# Guide

Manage asynchronous calls more easily in Vue apps

# What does an asynchronous call mean?

The asynchronous call mentioned here mainly refers to two things:

• Async component loading
• Send an async request to fetch data from the API

# Waiting for the loading of async components

In fact, in Vue the async component already supports displaying the loading component during the loading process. The following code is taken from the official website:

new Vue({
  // ...
  components: {
    'my-component': () => ({
        // async component
        component: import('./my-async-component'),
        // loading component
        loading: LoadingComponent,
        // delay
        delay: 200
    })
  }
})

But it has two problems:

• 1、The loading component is closely tied to the async component, and the loading component cannot be promoted to a higher level and used for loading multiple asynchronous components.
• 2、If the async component itself still has asynchronous calls, such as the request API, the hiding of the loading component will not wait for the API request to complete.

vue-async-manager provides the <Suspense> component to solve the above two problems.

# 1、Create an async component using the lazy function

In the past, the way we created an async component was：

const asyncComponent = () => import('./my-async.component.vue')

Now we use the lazy function provided by vue-async-manager to create the async component:

import { lazy } from 'vue-async-manager'
 
const asyncComponent = lazy(() => import('./my-async.component.vue'))

As shown in the code above, just pass the original asynchronous factory function as a parameter to the lazy function.

# 2、Wrap async components with the <Suspense> component

<template>
  <div id="app">
    <!-- Use the Suspense component to wrap a component tree that may have async components -->
    <Suspense>
      <!-- Loading slot -->
      <div slot="fallback">loading</div>
      <!-- Async component -->
      <asyncComponent1/>
      <asyncComponent2/>
    </Suspense>
  </div>
</template>

<script>
// Create async components
const asyncComponent1 = lazy(() => import('./my-async.component1.vue'))
const asyncComponent2 = lazy(() => import('./my-async.component2.vue'))
 
export default {
  name: 'App',
  components: {
    // Registration component
    asyncComponent1,
    asyncComponent2
  }
}
</script>

The loading component will only disappear after both <asyncComponent1/> and <asyncComponent2/> have been loaded.

# Use with vue-router

When we develop the Vue application, the most common way to use asynchronous components is to do code splitting with vue-router, for example:

const router = new VueRouter({
  routes: [
    {
      path: '/',
      component: () => import('./my-async-component.vue')
    }
  ]
})

In order for the <Suspense> component to wait for the loading of this async component, we can wrap this async component factory function with the lazy function:

const router = new VueRouter({
  routes: [
    {
      path: '/',
      component: lazy(() => import('./my-async-component.vue'))
    }
  ]
})

Finally, we only need to wrap the <router-view> component with the <Suspense> component:

<Suspense :delay="200">
  <div slot="fallback">loading</div>
  <router-view/>
</Suspense>

# How to display loading in the API request

In the past, we usually manually maintained whether to show loading, for example, when the "request to start", the loading was displayed. Hide the loading when "end of request". And if there are multiple requests for concurrency, you have to wait for all requests to complete before hiding loading. In short, you need to maintain the state of loading yourself, whether it is stored in the component or in store.

Now let's see how vue-async-manager solves the loading display problem in the API request process, assuming the following code:

<Suspense>
  <div slot="fallback">loading...</div>
  <MyComponent/>
</Suspense>

The <MyComponent> component is rendered inside the <Suspense> component, which is a normal component. Inside the <Suspense> component, an API request is sent, as shown in the following code:

<!-- MyComponent.vue -->
<template>
  <!-- Display data -->
  <div>{{ res }}</div>
</template>
 
<script>
import { getAsyncData } from 'api'
 
export default {
  data: {
    res: {}
  },
  async created() {
    // Fetch data
    this.res = await getAsyncData(id)
  }
}
</script>

This is the code we often see, usually we send async requests in the created or mounted hooks. However, for the <Suspense> component, it does not know that it needs to wait for the async requests to complete before hiding the loading. To solve this problem, we can create a resource manager using the createResource function provided by vue-async-manager:

