Vue Slots Explained
- Vue slots syntax with v-slot directive. With new v-slot directive, we can: – combine html layers: component tag and scope of the slot. – combine the slot and the scoped slot in a single directive.
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Tutorial
Quick Vue Tip: Fetching Data A common use-case we encounter is needing to show data fetched from an API using a Vue component. For example, say we have a Vue component called ViewAvenger.vue which shows the information of an Avenger based on its id. Vue allows us to do this by way of named slots. Named slots are simply slots with a name attribute lot to allow for namespaced content injection. Vue Js Slots Explained, filling fret slot ends, casino cidre, holdem poker winning hands. August 15, 2019.
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Oftentimes you will need to allow your parent Vue components to embed arbitrary content inside of child components. (Angular devs, you know this as transclusion or content projection.) Vue provides an easy way to do this via slots.
Basic Usage
To allow a parent component to pass DOM elements into a child component, provide a <slot></slot> element inside the child component. You can then populate the elements in that slot with <child-component><p>MyParagraph</p></child-component>.
Note: If there is no <slot></slot> element in the child’s template, any content from the parent will be silently discarded.
Fallback Content
If the parent does not inject any content into the child’s slot, the child will render any elements inside its <slot></slot> tag, like so:
Named Slots
Having one slot to inject content into is nice and all, but oftentimes it would be preferable to be able to inject various types of content at different locations in the child component. Say you’re making a burger component. You want the top and bottom buns to be predictably at the top and bottom of the burger, (don’t you?) but also want to be able spread things on the bun halves.
Vue allows us to do this by way of named slots. Named slots are simply slots with a name attribute <slot name='slotName'></slot> to allow for namespaced content injection.
Obviously I don’t have a clue as to how to make burgers, but that should at least serve as an understandable guide to Vue slots. Enjoy!
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Now it’s time to take a deep dive! One of Vue’s most distinct features is the unobtrusive reactivity system. Models are just plain JavaScript objects. When you modify them, the view updates. It makes state management simple and intuitive, but it’s also important to understand how it works to avoid some common gotchas. In this section, we are going to dig into some of the lower-level details of Vue’s reactivity system.
How Changes Are Tracked
When you pass a plain JavaScript object to a Vue instance as its data option, Vue will walk through all of its properties and convert them to getter/setters using Object.defineProperty. This is an ES5-only and un-shimmable feature, which is why Vue doesn’t support IE8 and below.
The getter/setters are invisible to the user, but under the hood they enable Vue to perform dependency-tracking and change-notification when properties are accessed or modified. One caveat is that browser consoles format getter/setters differently when converted data objects are logged, so you may want to install vue-devtools for a more inspection-friendly interface.
Every component instance has a corresponding watcher instance, which records any properties “touched” during the component’s render as dependencies. Later on when a dependency’s setter is triggered, it notifies the watcher, which in turn causes the component to re-render.
Change Detection Caveats
Due to limitations in JavaScript, there are types of changes that Vue cannot detect. However, there are ways to circumvent them to preserve reactivity.
For Objects
Vue cannot detect property addition or deletion. Since Vue performs the getter/setter conversion process during instance initialization, a property must be present in the data object in order for Vue to convert it and make it reactive. For example:
Vue does not allow dynamically adding new root-level reactive properties to an already created instance. However, it’s possible to add reactive properties to a nested object using the Vue.set(object, propertyName, value) method:
You can also use the vm.$set instance method, which is an alias to the global Vue.set:
Sometimes you may want to assign a number of properties to an existing object, for example using Object.assign() or _.extend(). However, new properties added to the object will not trigger changes. In such cases, create a fresh object with properties from both the original object and the mixin object:
For Arrays
Vue cannot detect the following changes to an array:
- When you directly set an item with the index, e.g.
vm.items[indexOfItem] = newValue - When you modify the length of the array, e.g.
vm.items.length = newLength
For example:
To overcome caveat 1, both of the following will accomplish the same as vm.items[indexOfItem] = newValue, but will also trigger state updates in the reactivity system:
You can also use the vm.$set instance method, which is an alias for the global Vue.set:
To deal with caveat 2, you can use splice:
Declaring Reactive Properties
Since Vue doesn’t allow dynamically adding root-level reactive properties, you have to initialize Vue instances by declaring all root-level reactive data properties upfront, even with an empty value:
If you don’t declare message in the data option, Vue will warn you that the render function is trying to access a property that doesn’t exist.
There are technical reasons behind this restriction - it eliminates a class of edge cases in the dependency tracking system, and also makes Vue instances play nicer with type checking systems. But there is also an important consideration in terms of code maintainability: the data object is like the schema for your component’s state. Declaring all reactive properties upfront makes the component code easier to understand when revisited later or read by another developer.
Async Update Queue
In case you haven’t noticed yet, Vue performs DOM updates asynchronously. Whenever a data change is observed, it will open a queue and buffer all the data changes that happen in the same event loop. If the same watcher is triggered multiple times, it will be pushed into the queue only once. This buffered de-duplication is important in avoiding unnecessary calculations and DOM manipulations. Then, in the next event loop “tick”, Vue flushes the queue and performs the actual (already de-duped) work. Internally Vue tries native Promise.then, MutationObserver, and setImmediate for the asynchronous queuing and falls back to setTimeout(fn, 0).
For example, when you set vm.someData = 'new value', the component will not re-render immediately. It will update in the next “tick”, when the queue is flushed. Most of the time we don’t need to care about this, but it can be tricky when you want to do something that depends on the post-update DOM state. Although Vue.js generally encourages developers to think in a “data-driven” fashion and avoid touching the DOM directly, sometimes it might be necessary to get your hands dirty. In order to wait until Vue.js has finished updating the DOM after a data change, you can use Vue.nextTick(callback) immediately after the data is changed. The callback will be called after the DOM has been updated. For example:
There is also the vm.$nextTick() instance method, which is especially handy inside components, because it doesn’t need global Vue and its callback’s this context will be automatically bound to the current Vue instance:
Vue Slots Explained App
Since $nextTick() returns a promise, you can achieve the same as the above using the new ES2017 async/await syntax: