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Ioc( Dependency Injection) - Teorie

Plecam de la principiul substitutiei a lui Liskovs:
Daca S este un subtip a lui T, atunci obiectul de tip T, in interiorul programului, poate sa fie înlocuit de obiectul de tip S fara sa se altereze funcționalitatea programului.
Sursa
Pe scurt, acesta se refera la faptul ca putem sa avem referinte la interfete si nu la tipuri concrete in interiorul aplicatiei, a.i. in orice moment putem sa înlocuim implementarea interfeței, fara a fi nevoiti sa schimbam codul.
Dependency injection se refera la faptul ca responsabilitatea initalizarii, construirii si a duratei de viata a unui obiect nu o are obiectul care contine referinta la obiectul nostru. Aceste elemente o sa fie controlate de catre un controler.
Exista trei tipuri prin care putem sa injectam dependentele prin:
  • constructor;
  • proprietati;
  • metode;
Prima varianta este cea mai des folosita. Prin acest mecanism o sa putem fi siguri ca dependența a fost injectata in interiorul obiectului nostru. Daca vrem sa injectam prin proprietati atunci trebuie sa avem grija ca fiecare propietate sa aibe set si valoarea ei sa fie setata inainte ca o portiune din cod sa aibe nevoie de aceasta referinta. Ultima varianta este mai rar folosita. Este folosita mai ales in momentele in care vrem sa folosim o dependenta doar pe perioada executiei metodei si atat( uneori se foloseste pentru initializarea obiectelor referite, dar acest caz intra in cazul unu pe care l-am descris).
Coupled code: Cand un obiect este responsabil de construirea, inițializarea si durata de viata a unui obiect de care el depinde.

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