In Vue 3, the KeepAlive component improves performance by caching inactive component instances. Its core mechanisms include LRU cache management, special handling of the virtual DOM, and lifecycle hook interception to preserve component state through hidden containers. It offers flexible configuration with include/exclude support and works seamlessly with Suspense and Transition components. Server-side rendering requires special handling, and the internal implementation includes multiple performance optimizations such as deferred unmounting and batch node movement. Developers can customize caching strategies but must consider memory management. The source code involves coordination between the renderer, reactivity system, and component lifecycle, implemented in approximately 500 lines of code.
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Vue3's dynamic components utilize the built-in `component` component and the `is` attribute to achieve runtime component switching. The core mechanism involves virtual DOM creation and component resolution. The `is` attribute supports various forms such as string component names, component objects, and asynchronous components. Dynamic components are often paired with `keep-alive` to enable state caching, managed through `shapeFlags` and `key` for cache strategies. The rendering and update process handles component switching and state changes. Vue3 optimizes dynamic components with static hoisting, patch flags, and event caching, while also addressing edge cases like transition animations, attribute inheritance, and asynchronous loading. In the Composition API, `ref` and `computed` can flexibly control dynamic components, and the compiler generates special flags to optimize rendering. SSR environments require special handling for asynchronous components. TypeScript provides robust type support, and custom resolution strategies can be configured via `app.config`. DevTools includes dedicated display optimizations for dynamic components. Dynamic components can also work seamlessly with other features like `Teleport`, `Suspense`, and `v-model`. Under the hood, VNode special attributes mark the state of dynamic components.
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In Vue 3, asynchronous components, implemented via `defineAsyncComponent`, enable on-demand loading to reduce initial load time, support configuring loading and error state components, and are built on ES module dynamic imports integrated with the reactive system. They offer advanced strategies like preloading and intelligent delay, can coordinate with `Suspense` to manage loading states, and provide multiple error-handling mechanisms. Performance is optimized through grouped loading and caching. In Vite environments, `import.meta.glob` enables flexible loading, while SSR requires special handling to ensure hydration matching. The API can also be extended for custom loading behavior, making it ideal for performance optimization in large-scale applications.
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The Vue 3 component update mechanism primarily relies on the reactivity system. When reactive data changes, it triggers component re-rendering. Modifying reactive object properties or changing ref values will trigger updates. Changes in props passed from parent components or slot content will also cause child components to update. You can force an update using `forceUpdate`, but this is generally not recommended. Vue uses an asynchronous update queue to batch multiple data changes within the same event loop. Computed property value changes will trigger updates in dependent components. Changes in elements controlled by conditional rendering (`v-if`) or list rendering (`v-for`) will also trigger updates. Changes in custom directive binding values will trigger the `update` hook. Global state management (e.g., Pinia state changes) will also trigger component updates. Functional component updates depend entirely on props changes. Asynchronous components trigger updates once loaded. `keep-alive` cached components do not re-render when activated but trigger the `activated` hook. Error boundaries trigger updates when errors are caught. Transition effect changes will also cause related components to update.
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Vue3's slot mechanism is implemented through the collaboration of the compiler and runtime. During the compilation phase, slot content in the template is transformed into specific data structures, including default slots, named slots, and scoped slots. At runtime, the `initSlots` function initializes the slot content and mounts it to the component instance. During rendering, the `renderSlot` function processes the slot content, supporting dynamic slot names and various performance optimizations such as static node hoisting and slot flags. Advanced techniques like render delegation, slot forwarding, and integration with the Composition API are also covered. Finally, Vue3 provides full type support for slots, including the implementation of generic slot components.
