The internal principles of composite functions

Vue 3's composable functions are a core feature that encapsulates reusable logic through plain JavaScript functions, leveraging Vue's provided APIs to achieve functionality. When composable functions are called within a component's `setup`, Vue creates a special execution context bound to the current component instance. Reactive state management is implemented by tracking the relationship between composable functions and component instances via `WeakMap`. Lifecycle hooks are collected and bound to the current component, allowing multiple composable functions to register their own hooks. The dependency injection mechanism uses `provide`/`inject` to enable cross-level communication, internally maintaining a component-tree-level injector chain. Asynchronous state handling is managed through a scheduler to control async updates. Composable function reuse strategies include reference-based sharing and the factory pattern for independent state. Reactive effects automatically track dependencies, and computed properties are implemented via `effect`. Type support is robust, with TypeScript automatically inferring return types. Compile-time optimizations enhance performance by caching frequently called results. They can also be used in render functions for unified processing. The `setup` return function provides error handling mechanisms to capture and prevent error propagation.

The implementation of getCurrentInstance

`getCurrentInstance` is a core method in Vue 3's Composition API, used to retrieve the current component instance context. It returns an instance object containing properties like `uid`, `parent`, and `appContext`. Typical use cases include accessing the component tree within the `setup` function and developing utility libraries. Its implementation relies on a global stack to manage the current instance, with `setCurrentInstance` handling the stack during component mounting. Asynchronous scenarios require manually saving the instance. Edge cases include returning `null` when called outside a component context and differences in server-side rendering. The article explores advanced applications like recursive component access and custom `provide`/`inject`, while also discussing performance optimization techniques, comparisons with Vue 2's `this` access, type system integration, and common troubleshooting methods. Finally, it provides debugging tips such as instance snapshots and component tree visualization to help developers use this API efficiently.

The exposure method of component instances

In Vue 3, the component instance exposure mechanism achieves precise control through the Composition API. By default, it exposes the setup return object and component options. The `expose` function can limit exposure to specific properties and methods. Template refs access DOM or child component instances via `ref`, and in TypeScript environments, component reference types can be declared. For handling multiple component references, an array is used for storage. Dynamic components also support instance exposure. Higher-order components (HOCs) forward child component instances, while render functions require manual ref handling. The component instance lifecycle affects its accessibility, and asynchronous scenarios require combining `nextTick` for access. During debugging, internal instances can be inspected. Compared to Vue 2, explicit `ref` declaration is required, with better support. Frequent instance access may cause performance issues, and edge cases require handling potentially `null` references.

The implementation principle of provide/inject

In Vue 3, `provide`/`inject` is a cross-level component communication API that allows ancestor components to inject dependencies into descendant components without passing them down layer by layer via props. Its core implementation is based on Vue's reactivity system and component instance dependency collection mechanism. Each component instance's `provides` property defaults to pointing to the parent component's `provides`, forming a prototype chain. When `provide` is called, a new object is created with its prototype set to the parent component's `provides`. `inject` traverses up the component tree to locate the dependency and supports reactive data handling—when provided reactive data changes, all injecting components update automatically. Compared to Vue 2, Vue 3 leverages the prototype chain for better performance, supports the Composition API, and offers improved type inference. Practical applications include global state sharing, form component design, and plugin development. Best practices recommend avoiding overuse, using Symbol keys for type safety, and checking for unprovided keys with warnings in development environments.

The registration mechanism of lifecycle hooks

Vue3's lifecycle hook registration mechanism is designed around the Composition API, with core logic centralized in the `component.ts` file. Unlike Vue2's Options API, Vue3 registers hooks by calling lifecycle APIs inside the `setup` function, enabling more precise type inference and better logic reuse. Internally, it uses the `LifecycleHooks` enum to define all supported hook types. Each component instance maintains a `hooks` object to store callback functions. When APIs like `onMounted` are called, the `injectHook` function is executed. For async components, special logic is added to lifecycle handling—when a `Suspense` context is detected, certain hooks are deferred. The execution order of lifecycle hooks is ensured by the `invokeArrayFns` function. In SSR scenarios, client-only lifecycle hooks are automatically skipped. Error-capturing lifecycles have independent handling logic and are deeply integrated with the `KeepAlive` component. In development mode, additional validation is applied to lifecycle usage. High-frequency lifecycle hooks are optimized and tightly coupled with the scheduler system. TypeScript type definitions ensure type safety while handling merges between the Options API and Composition API. Some lifecycles trigger specific reactive system operations. Custom renderers can extend or modify lifecycle behavior. The `Teleport` component specially handles certain lifecycles, and some hooks access VNode-related information. DevTools monitors lifecycle events for debugging purposes.

