www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Dependency injection pattern

reply Suliman <evermind live.ru> writes:
Could anybody give small example of Dependency injection pattern? 

sure how to use them.

Also I would like get some explanation/comments for code.
May 13 2018
next sibling parent Jesse Phillips <Jesse.K.Phillips+D gmail.com> writes:
On Sunday, 13 May 2018 at 07:42:10 UTC, Suliman wrote:

 Could anybody give small example of Dependency injection 

 am not quite sure how to use them.

 Also I would like get some explanation/comments for code.


Here is a quick example of the difference, myProgram.execute 
utilizes a database connection.
dInjection.execute utilizes a dependency injected database 
connection.

--------------------
         class dbConnection {}

         class myProgram {
                 void execute() {
                         auto db = new dbConnection();
                         //...
                 }
         }

         class dInjection {
                 void execute(dbConnection db) {
                         //...
                 }
         }
--------------------

What you should notice from the first execute function is that 
the dependency, in this case dbConnection, is created as part of 
the application execution. While the second the dependency is 
declared at the function's arguments allowing the caller to 
inject the needed dependency.

This could go a step further and accept an interface for DB 
connections, but is not necessary to meat dependency injection.

Dependency injection also applies to magic numbers.

     enum maxProcessingTime = 3582;

If this were declared inside a function rather than taken as a 
parameter, then the function would not correctly use dependency 
injection.

Additionally, you could inject the dbConnection as part of the 
class constructor:

----------------------
         class preInjection {
                 dbConnection db;
                 this(dbConnection data) { db = data}
                 void execute() {
                         //...
                 }
         }

----------------------

Now I think what trips a lot of people up is that frameworks are 
created to help you do this. They try to make it easy to define 
all your dependencies in a single location and then you request 
the object you need and the DI framework will do whatever it 
needs to for building that object.
May 13 2018
prev sibling next sibling parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 05/13/2018 12:42 AM, Suliman wrote:

 Could anybody give small example of Dependency injection pattern? I 

 to use them.
 
 Also I would like get some explanation/comments for code.


The name and the unnecessary confusion in its documentation are 
unfortunate. And if you read the Wikipedia article, you are confronted 
with pharses like "inversion of control", "passing of a dependency to a 
dependent object", etc. Lots of correct but confusing stuff...  The 
following article starts with that unfortunate confusion:

 
https://www.reddit.com/r/programming/comments/8cwa4o/dependency_injection_is_a_25dollar_term_for_a/

I think this part from the Wikipedia topic is what it actually is: "The 
client should accept values passed in from outside". And this is how I 
would describe it: Provide some part of an object's behavior from the 
outside, for example during its construction. (Kevlin Henney was 
popularizing the same idea as "parameterize from above.")

The following struct does not use dependency injection because it is 
printing the value in a hardcoded way:

struct S {
     int i;

     void foo() {
         import std.stdio;
         writeln("value: ", i);    // Hardcoded behavior
     }
}

void main() {
     auto s = S();
     s.foo();
}

The following struct uses dependency injection because it takes a 
printer object in its constructor and uses that object to print the value:

interface Writer {
     void write(int i);
}

class XmlWriter : Writer {
     void write(int i) {
         import std.stdio;
         writefln("<value>%s</value>", i);
     }
}

struct S {
     int i;
     Writer writer;

      disable this();

     this(Writer writer) {
         this.writer = writer;
     }

     void foo() {
         writer.write(i);    // Uses the dependency
     }
}

void main() {
     auto s = S(new XmlWriter());
     s.foo();
}

Ali
May 14 2018
prev sibling parent IntegratedDimensions <IntegratedDimensions gmail.com> writes:
On Sunday, 13 May 2018 at 07:42:10 UTC, Suliman wrote:

 Could anybody give small example of Dependency injection 

 am not quite sure how to use them.

 Also I would like get some explanation/comments for code.


Dependency injection is explicit dependencies are injected in to 
an object for use by the creator of the object rather than hard 
coding dependencies in to the object.

For example, if you are creating an object that will lightly 
depend on an interface I then you can the interface and require 
that the object for the interface be injected(specified) by the 
user of the object.

class X
{
     I i;
     void foo() { /* uses i */ };
}

Then X.i = _i; is a dependency injection of _i so that X uses _i. 
Usually one does not want the dependency to be freely changed as 
to avoid changing a dependency in the middle of it being used.

Various schemes can be create to avoid these issues such as 
allowing injection only during construction or some special 
synchronization techniques to avoid usages. e.g.,

void foo() { I _i = i; /* uses _i */ };

Then reassignment won't change the dependency when a function is 
using it.

Whatever means, the idea is simple: One does not want to hard 
code dependencies directly so that refactoring code is ultimately 
easier. Dependencies can be injected in to the working code by 
the user. This makes the code depend on the type of the 
dependency so any specific dependency can be injected.

Top answer here says pretty much exactly what I'm saying:

https://stackoverflow.com/questions/3058/what-is-inversion-of-control?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa
May 16 2018