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[email protected] the subject "subscribe".) This list has quite . With this book, the Gang of Four have made a seminalcontribution to software. Read "Gang of Four" by Liz Byrski available from Rakuten Kobo. Sign up today and get $5 off your first download. Gang of Four is a story of four very different. Read "The Gang Of Four" by Richard Lawther available from Rakuten Kobo. Sign up today and get £3 off your first download. An alien race that has been guiding.
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Out of Alice. Red Dust Dreaming. Eva Scott. Summer Secrets. Gone Fishing. Susan Duncan. Sisters and Brothers. Secrets of a Happy Marriage. Kakadu Sunset: The Porter Sisters 1. Annie Seaton. This has a number of advantages including: The freedom to have private functions and private members which can only be consumed by our module. As they aren't exposed to the rest of the page only our exported API is , they're considered truly private. Given that functions are declared normally and are named, it can be easier to show call stacks in a debugger when we're attempting to discover what function s threw an exception.
J Crowder has pointed out in the past, it also enables us to return different functions depending on the environment. In the past, I've seen developers use this to perform UA testing in order to provide a code-path in their module specific to IE, but we can easily opt for feature detection these days to achieve a similar goal.
Module Pattern Variations Import mixins This variation of the pattern demonstrates how globals e. This effectively allows us to import them and locally alias them as we wish.
Secondly, it supports private data - so, in the Module pattern, public parts of our code are able to touch the private parts, however the outside world is unable to touch the class's private parts no laughing! Oh, and thanks to David Engfer for the joke. Disadvantages The disadvantages of the Module pattern are that as we access both public and private members differently, when we wish to change visibility, we actually have to make changes to each place the member was used.
We also can't access private members in methods that are added to the object at a later point. That said, in many cases the Module pattern is still quite useful and when used correctly, certainly has the potential to improve the structure of our application. Other disadvantages include the inability to create automated unit tests for private members and additional complexity when bugs require hot fixes. It's simply not possible to patch privates.
Instead, one must override all public methods which interact with the buggy privates. Developers can't easily extend privates either, so it's worth remembering privates are not as flexible as they may initially appear. For further reading on the Module pattern, see Ben Cherry's excellent in-depth article on it.
The Revealing Module pattern came about as Heilmann was frustrated with the fact that he had to repeat the name of the main object when we wanted to call one public method from another or access public variables.
The result of his efforts was an updated pattern where we would simply define all of our functions and variables in the private scope and return an anonymous object with pointers to the private functionality we wished to reveal as public. It also makes it more clear at the end of the module which of our functions and variables may be accessed publicly which eases readability. Disadvantages A disadvantage of this pattern is that if a private function refers to a public function, that public function can't be overridden if a patch is necessary.
This is because the private function will continue to refer to the private implementation and the pattern doesn't apply to public members, only to functions. Public object members which refer to private variables are also subject to the no-patch rule notes above. As a result of this, modules created with the Revealing Module pattern may be more fragile than those created with the original Module pattern, so care should be taken during usage.
The Singleton Pattern The Singleton pattern is thus known because it restricts instantiation of a class to a single object. Classically, the Singleton pattern can be implemented by creating a class with a method that creates a new instance of the class if one doesn't exist. In the event of an instance already existing, it simply returns a reference to that object. Singletons differ from static classes or objects as we can delay their initialization, generally because they require some information that may not be available during initialization time.
They don't provide a way for code that is unaware of a previous reference to them to easily retrieve them.
FooSingleton above would be a subclass of BasicSingleton and implement the same interface. Why is deferring execution considered important for a Singleton? It is important to note the difference between a static instance of a class object and a Singleton: whilst a Singleton can be implemented as a static instance, it can also be constructed lazily, without the need for resources nor memory until this is actually needed.
If we have a static object that can be initialized directly, we need to ensure the code is always executed in the same order e. Both Singletons and static objects are useful but they shouldn't be overused - the same way in which we shouldn't overuse other patterns.
In practice, the Singleton pattern is useful when exactly one object is needed to coordinate others across a system. They're often an indication that modules in a system are either tightly coupled or that logic is overly spread across multiple parts of a codebase. Singletons can be more difficult to test due to issues ranging from hidden dependencies, the difficulty in creating multiple instances, difficulty in stubbing dependencies and so on.
Miller Medeiros has previously recommended this excellent article on the Singleton and its various issues for further reading as well as the comments to this article, discussing how Singletons can increase tight coupling. I'm happy to second these recommendations as both pieces raise many important points about this pattern that are also worth noting.
The Observer Pattern The Observer is a design pattern where an object known as a subject maintains a list of objects depending on it observers , automatically notifying them of any changes to state. When a subject needs to notify observers about something interesting happening, it broadcasts a notification to the observers which can include specific data related to the topic of the notification.