setup(props, { attrs, slots, emit, expose }) {
   ...
 }
 或
 setup(props, content) {
   const { attrs, slots, emit, expose } = content
 }

function setupStatefulComponent(instance, isSSR) {
    var _a;
    const Component = instance.type;
    {
        if (Component.name) {
            validateComponentName(Component.name, instance.appContext.config);
        }
        if (Component.components) {
            const names = Object.keys(Component.components);
            for (let i = 0; i < names.length; i++) {
                validateComponentName(names[i], instance.appContext.config);
            }
        }
        if (Component.directives) {
            const names = Object.keys(Component.directives);
            for (let i = 0; i < names.length; i++) {
                validateDirectiveName(names[i]);
            }
        }
        if (Component.compilerOptions && isRuntimeOnly()) {
            warn(`"compilerOptions" is only supported when using a build of Vue that ` +
                `includes the runtime compiler. Since you are using a runtime-only ` +
                `build, the options should be passed via your build tool config instead.`);
        }
    }
    // 0. create render proxy property access cache
    instance.accessCache = Object.create(null);
    // 1. create public instance / render proxy
    // also mark it raw so it's never observed
    instance.proxy = reactivity.markRaw(new Proxy(instance.ctx, PublicInstanceProxyHandlers));
    {
        exposePropsOnRenderContext(instance);
    }
    // 2. call setup()
    const { setup } = Component;
    if (setup) {
        const setupContext = (instance.setupContext =
            setup.length > 1 ? createSetupContext(instance) : null);
        setCurrentInstance(instance);
        reactivity.pauseTracking();
        const setupResult = callWithErrorHandling(setup, instance, 0 /* ErrorCodes.SETUP_FUNCTION */, [reactivity.shallowReadonly(instance.props) , setupContext]);
        reactivity.resetTracking();
        unsetCurrentInstance();
        if (shared.isPromise(setupResult)) {
            setupResult.then(unsetCurrentInstance, unsetCurrentInstance);
            if (isSSR) {
                // return the promise so server-renderer can wait on it
                return setupResult
                    .then((resolvedResult) => {
                    handleSetupResult(instance, resolvedResult, isSSR);
                })
                    .catch(e => {
                    handleError(e, instance, 0 /* ErrorCodes.SETUP_FUNCTION */);
                });
            }
            else {
                // async setup returned Promise.
                // bail here and wait for re-entry.
                instance.asyncDep = setupResult;
                if (!instance.suspense) {
                    const name = (_a = Component.name) !== null && _a !== void 0 ? _a : 'Anonymous';
                    warn(`Component <${name}>: setup function returned a promise, but no ` +
                        `<Suspense> boundary was found in the parent component tree. ` +
                        `A component with async setup() must be nested in a <Suspense> ` +
                        `in order to be rendered.`);
                }
            }
        }
        else {
            handleSetupResult(instance, setupResult, isSSR);
        }
    }
    else {
        finishComponentSetup(instance, isSSR);
    }
}

function createSetupContext(instance) {
    const expose = exposed => {
        if (instance.exposed) {
            warn(`expose() should be called only once per setup().`);
        }
        instance.exposed = exposed || {};
    };
    let attrs;
    {
        // We use getters in dev in case libs like test-utils overwrite instance
        // properties (overwrites should not be done in prod)
        return Object.freeze({
            get attrs() {
                return attrs || (attrs = createAttrsProxy(instance));
            },
            get slots() {
                return reactivity.shallowReadonly(instance.slots);
            },
            get emit() {
                return (event, ...args) => instance.emit(event, ...args);
            },
            expose
        });
    }
}

import { ref } from 'vue'
export default {
  setup(_, { expose }) {
    const count = ref(0)
 
    function increment() {
      count.value++
    }
    
    // 僅僅暴露 increment 給父組件
    expose({
      increment
    })
 
