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API / @angular/core

Optional

A constructor parameter decorator that marks a dependency as optional.

参见

说明

The DI framework provides null if the dependency is not found. For example, the following code allows the possibility of a null result:

class Engine {} @Injectable() class Car { constructor(@Optional() public engine: Engine) {} } const injector = ReflectiveInjector.resolveAndCreate([Car]); expect(injector.get(Car).engine).toBeNull();

选项