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47 lines
2.2 KiB
47 lines
2.2 KiB
6 years ago
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---
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title: setContext and getContext
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---
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The context API provides a mechanism for components to 'talk' to each other without passing around data and functions as props, or dispatching lots of events. It's an advanced feature, but a useful one.
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Take this example app using a [Mapbox GL](https://docs.mapbox.com/mapbox-gl-js/overview/) map. We'd like to display the markers, using the `<MapMarker>` component, but we don't want to have to pass around a reference to the underlying Mapbox instance as a prop on each component.
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There are two halves to the context API — `setContext` and `getContext`. If a component calls `setContext(key, context)`, then any *child* component can retrieve the context with `const context = getContext(key)`.
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Let's set the context first. In `Map.svelte`, import `setContext` from `svelte` and `key` from `mapbox.js` and call `setContext`:
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```js
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import { onMount, setContext } from 'svelte';
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import { mapbox, key } from './mapbox.js';
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setContext(key, {
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getMap: () => map
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});
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```
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The context object can be anything you like. Like [lifecycle functions](tutorial/onmount), `setContext` and `getContext` must be called during component initialisation; since `map` isn't created until the component has mounted, our context object contains a `getMap` function rather than `map` itself.
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On the other side of the equation, in `MapMarker.svelte`, we can now get a reference to the Mapbox instance:
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```js
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import { getContext } from 'svelte';
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import { mapbox, key } from './mapbox.js';
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const { getMap } = getContext(key);
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const map = getMap();
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```
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The markers can now add themselves to the map.
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> A more finished version of `<MapMarker>` would also handle removal and prop changes, but we're only demonstrating context here.
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## Context keys
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In `mapbox.js` you'll see this line:
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```js
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const key = {};
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```
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We can use anything as a key — we could do `setContext('mapbox', ...)` for example. The downside of using a string is that different component libraries might accidentally use the same one; using an object literal means the keys are guaranteed not to conflict in any circumstance, even when you have multiple different contexts operating across many component layers.
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