    return { increment, count }
  }
}

     if (shared.isPromise(setupResult)) {
            setupResult.then(unsetCurrentInstance, unsetCurrentInstance);
            if (isSSR) {
                // return the promise so server-renderer can wait on it
                return setupResult
                    .then((resolvedResult) => {
                    handleSetupResult(instance, resolvedResult, isSSR);
                })
                    .catch(e => {
                    handleError(e, instance, 0 /* ErrorCodes.SETUP_FUNCTION */);
                });
            }
            else {
                // async setup returned Promise.
                // bail here and wait for re-entry.
                instance.asyncDep = setupResult;
                if (!instance.suspense) {
                    const name = (_a = Component.name) !== null && _a !== void 0 ? _a : 'Anonymous';
                    warn(`Component <${name}>: setup function returned a promise, but no ` +
                        `<Suspense> boundary was found in the parent component tree. ` +
                        `A component with async setup() must be nested in a <Suspense> ` +
                        `in order to be rendered.`);
                }
            }
        }

else {
            handleSetupResult(instance, setupResult, isSSR);
        }

function handleSetupResult(instance, setupResult, isSSR) {
    if (shared.isFunction(setupResult)) {
        // setup returned an inline render function
        if (instance.type.__ssrInlineRender) {
            // when the function's name is `ssrRender` (compiled by SFC inline mode),
            // set it as ssrRender instead.
            instance.ssrRender = setupResult;
        }
        else {
            instance.render = setupResult;
        }
    }
    else if (shared.isObject(setupResult)) {
        if (isVNode(setupResult)) {
            warn(`setup() should not return VNodes directly - ` +
                `return a render function instead.`);
        }
        // setup returned bindings.
        // assuming a render function compiled from template is present.
        {
            instance.devtoolsRawSetupState = setupResult;
        }
        instance.setupState = reactivity.proxyRefs(setupResult);
        {
            exposeSetupStateOnRenderContext(instance);
        }
    }
    else if (setupResult !== undefined) {
        warn(`setup() should return an object. Received: ${setupResult === null ? 'null' : typeof setupResult}`);
    }
    finishComponentSetup(instance, isSSR);
}

function finishComponentSetup(instance, isSSR, skipOptions) {
    const Component = instance.type;
    // template / render function normalization
    // could be already set when returned from setup()
    if (!instance.render) {
        // only do on-the-fly compile if not in SSR - SSR on-the-fly compilation
        // is done by server-renderer
        if (!isSSR && compile && !Component.render) {
            const template = Component.template;
            if (template) {
                {
                    startMeasure(instance, `compile`);
                }
                const { isCustomElement, compilerOptions } = instance.appContext.config;
                const { delimiters, compilerOptions: componentCompilerOptions } = Component;
                const finalCompilerOptions = shared.extend(shared.extend({
                    isCustomElement,
                    delimiters
                }, compilerOptions), componentCompilerOptions);
                Component.render = compile(template, finalCompilerOptions);
                {
                    endMeasure(instance, `compile`);
                }
            }
        }
        instance.render = (Component.render || shared.NOOP);
        // for runtime-compiled render functions using `with` blocks, the render
        // proxy used needs a different `has` handler which is more performant and
        // also only allows a whitelist of globals to fallthrough.
        if (installWithProxy) {
            installWithProxy(instance);
        }
    }
    // support for 2.x options
    {
        setCurrentInstance(instance);
        reactivity.pauseTracking();
        applyOptions(instance);
        reactivity.resetTracking();
        unsetCurrentInstance();
    }
    // warn missing template/render
    // the runtime compilation of template in SSR is done by server-render
    if (!Component.render && instance.render === shared.NOOP && !isSSR) {
        /* istanbul ignore if */
        if (!compile && Component.template) {
            warn(`Component provided template option but ` +
                `runtime compilation is not supported in this build of Vue.` +
                (``) /* should not happen */);
        }
        else {
            warn(`Component is missing template or render function.`);
        }
    }
}