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This article mainly introduces "how to use Solidity event Event". In daily operation, I believe many people have doubts about how to use Solidity event Event. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "how to use Solidity event Event". Next, please follow the editor to study!

What is an event Evnet

Events are a convenient interface provided by the ethernet virtual machine (EVM) logging infrastructure. When an event is sent (called), the trigger parameters are stored in the transaction log (a special data structure on the block chain). These logs are associated with the address of the contract and recorded in the block chain. To straighten out the relationship: a blockchain is a chain of blocks that package a series of transactions, and each transaction "receipt" contains 0 or more log records, which represent events triggered by smart contracts.

In the application of DAPP, if an event is monitored, a callback will be made when the event occurs. Note, however: logs and events are not accessible within the contract, even the contract that created the log.

In Solidity code, use the event keyword to define an event, such as:

Event EventName (address bidder, uint amount)

This usage is the same as defining a function, and events can also be inherited in a contract. Trigger an event using emit (note that emit was not required in previous versions), such as:

Emit EventName (msg.sender, msg.value)

Trigger events can be called in any function, such as:

Function testEvent () public {/ / triggers an event emit EventName (msg.sender, msg.value);} listens for events

Through the above introduction, you may still not know what the effect of the event is. If you have followed this article about the interaction between Web3 and Smart contract, you will find that after clicking the "Updata Info" button, although the call to Smart contract is successful, the current interface has not been updated. Using event monitoring, you can solve this problem very well, let's see how to implement it.

Modify contracts, define events and trigger events

Let's review the contract code:

Pragma solidity ^ 0.4.21 X contract InfoContract {string fName; uint age; function setInfo (string _ fName, uint _ age) public {fName = _ fName; age = _ age;} function getInfo () public constant returns (string, uint) {return (fName, age);}}

First, you need to define an event:

Event Instructor (string name, uint age)

In this event, you accept two parameters: name and age, that is, two pieces of information that need to be tracked.

Then, you need to trigger the Instructor event in the setInfo function, such as:

Function setInfo (string _ fName, uint _ age) public {fName = _ fName; age = _ age; emit Instructor (_ fName, _ age);}

When Web3 interacts with the smart contract, after clicking the "Updata Info" button, the setInfo function is called and the Instructor event is triggered when the function is called.

Refresh UI using Web3 listening events

Now you need to use Web3 to listen for events and refresh the UI. Let's review the previous code that interacts with smart contracts using Web3:

If (typeof web3! = = 'undefined') {web3 = new Web3 (web3.currentProvider);} else {/ / set the provider you want from Web3.providers web3 = new Web3 (new Web3.providers.HttpProvider ("http://localhost:7545"));} web3.eth.defaultAccount = web3.eth.accounts [0]; var infoContract = web3.eth.contract (ABI INFO); var info = infoContract.at (' CONTRACT ADDRESS')) Info.getInfo (function (error, result) {if (! error) {$("# info") .html (result [0] +'('+ result [1] + 'years old)'); console.log (result);} else console.error (error);}) $("# button") .click (function () {info.setInfo ($("# name") .val (), $("# age") .val ());})

Instead of needing info.getInfo () to get the information, you can use the listening event to get the information, first defining a variable reference event:

Var instructorEvent = info.Instructor ()

Then use the * .watch () * * method to add a callback function:

InstructorEvent.watch (function (error, result) {if (! error) {$("# info") .html (result.args.name +'('+ result.args.age + 'years old)');} else {console.log (error);}})

After the code is updated, you can view the effect in the browser. After clicking the "Updata Info" button, the interface will be updated in time.

Advanced usage of events-filter

Sometimes we have a need to get all current names and age records, or what should we do to filter out records that are 28 years old? And another common scenario: if you want to get all the transfer records in a token contract, you also need to use the event filter function. Please subscribe to the small column Block chain Technology to read.

At this point, the study on "how to use the Solidity event Event" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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