<template>
  <!-- Display data -->
  <div>{{ $rm.$result }}</div>
</template>
 
<script>
import { getAsyncData } from 'api'
import { createResource } from 'vue-async-manager'
 
export default {
  created() {
    // Create a resource manager(rm)
    this.$rm = createResource((params) => getAsyncData(params))
    // Read data
    this.$rm.read(params)
  }
}
</script>

Pass a factory function to the createResource function, we create a resource manager: $rm, and then call the resource manager's $rm.read() function to read the data. Note that the above code is written in a synchronous manner, and the <Suspense> component knows that the component is making an async call, so the <Suspense> component will wait for the async call to complete before hiding loading .

In addition, we see the template part of the above code, the data we show is $rm.$result, in fact, after the async data is successfully acquired, the obtained data will be saved in the $rm.$result property, it's important to note that the property itself is reactive, so you don't need to declare it in the data option of the component.

# Use with vuex

With vuex it's very simple, just use mapActions to map actions to methods:

export default {
  name: "AsyncComponent",
  methods: {
    ...mapActions(['increase'])
  },
  created() {
    this.$rm = createResource(() => this.increase())
    this.$rm.read()
  }
};

# Capture all async calls in the component tree

The <Suspense> component not only captures the loading of async components. If an async component itself has other async calls, such as reading data through the Resource Manager, the <Suspense> component can also capture these async calls and wait for all async calls to end before hiding loading .

Let's look at an example：

<Suspense>
  <div slot="fallback">loading</div>
  <!-- MyLazyComponent is a component created by the lazy function -->
  <MyLazyComopnent/>
</Suspense>

The <MyLazyComopnent/> component is a component created by the lazy function, so the <Suspense> component can wait for the async component to load, whereas the async component itself reads the data through the resource manager:

// Async component
export default {
  created() {
    // Read data through the resource manager
    this.$rm = createResource((params) => getAsyncData(params))
    this.$rm.read(params)
  }
}

At this point, the <Suspense> component will wait for the completion of both asynchronous calls before hiding loading, which are:

• 1、Async component loading
• 2、Async requests sent by Resource Manager within the async component

# Resource manager

We have been emphasizing a word: Resource Manager，the return value of the createResource function is a resource manager, which is an object, we usually use $rm to name.

The complete form of the resource manager is as follows:

this.$rm = createResource(() => getAsyncData())

this.$rm = {
    read(){},   // A function that calls this function to actually send an async request to fetch data
    $result,    // The initial value is null. After the async data is successfully acquired, the obtained data is stored.
    $error,     // The initial value is null, which holds the err data when the async request fails.
    $loading,   // A boolean value with an initial value of false, indicating whether the request is in progress
    fork()      // Create a new resource manager based on an existing resource manager
}

The $rm.read() function is used to send async requests to fetch data, which can be called multiple times, such as clicking the button to call it again. $rm.$result we have also seen it, which is used to store data obtained asynchronously. $rm.$loading is a Boolean value that indicates whether the request is in progress. Usually we can customize the loading display like this:

<template>
  <!-- Control loading display -->
  <MyButton :loading="$rm.$loading" @click="submit" >提交</MyButton>
</template>
 
<script>
import { getAsyncData } from 'api'
import { createResource } from 'vue-async-manager'
 
export default {
  created() {
    // Create a resource manager
    this.$rm = createResource((params) => getAsyncData(params))
  },
  methods: {
    submit() {
      this.$rm.read(params)
    }
  }
}
</script>

If the resource manager has encountered an error while fetching data, the error data is stored in the $rm.$error property. The $rm.fork() function is used to create an identical resource manager based on the existing resource manager.

