Dependency Injection / IoC — Simple

flowchart LR
  Caller[App composition root]
  Service[Service]
  DepA([Dependency A])
  DepB([Dependency B])
  Caller -->|new with deps| Service
  Caller -->|new| DepA
  Caller -->|new| DepB
  Service -.uses.-> DepA
  Service -.uses.-> DepB

  classDef p fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
  classDef s fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
  classDef r fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
  class Caller p;
  class Service s;
  class DepA,DepB r;

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class Caller,Service,DepA,DepB service;

A class should receive its collaborators, not create them. The component that wires everything together is the "composition root" — usually main() or a DI container's config.