I'm working on an app in React Native, and am having trouble accessing props that I feed into a component I made.
If I do console.log(this.props) within the constructor, I can see the props display in the console as desired, however if I put it in any other method, it prints undefined. How can I access the props that are clearly being sent to the component from outside of the constructor method?
You are probably adding new methods that are not binding this.
Check if you are writing the method like this:
myMethod(){
//Code
}
and just change it to:
myMethod = () => {
//Code
}
Edit: Like #Li357 says, these are called arrow functions. Arrow functions don't bind this automatically, and as a consequence receive the this of the surrounding class. In your case it will solve your issue as you want to access the properties of that class but you might want to read about it and how binding works in JS classes.
Another option is to write function.bind() but either way should work.
Related
I created a form component where i pass an object prop to it containing strings and an object array. I then i transfer the elements of the props to the component variables so i can edit them without interacting with the props directly like so:
beforeMount(){
this.var1 = this.props.var1
this.var2 = this.props.var2
this.array = this.props.array
}
when i edit all the other variables, close my component and open it again everything resets BESIDES the array. every time i try to interact with it the reset never happens. i know i am not interreacting directly with the props either in any part of my code so i don't really why this happens when when everything works fine. To fix that i had to do the following:
beforeMount(){
this.var1 = this.props.var1
this.var2 = this.props.var2
this.props.array.forEach(element => {
this.array.push(element)
})
}
why does this work exactly?
I have also been having similar problems with method specific variables not resetting after the function is already over so i find these behaviors a little weird. I am also using "v-if" and also tried to use component keys to reset the component but it doesn't work for the array for whatever reason.
try to copy your array like that :
this.array = [...this.props.array]
I'm not sure about it, but as you pass it within an object, you might actually use you array as reference and not values as you 'd like to.
I have a quite decent background in android but now I am starting digging into react native and I am really confused with the ways to change the state of a component through hooks and set state function.
To the point, I have my main screen and I have many small components which change visibility. This is done by letting the user change some filter settings within dialogs. So the suggested way to do that is by having a hook in my main screen with a list that holds the values for the visibility of each component. But since I change the visibility of the components from inside the modals, every time I want to show a modal I will have to pass in a different function(for example setComponentEnable or setComponentDisabled) to set the state for each component. So my main screen will be polluted from all these small functions. Also I should not forget to mention that my modals are consisted from many smaller components and I will have to pass as deep as it goes the proper function to match the user action.
So my question is, is there a way to do this thing without polluting my main with all these small functions and make it possible for the main screen to update every time the user change the filters within the modals?
I already read about context but the docs say:
Context is designed to share data that can be considered “global” for a tree of React components, such as the current authenticated user, theme, or preferred language.
So I dont think that this should be a great case for context use.
What I am trying to do now is create a hook with a list
const [isibility, setVisibility] = useState([]);
create visibility handler functions
const setVisibilityEnable = () => {
...
}
and pass it into my modal.
<MyModal
visibilityHandler={setVisibilityEnable}/>
Is there a way to manipulate the state without passing all these callbacks to the modals? Or maybe is there anyone that can suggest a better and clean solution to avoid end up having a really huge main screen?
you can include all the settings in one object and pass that object to all the components. Then each component will then modify that object accordingly.
const defaultVisibility = {
childComponentOne: true,
childComponentTwo: true,
};
const [visibilityObject, setVisibilityObject] = useState(defaultVisibility);
pass both the function and the object into your child components:
<ChildComponentOne visibilityObject={visibilityObject} setVisibilityObject={setVisibilityObject} />
Then in your child component, you set the visibility like so:
setVisibilityObject({...visibilityObject, childComponentOne: false});
Why you don't just pass a property to your modal and check if changed in oncomponentdidchange method?
componentDidUpdate(prevProps) {
if (this.props.yourPoperty!== prevProps.yourPoperty) {
//do your visibility stuff
}
}
Alternatively you can do it with redux when you connect your components to the store.
I'm trying to understand the solution in this SO post. The solution allows the user to keep track of the previous route in the current route.
Below is the snippet of Vue code that I'm trying to understand. If I understand correctly, next accepts a callback function that receives the current component's vue instance. We then set the prevRoute data property of this vue instance to from. Is this interpretation correct? If not, what is actually happening?
