Now, let’s imagine that we want to track the mouse position in another component as well. Can we reuse any of the code from our
paradigm, the problem of code reuse was solved using a technique called “mixins”. Let’s create a
that anyone can use to track the mouse position.
Problem solved, right?! Now anyone can simply “mix in”
to their component class to get the
of the mouse in
Then last year, arrived and ultimately the React team decided to move away from using
But there was a problem: ES6 classes don’t support mixins . Also, besides the fact they aren’t part of the ES6 class spec, mixins have other shortcomings too, many of which discussed at length in a post on the React blog .ES6 classes don’t support mixins
To summarize, problems with mixins are:ES6 classes Indirection Naming collisions
So instead of using mixins, many in the React community eventually settled on an alternative technique for code reuse known as higher-order components , or HOCs. In this paradigm, code is shared using a similar technique to decorators ; you start with the component that defines the bulk of the markup to be rendered and then wrap it in more components that contain the behavior you’d like to share. Instead of mixing in the behavior you need, you can just decorate your component! 😎decorators
It was a good solution that solved the problem of code reuse elegantly in the brave new world of ES6 classes, and the community adopted it in droves.
At this point I’d like to stop and ask: what did we gain by moving to higher-order components? Did we solve any of the problems we had with mixins?
Another problem that both mixins and HOCs share is that they use
. Ask yourself: where is the composition happening in the HOC paradigm? Static composition happens once, when the component class is created (e.g.
in the previous example).
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By Cassandra Fernandez, April 03, 2018
I’ve taken the opportunity to demonstrate the power of a serializable transaction, but in real life you’d want to put in a
will protect you from a duplicate insert, an added
will act as one more check to protect your application against incorrectly invoked transactions or buggy code. It’s worth having it in there.
It’s a common pattern to add jobs to a background queue during an HTTP request so that they can be worked out-of-band and a waiting client doesn’t have to block on an expensive operation.
Let’s add one more step to our user service above. In addition to creating user and user action records, we’ll also make an API request to an external support service to tell it that a new account’s been created. We’ll do that by queuing a background job because there’s no reason that it has to happen in-band with the request.
If we used a common job queue like Sidekiq to do this work, then in the case of a transaction rollback (like we talked about above where two transactions conflict), we could end up with an invalid job in the queue. It’s referencing data that no longer exists, so no matter how many times job workers retried it, it can never succeed.
A way around this is to create a job staging table into our database. Instead of sending jobs to the queue directly, they’re sent to a staging table first, and an enqueuer pulls them out in batches and puts them to the job queue.enqueuer
The enqueuer selects jobs, enqueues them, and then removes them from the staging table 2 . Here’s a rough implementation:
Because jobs are inserted into the staging table from within a transaction, its isolation property (ACID’s “I”) guarantees that they’re not visible to any other transaction until after the inserting transaction commits. A staged job that’s rolled back is never seen by the enqueuer, and doesn’t make it to the job queue.
I call this pattern a
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It’s also possible to just put the job queue directly in the database itself with a library like , but
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, this probably isn’t as good of an idea.
Increment/decrement operators increment or decrement the value of the object.
Pre-increment and pre-decrement operators increments or decrements the value of the object and returns a reference to the result.
Post-increment and post-decrement creates a copy of the object, increments or decrements the value of the object and returns the copy from before the increment or decrement.
The prefix increment and decrement expressions have the form
The operand expr of a built-in prefix increment or decrement operator must be a modifiable (non-const) lvalue of non-boolean arithmetic type or pointer to complete object type . For non-boolean operands, the expression ++ x is exactly equivalent to x + = 1 , and the expression -- x is exactly equivalent to x - = 1 , that is, the prefix increment or decrement is an lvalue expression that identifies the modified operand. All arithmetic conversion rules and pointer arithmetic rules defined for arithmetic operators apply and determine the implicit conversion (if any) applied to the operand as well as the return type of the expression.
If the operand of the pre-increment operator is of type bool , it is set to true (deprecated) . (until C++17)
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, for every optionally volatile-qualified arithmetic type
, and for every optionally volatile-qualified pointer
to optionally cv-qualified object type, the following function signatures participate in overload resolution:
The postfix increment and decrement expressions have the form
The operand expr of a built-in postfix increment or decrement operator must be a modifiable (non-const) lvalue of non-boolean arithmetic type or pointer to complete object type . The result is prvalue copy of the original value of the operand. As a side-effect, for non-boolean operands, the expression x ++ modifies the value of its operand as if by evaluating x + = 1 , and the expression x -- modifies the value of its operand as if by evaluating x - = 1 . All arithmetic conversion rules and pointer arithmetic rules defined for arithmetic operators apply and determine the implicit conversion (if any) applied to the operand as well as the return type of the expression.