement.cloneNode(false); ul.id = "program-" + program.id; for(i = 0; i < len; i++) { li = phaseElement.cloneNode(false); li.id = "program-" + program.id + "-phase-" + phases[i].name + ""; li.style.width = getPhaseWidth(phases[i], program.dateRange.duration); ul.appendChild(li); } div.appendChild(ul); return div.innerHTML; } function buildCampaignHTML() { return"";
"
- "
+ "
- fixme:\u00A0buildCampaignHTML" + "
";
}
function getPhaseWidth(phase, programDuration) { var phaseDateRange = new A.DateRange( phase.startDate, phase.endDate); var rationalValue = phaseDateRange.duration/programDuration; return (0|100 * rationalValue) + "%"; }
function getOrCreateProgram(config) { return programProxies[config.id] || (programProxies[config.id] = new ProgramProxy(config.id, config)); }
function removeProgram(id) { if(!programs[id]) { throw new ReferenceError("Program: " + id + " does not exist." ); } delete programs[id]; delete programProxies[id]; }
return { getOrCreate: getOrCreateProgram, remove: removeProgram };
}({
DateRange: DateRange // defined elsewhere.
}); </source>
Program.getOrCreate
returns a ProgramProxy
.
ProgramProxy
has two methods, both of which delegate to Program
instances.
The body of buildHTML: <source lang="javascript"> setPhases: function(phases) {
if(phases && phases.slice) { programs[id].phases = phases.slice(); }
} </source>
Because ProgramProxy
and each Program
have the same id, any Program
or ProgramProxy
instance counterpart can access its corresponding counterpart from within any of its instance methods.
This is implemented by both Program
and ProgramProxy
instances being stored in a corresponding object for, keyed by id.
See also: Proxy Design Pattern.