If someone could also add a brief explanation as to what the Vue API is doing behind the scenes that would also be very helpful for me to actually understand the snippet of code.
...
data() {
return {
...
prevRoute: null
}
},
beforeRouteEnter(to, from, next) {
next(vm => {
vm.prevRoute = from
})
},
...
As per the documentation...
The beforeRouteEnter guard does NOT have access to this, because the guard is called before the navigation is confirmed, thus the new entering component has not even been created yet.
However, you can access the instance by passing a callback to next. The callback will be called when the navigation is confirmed, and the component instance will be passed to the callback as the argument
So vm is the component instance assigned to the destination route.
From your question...
We then set the prevRoute data property of this vue instance to from. Is this interpretation correct?
Almost. All you're doing is setting a direct object property on the Vue component which is after all, just a JavaScript object at heart. For example
const vm = { name: 'I am totally a Vue component' }
vm.prevRoute = from
This property will not be reactive but you can certainly access it within your component via this, just as you can other non-data properties like $el, $refs, etc.
I am using react-native and react-redux to create a simple app. Thus, all of my props are passed in from mapStateToProps, coming from redux.
I should have to use mapDispachToProps to manipulate the props but that is not what is happening.
I have an event handler that allows me to directly manipulate the props. Why is this behavior occurring?
nameHandler(text) {
console.log(this.props.group.name); //output: Billy
this.props.group.name = 'illegal!';
console.log(this.props.group.name); // output: illegal!
}
I am binding 'this' to 'nameHandler' within the constructor, but that doesn't seem relevant.
I've mostly gotten around this, but shouldn't this not be possible?
I am trying to make my Vue Component reusable but there is a part in it which requires to run a function on button click which I have defined in the parent component.
The component's button will always run a parent function and the parameter it passes is always the same (its only other property).
Right now I am passing 2 properties to the component: 1) an object and 2) the parent function reference, which requires the object from 1) as a parameter.
The Child-Component looks like this (stripped unnecessary code):
<button v-on:click="parentMethod(placement)">Analyze</button>
Vue.component('reporting-placement', {
props: ['placement', 'method'],
template: '#reporting-placement',
methods: {
parentMethod: function(placement) {
this.method(placement);
}
}
});
The parent is making use of the child like this:
<reporting-placement v-bind:placement="placement" v-bind:method="analyzePlacement"></reporting-placement>
methods: {
analyzePlacement: function(placement) {
this.active_placement = placement;
},
}
As you can see, the child has only one property, placement, and the callback reference. The placement must be put in as a parameter to the reference function from the parent.
But since the parent defines the parameters, the child shouldn't concern itself with what it needs to pass to the parent function. Instead I would prefer to already pass the parameter along in the parent.
So instead of
<reporting-placement v-bind:placement="placement" v-bind:method="analyzePlacement"></reporting-placement>
I would prefer
<reporting-placement v-bind:placement="placement" v-bind:method="analyzePlacement(placement)"></reporting-placement>
(including appropriate changes in the child).
But passing the parameter along does not work that way.
Is it possible (maybe in other syntax) to 'bind' the variable to the function reference so that it is automatically passed along when the callback is called?
Info: I don't get an error message if I write it down as above but the whole Vue screws up when I pass the parameter along to the component.
Hope the issue is clear :-) Thanks a lot!
By reading your proposal I've found out that you are overusing the props passing.
Your concern that child component should not have any knowledge about the way that the parent component uses the data is completely acceptable.
To achieve this you can use Vue's event broadcasting system instead of passing the method as props.
So your code will become something like this:
Vue.component('reporting-placement', {
props: ['placement', 'method'],
template: '#reporting-placement',
methods: {
parentMethod: function(placement) {
this.$emit('reporting-placement-change', placement)
}
}
});
And you can use it like this:
<reporting-placement v-bind:placement="placement" #reporting-placement-change="analyzePlacement($event)"></reporting-placement>
But if you need the data which is provided by the method from parent it's better to consider using a state management system (which can be a simple EventBus or event the more complex Vuex)
And finally, if you really like/have to pass the method as a prop, You can put it in an object, and pass that object as prop.