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This article is fully based on Gamma's Design Patterns, Elements of Reusable Object-Oriented Software [1].

Intend

Provide a surrogate or placeholder for another object to control access to it. Also know as Surrogate.

Motivation

One reason for controlling access to an object is to defer the full cost of its creation and initialization until we actually need to use it. Consider a document editor that can embed graphical objects in a document. Some graphical objects, like large raster images, can be expensive to create. But opening a document should be fast, so we should avoid creating all the expensive objects at once when the document is opened. This isn't necessary anyway, because not all of these objects will be visible in the document at the same time.

These constraints would suggest creating each expensive object on demand, which in this case occurs when an image becomes visible. But what do we put in the document in place of the image? And how can we hide the fact that the image is created on demand so that we don't complicate the editor's implementation? This optimization shouldn't impact the rendering and formatting code, for example.

The solution is to use another object, an image proxy, that acts as a stand-in for the real image. The proxy acts just like the image and takes care of instantiating it when it's required.

Proxy-Gamma-001.png

See also

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Reference

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  1. Design Pattern, Elements of Reusable Object-Oriented Software, Erich Gamma, Ralph Johnson, Richard Helm, John Vlissides, First Published: October 21, 1994