# Fork a resource manager

When an API is used to fetch data, and we need to fetch data twice, we only need to call $rm.read() twice:

<script>
import { getAsyncData } from 'api'
import { createResource } from 'vue-async-manager'
 
export default {
  created() {
    // Create a resource manager
    this.$rm = createResource((type) => getAsyncData(type))
     
    // Continuous fetch data twice
    this.$rm.read('top')
    this.$rm.read('bottom')
  }
}
</script>

But doing so will create a problem, since one resource manager is associated with only one $rm.$result, so in the above code, $rm.$result will only save the data of $rm.read('bottom'). Of course, sometimes this is expected, but if you need to store the data twice called, you need fork to create a new resource manager:

<script>
import { getAsyncData } from 'api'
import { createResource } from 'vue-async-manager'
 
export default {
  created() {
    // Create a resource manager
    this.$rm = createResource((type) => getAsyncData(type))
    // Fork a new resource manager based on the existing resource manager
    this.$rm2 = this.$rm.fork()
    
    // The data read twice will be stored separately
    this.$rm.read('top')
    this.$rm2.read('bottom')
  }
}
</script>

Thus, since $rm and $rm2 are two separate resource managers, they do not affect each other.

# prevent option & duplicate submissions

Suppose we are submitting a form. If the user clicks the button twice, it will cause duplicate submissions, as in the following example:

<template>
  <button @click="submit">Submit</button>
</template>
<script>
import { getAsyncData } from 'api'
import { createResource } from 'vue-async-manager'
 
export default {
  created() {
    // Create a resource manager
    this.$rm = createResource((type) => getAsyncData(type))
  },
  methods: {
    submit() {
      this.$rm.read(data)
    }
  }
}
</script>

In fact, we can provide the prevent option when creating the resource manager, so the created resource manager will automatically prevent duplicate submissions for us:

<template>
  <button @click="submit">Submit</button>
</template>
<script>
import { getAsyncData } from 'api'
import { createResource } from 'vue-async-manager'
 
export default {
  created() {
    // Create a resource manager with the prevent option
    this.$rm = createResource((type) => getAsyncData(type), { prevent: true })
  },
  methods: {
    submit() {
      this.$rm.read(data)
    }
  }
}
</script>

When a button is clicked for the first time, a request is sent and all new requests that occur before the request is completed are ignored.

# loading style

The style of loading can be divided into two types: one is to display only loading and not to display other content; the other is to display loading while rendering other content normally, such as a long loading bar at the top of the page. This load bar does not affect the normal rendering of other content.

So vue-async-manager provides two rendering modes:

import VueAsyncManager from 'vue-async-manager'
Vue.use(VueAsyncManager, {
  // Specify the rendering mode, the optional value is 'visible' | 'hidden', the default value is: 'visible'
  mode: 'visible'
})

The rendering mode of 'visible' is used by default. This means that loading can coexist with other content. If you don't want this rendering mode, you can specify mode to 'hidden'.

So far, we have only seen the use of <Suspense> components to control the display of loading, and the contents of loading are determined by the fallback slot of the <Suspense> component. But sometimes we want to be more flexible, we often encounter such a scenario: when you click the button and display a tiny loading icon on the button, our code might look like this:

<MyButton :loading="isLoading" >Submit</MyButton>

The style of loading is provided by the <MyButton> component，in other words, we abandoned the fallback slot of <Suspense> as loading to show. Therefore, we need to know if it is currently loading. We have already introduced the solution to this problem, we can use the resource manager's $rm.$loading property:

<MyButton :loading="$rm.$loading" >Submit</MyButton>

# Error handling

When the lazy component fails to load, the error slot of the <Suspense> component is displayed. You can also customize the error handling by listening to the rejected event of <Suspense>.

In addition to displaying the error slot when an error occurs, you can also customize the processing by listening to the rejected event of the <Suspense> component:

<template>
  <Suspense :delay="200" @rejected="handleError">
    <p class="fallback" slot="fallback">loading</p>
    <AsyncComponent/>
  </Suspense>
</template>
<script>
export default {
  // ......
  methods: {
    handleError() {
      // Custom behavior
    }
  }
};
</script>

# About LRU Cache

React Cache uses the LRU algorithm to cache resources, which requires the API to be idempotent. However, in my working environment, there are very few APIs that are really idempotent in a given time period, so there is no provision for caching resources ability.