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| 1 | +- Start Date: 2020-03-25 |
| 2 | +- Target Major Version: 3.x |
| 3 | +- Reference Issues: https://github.com/vuejs/rfcs/pull/28 |
| 4 | + |
| 5 | +# Summary |
| 6 | + |
| 7 | +Introduce a dedicated API for defining async components. |
| 8 | + |
| 9 | +# Basic example |
| 10 | + |
| 11 | +```js |
| 12 | +import { createAsyncComponent } from "vue" |
| 13 | + |
| 14 | +// simple usage |
| 15 | +const AsyncFoo = createAsyncComponent(() => import("./Foo.vue")) |
| 16 | + |
| 17 | +// with options |
| 18 | +const AsyncFooWithOptions = createAsyncComponent({ |
| 19 | + loader: () => import("./Foo.vue"), |
| 20 | + loading: LoadingComponent, |
| 21 | + error: ErrorComponent, |
| 22 | + delay: 200, |
| 23 | + timeout: 3000 |
| 24 | +}) |
| 25 | +``` |
| 26 | + |
| 27 | +# Motivation |
| 28 | + |
| 29 | +Per changes introduced in [RFC-0008: Render Function API Change](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0008-render-function-api-change.md), in Vue 3 plain functions are now treated as functional components. Async components must now be explicitly defined via an API method. |
| 30 | + |
| 31 | +# Detailed design |
| 32 | + |
| 33 | +## Simple Usage |
| 34 | + |
| 35 | +```js |
| 36 | +import { createAsyncComponent } from "vue" |
| 37 | + |
| 38 | +// simple usage |
| 39 | +const AsyncFoo = createAsyncComponent(() => import("./Foo.vue")) |
| 40 | +``` |
| 41 | + |
| 42 | +`createAsyncComponent` can accept a loader function that returns a Promise resolving to the actual component. |
| 43 | + |
| 44 | +- If the resolved value is an ES module, the `default` export of the module will automatically be used as the component. |
| 45 | + |
| 46 | +- **Difference from 2.x:** Note that the loader function no longer receives the `resolve` and `reject` arguments like in 2.x - a Promise must always be returned. |
| 47 | + |
| 48 | + For code that relies on custom `resolve` and `reject` in the loader function, the conversion is straightforward: |
| 49 | + |
| 50 | + ```js |
| 51 | + // before |
| 52 | + const Foo = (resolve, reject) => { |
| 53 | + /* ... */ |
| 54 | + } |
| 55 | + |
| 56 | + // after |
| 57 | + const Foo = createAsyncComponent(() => new Promise((resolve, reject) => { |
| 58 | + /* ... */ |
| 59 | + })) |
| 60 | + ``` |
| 61 | + |
| 62 | +## Options Usage |
| 63 | + |
| 64 | +```js |
| 65 | +import { createAsyncComponent } from "vue" |
| 66 | + |
| 67 | +const AsyncFooWithOptions = createAsyncComponent({ |
| 68 | + loader: () => import("./Foo.vue"), |
| 69 | + loading: LoadingComponent, |
| 70 | + error: ErrorComponent, |
| 71 | + delay: 100, // default: 200 |
| 72 | + timeout: 3000, // default: Infinity |
| 73 | + suspensible: false // default: true |
| 74 | +}) |
| 75 | +``` |
| 76 | + |
| 77 | +Options except `loader` and `suspensible` works exactly the same as in 2.x. |
| 78 | + |
| 79 | +**Difference from 2.x:** |
| 80 | + |
| 81 | +The `component` option is replaced by the new `loader` option, which accepts the same loader function as in the simple usage. |
| 82 | + |
| 83 | +In 2.x, an async component with options is defined as |
| 84 | + |
| 85 | +```ts |
| 86 | +() => ({ |
| 87 | + component: Promise<Component> |
| 88 | + // ...other options |
| 89 | +}) |
| 90 | +``` |
| 91 | + |
| 92 | +Whereas in 3.x it is now: |
| 93 | + |
| 94 | +```ts |
| 95 | +createAsyncComponent({ |
| 96 | + loader: () => Promise<Component> |
| 97 | + // ...other options |
| 98 | +}) |
| 99 | +``` |
| 100 | + |
| 101 | +## Using with Suspense |
| 102 | + |
| 103 | +Async component in 3.x are *suspensible* by default. This means if it has a `<Suspense>` in the parent chain, it will be treated as an async dependency of that `<Suspense>`. In this case, the loading state will be controlled by the `<Suspense>`, and the component's own `loading`, `error`, `delay` and `timeout` options will be ignored. |
| 104 | + |
| 105 | +The async component can opt-out of Suspense control and let the component always control its own loading state by specifying `suspensible: false` in its options. |
| 106 | + |
| 107 | +# Adoption strategy |
| 108 | + |
| 109 | +- The syntax conversion is mechanical and can be performed via a codemod. The challenge is in determining which plain functions should be considered async components. Some basic heuristics can be used: |
| 110 | + |
| 111 | + - Arrow functions that returns dynamic `import` call to `.vue` files |
| 112 | + - Arrow functions that returns an object with the `component` property being a dynamic `import` call. |
| 113 | + |
| 114 | + Note this may not cover 100% of the existing usage. |
| 115 | + |
| 116 | +- In the compat build, it is possible to check the return value of functional components and warn legacy async components usage. This should cover all Promise-based use cases. |
| 117 | + |
| 118 | +- The only case that cannot be easily detected is 2.x async components using manual `resolve/reject` instead of returning promises. Manual upgrade will be required for such cases but they should be relatively rare. |
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