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Vue3's Props system is the core mechanism for component communication, supporting features like type checking and default values. During component instantiation, Props resolution occurs in the `setupComponent` phase. Vue matches the props passed from the parent component with the child component's defined options, standardizing the process by converting various forms of prop definitions into a unified object format. It supports basic type checking, multiple possible types, required fields, and custom validation functions. When a prop is not provided, default values are handled, with special conversion rules for boolean types. Vue3 uses Proxy to create reactive props while maintaining read-only properties to ensure unidirectional data flow. Custom validation can be implemented for complex logic, and stricter type checking can be achieved with TypeScript. Compared to Vue2, Vue3 introduces many improvements, such as removing the `sync` modifier and offering better TypeScript integration. During development, `toRefs` can be used for debugging props. Edge cases require attention to the distinction between `undefined` and `null`, as well as shared reference issues with default values for objects and arrays. In the Composition API, maintaining reactivity is crucial, and dynamic prop binding should be combined with TypeScript for precise type inference.
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The creation of a Vue 3 component instance involves multiple key steps from virtual DOM to real DOM conversion. First, a virtual node containing component structure information is created via `createVNode`. Then, `setupComponent` initializes props and slots and processes the composition state returned by the `setup` function. Template-based components undergo compilation to generate a render function, and the instantiation process establishes reactive associations. Dependencies are tracked via `effect` to trigger updates. Lifecycle hooks are called in a specific order at designated stages. Parent components recursively process child components to ensure proper initialization of the component tree. Async components follow a special loading flow, while higher-order components undergo additional wrapping. An error-handling mechanism captures various exceptions. Performance optimizations include static hoisting and event caching. Compared to Vue 2, Vue 3's improvements include the Composition API, finer-grained reactivity tracking, and better TypeScript integration.
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In Vue 3, when a component is defined through an options object containing various configurations such as data, methods, props, etc., it is converted into an internal representation. The component options undergo normalization to standardize them into a unified format, applying global configurations and mixins. The internal representation includes properties like uid, type, name, render, setup, props, and emits. The props definition is normalized for consistency. The `setup` function, as the core of the Composition API, executes before instance creation and returns either a render function or reactive data. The render function is ultimately converted into a VNode. Functional components and asynchronous components have special internal representations, while built-in components like `KeepAlive` have unique implementations. Component types are marked using bitmask flags, and caching optimizations are implemented via WeakMap. Component inheritance is based on the prototype chain, and directives and slots have corresponding internal handling. Lifecycle hooks are transformed into array form and tightly integrated with the reactivity system. The template is compiled into a render function, and the instance accesses data through a proxy. The internal representation includes optimization-related markers for updates.
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Vue 3 achieves the coexistence of the Options API and Composition API through a unified reactivity system and component instance management mechanism. The Composition API is transformed into an equivalent Options API form via the `setup` function. A property access proxy layer resolves attributes in the order of `setupState`, `data`, `props`, etc. Lifecycle hooks employ a merging strategy to ensure execution order. Computed properties are uniformly converted into Composition API implementations. Method binding is handled via `bind` to address context differences while sharing the same reactivity system. The template compilation adaptation layer uniformly processes property access. The type system achieves compatibility through generics. Custom directives are uniformly handled via `withDirectives`. `provide` and `inject` enable cross-API communication using the same logic. Async components are uniformly processed with `defineAsyncComponent`. Global APIs adapt based on component type. The rendering context encapsulates and integrates attributes from both APIs.
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Vue 3 introduced the Composition API to address the code organization issues of the Options API in complex components by centralizing related logic, improving code readability and maintainability. The Composition API leverages Proxy for its reactivity system, which is more efficient than Vue 2's Object.defineProperty, supporting dynamic property addition. Compile-time optimizations generate more efficient render functions for templates. The Composition API employs fine-grained dependency tracking, reducing unnecessary overhead, making it particularly suitable for functional components with more efficient memory management, as it automatically cleans up reactive state when components are unmounted. It works seamlessly with Suspense to optimize asynchronous loading and disables the reactivity system in SSR environments to reduce overhead. Deep integration with Vite enhances the development experience. Performance tests show that the Composition API delivers significant advantages in large-scale applications, typically improving computation speed and memory usage by 10-30%.
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