Angular 依赖注入机制根据providers定义生成注入实例的框架代码

/**
 * Converts a `SingleProvider` into a factory function.
 *
 * @param {?} provider provider to convert to factory
 * @param {?=} ngModuleType
 * @param {?=} providers
 * @return {?}
 */
function providerToFactory(provider, ngModuleType, providers) {
    /** @type {?} */
    let factory = undefined;
    if (isTypeProvider(provider)) {
        /** @type {?} */
        const unwrappedProvider = resolveForwardRef(provider);
        return getFactoryDef(unwrappedProvider) || injectableDefOrInjectorDefFactory(unwrappedProvider);
    }
    else {
        if (isValueProvider(provider)) {
            factory = (/**
             * @return {?}
             */
            () => resolveForwardRef(provider.useValue));
        }
        else if (isFactoryProvider(provider)) {
            factory = (/**
             * @return {?}
             */
            () => provider.useFactory(...injectArgs(provider.deps || [])));
        }
        else if (isExistingProvider(provider)) {
            factory = (/**
             * @return {?}
             */
            () => ɵɵinject(resolveForwardRef(provider.useExisting)));
        }
        else {
            /** @type {?} */
            const classRef = resolveForwardRef(provider &&
                (((/** @type {?} */ (provider))).useClass || provider.provide));
            if (!classRef) {
                throwInvalidProviderError(ngModuleType, providers, provider);
            }
            if (hasDeps(provider)) {
                factory = (/**
                 * @return {?}
                 */
                () => new (classRef)(...injectArgs(provider.deps)));
            }
            else {
                return getFactoryDef(classRef) || injectableDefOrInjectorDefFactory(classRef);
            }
        }
    }
    return factory;
}

UseValue和UseExisting可以在此处打断点调试：

而useClass的分支在下图，由此可见，useClass就是简单的new操作：

factory其实就是构造函数，只是由于Chrome开发者工具的限制，无法正确显示出来：

这里可见，useExisting如果没有实例，还是会调用构造函数new一个新的出来。

更多Jerry的原创文章，尽在：“汪子熙”: