I could successfully emitted event, but I don't know how to show those event.
My event is this
event Sell (
address indexed _buyer,
uint256 indexed _value
);
I could get all event in console.
const event = await tokenSale.events.Sell({}, {
fromBlock: 0,
toBlock: 'latest',
})
event.on("data", (data) => console.log("data", data))
I want to show _value in frontend.
Could you give me any advise, please?
There are lots of ways to render values in the front end. It depends on what language/library you are using. If you are using React, a nice tool for passing contract data to the front end is Drizzle
Related
Suppose I want to design a game where the user places a bet (using the ERC20 token) for a coin toss, and suppose the solidity function within our game contract called to place the bet is bet(uint256 _betAmount, int256 pick).
When playing around with some code, I realized that if the bet() function contains the token.transfer() call (used to place the initial bet before simulating the coin toss), the funds would be transfered from the game contract, and not from the user, even if the caller of bet() is the user. Thus, I would need to use the token.transferFrom() function, but the user would need to set the allowance for the game contract to spend the funds on their behalf.
My question is, how could this be handled in the frontend? Is there a way for a pop-up to show up asking for permission to spend the tokens (thinking of somehting similar to metamask when executing a transaction)?
Basically the user will need to accept two transactions.
But before calling the bet function you need also to wait until the approve function is confirmed.
In a project i developed i also had to handle your exact same case and i did something like this (i used react-moralis to interact with the contract):
const handleBet = async () => {
await approve({
onSuccess: async (tx) => {
await tx.wait(1) // Wait until the transaction is mined
await bet() // Call the bet function
},
onError: () => {
console.log("error") // Handle error however you want
}
})
}
I want to be able to watch the state/storage of an ethereum contract and know for instance when some conditions are satisfied. For instance, one account value is greater than 200, another map has more than 30 keys, etc.
What can I use for this? Is there a SaaS platform that can be used for this purpose?
EDIT: I forgot to mention that I am NOT the owner of the contract. I simply want to watch somebody's else contract.
Events are supported for this purpose.
Event Documentation
Example:
contract SomeContract {
struct AccountData {
address account;
uint value;
}
...
event AccountValueChange(address account, uint value, bytes msg);
...
function someAction() {
AccountData accountData;
// some activity with accountData...
if (accountData.value > 200) {
AccountValueChange(accountData.address, accountData.value, "Some message");
}
}
}
You can listen for the events in your client
const someContract = web3.eth.contract(abi).at(address);
myContract.events.AccountValueChange({
filter: {//Optional event filters},
fromBlock: 0
}, function(error, event){ console.log(event); })
.on('data', function(event){
console.log(event);
})
.on('error', console.error);
EDIT: Updated client code to listen to all events instead of just a single fired event.
Short answer is that you'll have to probe the EVM yourself to make this happen and then run your own node on the network that you can monitor. There is no public project I can find that will do this, and the RPC api does not provide any way to accomplish it. I used a modified Geth binary with custom transaction filters coded in and mailgun to send me emails when my conditions were met.
Events will not solve your issue because events must be included in the contract at the time of creation.
I am working in a meteor project(1.5.1) and using mdg:geolocation#1.3.0, and i am trying to get the value of Geolocation.currentLocation() but it is giving me null value and the behaviour is not the same for all the time, sometime it is giving me null value and sometime giving me proper value.
I have research for the long time, but not got the solution yet, if possible please provide solution with meteor#1.5.1 and with use of mdg:geolocation package.
Thanks in advance...
MDG has implemented mdg:geolocation for continuous geolocation and functions in this Meteor package return a reactive var. You can use the package this way (for example):
// Continous geolocation with mdg:geolocation
// This code will run every time user location changes.
Tracker.autorun(() => {
const position = Geolocation.currentLocation();
Tracker.nonreactive(() => {
if (position) {
Meteor.call('user.update.position', position);
}
});
});
If you don't need continous geolocation, You can use javascript method instead of meteor package. See this answer for other options with windows.navigator.geolocation. In this case you may need to add cordova-plugin-geolocation in case you are building Android or iOS apps
// Navigation geolocation to get geolocation only once
let errorCallback;
let successCallback;
successCallback = position => Meteor.call('user.update.position', position);
errorCallback = err => console.log(err);
navigator.geolocation.getCurrentPosition(successCallback, errorCallback, {
maximumAge: 60000,
timeout: 20000
});
I want to create a form on an index page that can store data via session storage. I also want to make sure that whatever data(let's say name) ... is remembered and used throughout the site with angular. I have researched pieces of this process but I do not understand how to write it or really even what it's called.
Any help in the right direction would be useful as I am in the infant stages of all of this angular business. Let me know.
The service you want is angular-local-storage.
Just configure it in your app.js file:
localStorageServiceProvider
.setStorageType('sessionStorage');
And then use it in the controller that contains whatever data you want to remember. Here is an example of a controller that loads the session storage data on initialization, and saves it when a user fires $scope.doSearch through the UI. This should give you a good place to start.
(function () {
angular.module("pstat")
.controller("homeCtrl", homeCtrl);
homeCtrl.$inject = ['$log', 'dataService', 'localStorageService', '$http'];
function homeCtrl ($log, dataService, localStorageService, $http) { {
if (localStorageService.get("query")) { //Returns null for missing 'query' cookie
//Or store the results directly if they aren't too large
//Do something with your saved query on page load, probably get data
//Example:
dataService.getData(query)
.success( function (data) {})
.error( function (err) {})
}
$scope.doSearch = function (query) {
vm.token = localStorageService.set("query", query);
//Then actually do your search
}
})
}()
I'm working on my first react/reflux app so I may be approaching this problem in completely the wrong way. I'm trying to return a promise from a reflux store's action handler. This is the minimum code that represents how I'm trying to do this. If I display this in the browser, I get an error saying that the promise is never caught, because the result of the onLogin function is not passed back when the action is initiated. What is the best way to do this?
var Reflux = require('reflux');
var React = require('react/addons')
const Action = Reflux.createAction();
const Store = Reflux.createStore({
init: function() {
this.listenTo(Action, this.onAction);
},
onAction: function(username, password) {
var p = new Promise((resolve, reject) => {
reject('Bad password');
});
return p;
}
});
var LoginForm = React.createClass({
mixins: [Reflux.connect(Store, 'store')],
login: function() {
Action('nate', 'password1').catch(function(e) {
console.log(e); // This line is never executed
});
},
render: function() {
return (
<a onClick={this.login} href="#">login</a>
)
}
});
React.render(<LoginForm />, document.body);
Several things seem a bit confused here.
Reflux.connect(Store, 'store') is a shorthand for listening to the provided store, and automatically set the "store" property of your component state to whatever is passed in your store's this.trigger() call. However, your store never calls this.trigger so "store" in your component's state will never be updated. Returning a value from your store's action handlers doesn't trigger an update.
Stores should listen to actions to update their internal state, and typically then broadcast this state update by calling this.trigger. No component is going to get your returned promise from the store's onAction unless it explicitly calls Store.onAction (and then it doesn't matter if the actual action was invoked or not).
Async work should typically happen in the action's preEmit hook, not in the store. You should then also declare the action as async in createAction by setting the asyncResult option to true to automatically create "completed" and "failed" child actions. Check out the Reflux documentation here to learn about async events. Async actions automatically return promises, whose resolve and reject are called when the "completed" and "failed" sub-actions are called respectively. This is a bit opinionated, but that is definitely what I perceive is the intended Reflux way.