Inject

The inject package contains the XP framework's dependency injection API. Its entry point class is the "Injector".

Binding

Values can be bound to the injector by using its bind() method. It accepts the type to bind to, an optional name and these different scenarios:

• Binding a class: The typical usecase, where we bind an interface to its concrete implementation.
• Binding an instance: By binding a type to an existing instance, we can create a singleton model.
• Binding a provider: If we need more complicated code to create an instance, we can bind to a provider.
• Binding a named lookup: If we want control over the binding lookups for a type, we can bind to a Named instance.

use inject\{Injector, Bindings};
use com\example\{Report, HtmlReport, Storage, InFileSystem};

// Manually
$injector= new Injector(Bindings::using()
  ->typed(Report::class, HtmlReport::class)
  ->singleton(Storage::class, new InFileSystem('.'))
  ->named('title', 'Report title')
);

// Reusable via Bindings instances
class ApplicationDefaults extends Bindings {

  public function configure($injector) {
    $injector->bind(Report::class, HtmlReport::class);
    $injector->bind(Storage::class, new InFileSystem('.'));
    $injector->bind('string', 'Report title', 'title');
  }
}

$injector= new Injector(new ApplicationDefaults());

Instance creation

Keep in mind: "injector.get() is the new 'new'". To create objects and perform injection, use the Injector's get() method instead of using the new keyword or factories.

use inject\Injector;

$injector->bind(Report::class, HtmlReport::class);

// Explicit binding: Lookup finds binding to HtmlReport, creates instance.
$instance= $injector->get(Report::class);

// Implicit binding: No previous binding, TextReport instantiable, thus created.
$instance= $injector->get(TextReport::class);

Manual calls are usually not necessary though, instead you'll use injection:

Injection

Injection is performed by looking at a type's constructor. Bound values will be passed according to the given type hint.

class ReportImpl implements Report {

  public function __construct(ReportWriter $writer) { ... }
}

You can supply the type by using parameter attributes in case where the PHP type system is not concise enough. If the bound value's name differs from the parameter name, you can supply a name argument.

use inject\Inject;

class ReportImpl implements Report {

  public function __construct(
    ReportWriter $writer,
    Format $format,
    #[Inject(type: 'string[]', name: 'report-titles')]
    $titles
  ) { ... }
}

When a required parameter is encountered and there is no bound value for this parameter, an inject.ProvisionException is raised.

class ReportWriter implements Writer {

  public function __construct(Storage $storage) { ... }
}

$injector= new Injector();
$report= $injector->get(ReportWriter::class);  // *** Storage not bound

Method and field injection are not supported.

Configuration

As seen above, bindings can be used instead of manually performing the wiring. You might want to configure some of your app's settings externally instead of harcoding them. Use the inject.ConfiguredBindings class for this:

$injector= new Injector(
  new ApplicationDefaults(),
  new ConfiguredBindings(new Properties('etc/app.ini'))
);

The syntax for these INI files is simple:

web.session.Sessions=web.session.InFileSystem("/tmp")
name="Application"

Providers

Providers allow implementing lazy-loading semantics. Every type bound to the injector can also be retrieved by a provider. Invoking its get() method will instantiate it.

$provider= $injector->get('inject.Provider<com.example.writers.ReportWriter>');

// ...later on
$instance= $provider->get();

Named lookups

If we need control over the lookup, we can bind instances of Named:

use inject\{Injector, Named, InstanceBinding};
use com\example\Value;

$inject= new Injector();
$inject->bind(Value::class, new class() extends Named {
  public function provides($name) { return true; }
  public function binding($name) { return new InstanceBinding(new Value($name)); }
});

$value= $inject->get(Value::class, 'default');  // new Value("default")