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2020-03-15 02:12:16 +00:00
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@@ -122,16 +122,26 @@ https://stackoverflow.com/a/44350932
With computed you are creating getters/setters. You do this by writing the function in the `computed` directive.
Here we have written a computed property for `emailRules()` - we are using this function to check input on a form is correct.
Computed properties don't take any arguments - but you can access the `data` scope through `$this.` if you need access to the data in the scope.
They are better than using watchers/methods if you don't need the extra functionality. A good example is https://michaelnthiessen.com/most-important-feature-vue/
The main difference between `computed` and a `method` is that `computed` will cache automatically. It will only update, if the value updates. Everytime you call it it will use its cache. Expensive operations can be done this way.
They are properties (like in Python). Computed properties will update whenever one of the values needed to make it is also updated. Whenever you want to filter or transform your data, you can use a Computed property.
E.g if you have a list of names, and you want just those names that begin with 'b', you can write it as a computed property that returns this list. The stack overflow answer above demonstrates this.
Remember: anything in `data` you can access in the `<script>` with `$this.var`. In the html you can access the var name directly `{{var}}`
Watchers
https://vuejs.org/v2/guide/computed.html#Watchers
The watch directive can be used to watc variables in the component. You create a function that is the same name as the variable you want to watch. This function should take two arguments: new and old. (You can use this to compare inputs for exmaple).
Watchers are like computed but they are a more generic way to watch for datachanges. If you need to add more functionality (rate limiting) or refer to many methods or take many arguments - a watcher might be more appropiate.
There is a good example of using this functionality to do an ajax call (with rate limiting) in the documentation for watches above.
The watch directive can be used to watch variables in the component. You create a function that is the same name as the variable you want to watch. This function should take two arguments: new and old. (You can use this to compare inputs for exmaple).
There is a good example of using this functionality to do an ajax call (with rate limiting) in the documentation for watchers above.
@@ -151,7 +161,7 @@ Usage of `$this`
The following (under the "No separate this" header) explains well.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions
When you write a function and define a variable, you might want to write another function inside. If you use `$this` inside the second function, you will find it is bound to the global scope, not the parent function!
When you write a function and define a variable, you might want to write another function inside. If you use `$this` inside the second function, you will find it is bound to the global scope, not the parent function!
To fix this behaviour, you should _close over_ the variable `$this`:
@@ -171,3 +181,155 @@ function Person() {
This is similar to "closure" in python.
Events system and having functions as props
https://michaelnthiessen.com/pass-function-as-prop/
https://vuejs.org/v2/guide/events.html
```javascript
<!-- Parent -->
<template>
<ChildComponent @send-message="handleSendMessage" />
</template>
// Parent
export default {
methods: {
handleSendMessage(event, value) {
// Our event handler gets the event, as well as any
// arguments the child passes to the event
console.log('From the child:', value);
}
}
}
```
And in the child component we emit the event:
```javascript
// Child
export default {
props: {
method: { type: Function },
},
data() {
return { value: 'I am the child.' };
},
mounted() {
// Instead of calling the method we emit an event
this.$emit('send-message', this.value);
}
}
```
When the child component is mounted, it emits the event 'send-message'. The parent can then listen for this 'send-message' event and call a corresponding function. As the data flows in only one way, it makes for much easier debugging too. We can also pass up variables from the child to the parent in this way.
Object destructuring
When you define a function and specifiy attributes, you can use javascripts object destructing.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment
For example, if you write a function that takes a dictionary, but in the function you only need one of the key-value pairs and not the rest, you can write your function arguments to take this using `{}` inside the arguments. Then when you pass in the whole dictionary as an argument to the function, it will only use what you've told it to.
Listening to events
In a parent, you can use the `@` syntax on a html tag to listen to an event.
`<ChildComponent @send-message="handleSendMessage" />`
This will listen for a `send-message` event from this child, and when it receives it it will run the function handleSendMessage.
```javascript
mounted() {
this.$emit('send-message', this.value);
}
```
This will trigger the handler in the parent, and pass the `this.value` parameter up with it.
Slots
https://vuejs.org/v2/guide/components-slots.html
Slots are useful if you want to send data from a parent down to a child (or vice versa) and want to include more advanced things.
For example, you can use a slot to send html from a parent down to a child to be rendered.
```javascript
// parent
<navigation-link url="/profile">
<!-- Add a Font Awesome icon -->
<span class="fa fa-user"></span>
Your Profile
</navigation-link>
```
```javascript
// child (navigation-link)
<a
v-bind:href="url"
class="nav-link"
>
<slot></slot>
</a>
```
The content between the `<slot>` will be replaced by what is between the `<navigation-link>`.
Summary:
Send data
From parent to child: use props.
From child to parent: use events.
Send data parent to child:
If you want to send data from a parent to a child, but the data is defined on the child but you want to send a reference to it from the parent then have the parent send an arrow function as a prop, which returns the statement/expression you want to be rednered in the child.
As its a function, you can have it accept an argument. In the child you would refer to the prop name and pass in the arguments.
In the child:
`{ prop-name(argument)}`
Scoped Slots
https://www.youtube.com/watch?v=GWdOucfAzTo
If you want to send more than a string down, and want to send more advanced things like whole functions or even html, then you can use a scoped slot.
A scoped slot is a special kind of slot designed for this purpose. They are really powerful in Vue, and using them in design patterns allows you to decouple components.
To use a scoped slot you define a normal slot, but you pass add attributes to the slot tag in the html.
`<slot :attr1="param"></slot>`
You are passing in a key-value pair. This is slightly different to when you use arrow functions with arguments to render the data (unless your arrow function takes an object of a dictionary with the same key-value pairs).
So if we need to send data from a child to a parent:
In the child: write a `<scope :something="a value">` that takes arguments.
In the parent: write a `<template>` directive that has a `slot-scope` attribute on it: `<template slot-scope="name"> {{name.something}} </template>`.
You don't have to use a template, you can use any html tag - you just need some html tag to accept the `slot-scope` - (an `<a>` for example). You can fall back on template if needed as a default.
Think of a scoped slot as a function that returns template data.
Regular slots are just regular props.
Scoped slots are function props.
The video from adam linked above has a really good demonstration of this.
Portals
https://www.youtube.com/watch?v=1yWAxrpL3zU&list=PL7CcGwsqRpSOZAiNYyVvgTKSyARERvvij
Portals are useful if you want to render something in a child somewhere in a specific place in the DOM.
This is useful if you have a child component somewhere that is toggling a modal, and you want that model to actually be placed in the parent in a specific place in the DOM for formatting.
Portals will literally render the data where it needs to, then transport it to wherever you want it to actually be placed in the HTML.
Advanced Vue Component Design course
https://adamwathan.me/advanced-vue-component-design/