The execution timing of the setup function

The `setup` function is the core of Vue 3's Composition API. It executes during the component instance creation process, earlier than template compilation and DOM mounting, equivalent to the phase between `beforeCreate` and `created` in the Options API. At this stage, the component instance is partially initialized, allowing access to props and context but not `this` or template refs. It serves as the primary place to declare reactive state, ensuring the reactivity system initializes before rendering. For async components, `setup` delays execution until after loading completes. During server-side rendering, the client does not re-execute `setup`. Synchronous errors are caught immediately, while async errors require manual handling. Normally, `setup` runs only once, though it may re-execute during hot updates. It is the ideal location for `provide` and should avoid heavy operations to optimize performance. Its early execution also impacts how TypeScript type inference works.

Compilation processing of custom directives

In Vue 3, the compilation process of custom directives includes template parsing, AST transformation, and code generation. The compiler uses regular expressions to match directives and generates AST node attributes, including the directive name, argument, modifiers, and value. During the AST transformation phase, directive nodes are standardized, their names are validated for legality, and dynamic arguments are processed. In the code generation phase, directives are converted into `withDirectives` function calls. Built-in directives like `v-show` and `v-model` receive special optimizations. At runtime, directives are applied via `withDirectives`, and their lifecycle hooks are invoked during the patch process. Dynamic arguments require additional handling for expression changes. The directive lifecycle remains synchronized with the component lifecycle. When the same directive is used multiple times, the latter definition overrides the former. Static directives are marked for optimization and skipped during the diff phase.

The collaboration between the compiler and the runtime

Vue3's reactivity system achieves high performance through close collaboration between the compiler and runtime. The compiler transforms templates into render functions and optimizes the AST, while the runtime executes these functions and manages reactive state. The compiler processes directives, events, and interpolations to generate standard JavaScript code. The runtime leverages Block Tree and Patch Flags to optimize performance, with static nodes hoisted to avoid repeated creation. The reactivity system uses Proxy to trigger updates on data changes. Directives like `v-if` are compiled into ternary expressions, and event handlers are inlined. Slot content is transformed into functions, while static content is hoisted to reduce VNode creation. Custom directives are converted into configuration objects, and dynamic components are resolved via functions. The Composition API automatically handles `ref` value access, and type-safe templates utilize TypeScript for type inference. Server-side rendering distinguishes between client and server processing. This collaborative mechanism allows Vue3 to maintain developer convenience while ensuring efficient runtime performance.

The compilation result of dynamic binding

In Vue 3, dynamic binding is achieved through `v-bind` or its shorthand, enabling attribute binding where the value can be reactive data. When the data changes, the DOM updates automatically. The compiler transforms dynamic bindings into specific JavaScript code and generates `patchFlag`, a bitmask used to optimize the update process. `patchFlag` marks the type of dynamic binding (e.g., `CLASS`, `STYLE`, `PROPS`, etc.), allowing the diff algorithm to quickly determine which properties need comparison, avoiding full diffing. The compiler hoists static content and applies special handling for dynamic components, directives, and scoped slots. The compilation process includes static analysis, type validation, and performance optimization strategies. Proper use of dynamic binding can enhance application performance.

Compatibility issues between Android and iOS

In cross-platform development between Android and iOS, numerous compatibility challenges arise, primarily due to differences in system APIs, UI component rendering, style presentation, and native feature invocation. Under the uni-app framework, developers need to pay attention to platform-specific issues such as device information retrieval, permission handling, button styles, and scrolling behavior. Most compatibility issues can be resolved through conditional compilation and platform-specific code adjustments. Additionally, performance optimization, third-party plugin adaptation, and new issues introduced by version updates require special attention. Maintaining a consistent user experience is key. Leveraging platform-specific features wisely and establishing a robust testing process can enhance application quality. Community resources offer abundant solutions to address compatibility problems.