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angulardependency-injectionangular-di

useClass vs useExisting

When should we use useExisting provider instead of useClass?

providers: [
{provide: Class1, useClass: Class1}, 
{provide: Class2, useExisting: Class2}]

REMARK: I have not found an exact question on SO. And for better indexing decided to create this specific one here, although I found this answers:

but would like to have more real examples

Solution

  • Angular creates factory for providers that will be used to instantiate provider.

    I usually use the following table to understand the difference between types of providers.

    enter image description here

    As we can see in the picture above all of providers can be presented similar useFactory. When it's the time to get an instance of provider angular just calls factory function.

    So for useClass angular resolves dependency from parameters array and then calls constructor with parameters while for useExisting angular gets existing resolved instance and returns it.

    Use cases:

    1) Don't expose full functionallity

    { provide: PublicApi, useExisting: PrivateImpl }

{ provide: MinimalLogger, useExisting: LoggerService }

{ provide: PlatformRef, useExisting: PlatformRef_ }

{ provide: ApplicationRef, useExisting: ApplicationRef_}

{ provide: Sanitizer, useExisting: DomSanitizer },

{ provide: Compiler, useExisting: JitCompiler }

    2) Build tree

    { provide: Parent, useExisting: forwardRef(() => TreeViewComponent) }

    3) Avoid circular dependency

    { provide: BaseComponent, useExisting: forwardRef(() => MyComponent) }

    4) Provide common tokens

    { provide: NG_VALIDATORS, useExisting: ForbiddenValidatorDirective, multi: true }

{ provide: NG_VALIDATORS, useExisting: forwardRef(() => EmailValidator),  multi: true }

{ provide: NgControl, useExisting: forwardRef(() => NgModel) }

{ provide: ControlContainer, useExisting: forwardRef(() => FormGroupDirective) }

{ provide: NG_VALUE_ACCESSOR, multi: true, useExisting: MyDatePickerComponent }

    If we replace useExisting with useClass, then we’d be registering a new class instance