How To Build a Simple Ethereum Game – Live Coding

How To Build a Simple Ethereum Game – Live Coding

– Hey guys, welcome to this webinar, where we show you some cool coding that you can do with solidity. And this specific webinar,
we are going to be looking at the ERC721 interface also known as non fungible tokens. So ERC721 is a standard
interface which came from something called an EIP an ethereum improvement proposal, where anyone can suggest an improvement and one of the improvement
was this ERC721 interface that would define a
standard set of methods that you can call on a ERC721
on a non fungible token in order to faster
interoperability between different tokens in the ethereum ecosystem. So first before we are going to the code, let’s take a second to
appreciate the difference between a fungible token
and a non-fungible token. So a fungible token, you can think of as something
that can be interchanged, right, so if you think of
a physical fungible asset, you can think of a dollar, the US dollar. So each dollar bill is interchangeable for any other dollar bill and it doesn’t make a difference, you still have the same value. But an example of a
non-fungible physical token would be something like Pokemon cards or trading cards because each of those has you know specific set of powers or skills that are associated to
that card, that token, and that’s why it’s non-fungible, it’s not interchangeable for each other like each of them is unique
and has a set of unique sort of powers or abilities
or characteristics. Whereas with fungible tokens,
they are interchangeable and it doesn’t matter
which specific one you have because it doesn’t make a difference. So what I talked about
were two physical examples, but in the digital world you can also find these examples. So something like bit coin or ether or any crypto currency is interchangeable, not any cryptocurrency but
ethereum and bit coin have sort of fungible tokens,
right, so it doesn’t matter which bit coin you have as long as you have one bit coin. Similarly with ether, it doesn’t matter which
specific ether you have, but as long as your
balance says that you have one ether, the value is the same. But if you look a non-fungible
token in the digital world, so an example of non-fungible token, the most popular example
is a CryptoKitties. So CryptoKitties you can
think of as a set of unique trading cards where
each of them represents a digital kitty that has
some special characteristics. It has you know a special look to it, it might have some super powers, it might have some other cool stuff that you can add in your game. But the point is that they
are not interchangeable. Each of them is unique like a trading card and has its own unique value and therefore you can just
trade CrptoKitty for another, because each of them is unique. So that being said, one of the fungible interface examples, most popular one is ERC20. So if you are doing an ICO for a token that you want to be, you wanted to be listed
on other exchanges, you would probably
implement the ERC20 token, because that would allow other operators or other exchangers to
operate against an interface without having to know
the particular details about your specific token. So the reasons behind these interfaces are to faster interoperability
and allow these contracts and tokens to talk to each other and also with services
outside of ethereum. So one organization that
is a part of building these standards in ethereum
community is OpenZeppelin. So if you just go to, we can see that it’s a
framework ,it’s a open source, and it’s a framework of reusable and audited smart contract. So that’s what makes them secure. They are audited by the community and they provide these
open source implementations for developers like yourself
to use in your projects and avoid common sort of security mistakes and just use their code as
sort of like a starting point. So one of the things they provide, so people can get started, it
will take us to their get hub and if we just look at
their contracts folder, you’ll see that they have a
bunch of other stuff as well but if it’s over move over
to the contracts folder we’ll see a bunch of contracts and if we go into the token folder, we’ll see folders for each
of these various formats. So we mentioned ERC20 and ERC721, 827 is another interface,
but for this webinar, we are only gonna be looking at ERC721. And we can that so if we
start from this ERC721 basic, so this just defines the interface, so this is an abstract class
that defines the functions that your contract needs to implement if it’s going to be ERC721 compliant. And you can see there is ERC721 basic, which consist of these functions. So where do these functions come from? They came from the EIP and just at the top you can see a link to the
EIP, which you can copy paste. Let’s go and take a look
at the EIP real quick. And it’s the complete proposal explaining you know what are non-fungible tokens, why is this interface needed, and what are some of the use cases for us? So I encourage you to read through this to learn more about the proposal, but for now we are just gonna
be looking at this code. But this interface is basically
defined in that proposal and it defines these functions that you should be able to
execute on your token. So at the top, we see
there is a bunch of events that basically indicates
transfers of ownership, so if you have an ERC721 token that you wanna give to another person then there would be events related to that particular action and we also have these approval events. An approvals are basically, you are authorizing someone else to you know act on your behalf. And we will talk about why
you might wanna do that but that is the basic idea
behind these approval events, where you’re giving
someone else approval to you know make transfers or
take actions on your behalf. So some of the functions that we have, just quickly to query, what is the balance of
a particular account, who is the owner of a particular token, so each token has an ID that you can use to query
and get information about who the owners and other
information as well. And you can also just check whether a token actually exist or not. Then we have a bunch of
functions around approvals. So you can see with
approve you can specify which address you wanna
approve and a particular token ID that you want
to approve them for. And getapprove will just
return whoever is the currently approved person for a particular token so you can check if someone
is actually approved. And approval, so the difference between approve and approval for all is that you are giving approval
for all of your tokens instead of a particular one in approve. So in approve for all,
you can give someone authority to act on all of your tokens and one use case for that
might be if you are creating a new let’s say a board game where you have a bunch
of sort of magic cards and you want to give someone a deck of like 20 magic cards and
you want to approve them to be able to use that. So that’s one example of where you could use approvals. And you also have three functions related to transferring
and we will take a look at these implementations in a bit and what the differences between safe transfer and non-safe transfers. But this is just the interface, this is just the events and functions that we need to implement. And so opens up link provides us a basic implementation
called ERC721 basic token. And as you can see it extends the ERC721 basic interface
we were just looking at and it also use some other
libraries to do safe math and to provide some utility
functions on addresses. But so we’re not go into too much details in this particular implementation, but we are just gonna
take a look at what sort of information is being
tracked in this basic token. So we are keeping track of who were the token owners are, so it’s a mapping from a
token ID to an address, right, so we can quickly query, who the owner is of a particular token. We are keeping track of
token ID to approvals, right to know whether
someone is authorized to act on a particular token. And we are also keeping track of how many tokens a particular address owns. And we also have another mapping to keep track of more approvals, you can have multiple approvals, so this mapping helps
us keep track of that. And we have some modifiers that we’ll see how to use in a second, but this is just implementing
the interface that we saw and it provides us some
basic functionality that we can extend to create our game. So let’s see what that
would look like, right, well let’s try to create our own game, which is creating an ERC721
token, a non-fungible token and we will call it cryptomonsters. And let’s see how we can
use this code provided by OpenZeppelin to implement our own CryptoKitties like game. So I am just gonna go over to remix and let’s see what we have
here, so I have some of the code that I have loaded in from here, so I basically got these source code directly from this github. So these files over here I just downloaded and I opened them in remix and I just the main one
that we’re interested in is this basic token, but the rest are dependencies
for this contract. But we are gonna write a new one called cryptomonsters, so let’s open up our new contract. We are gonna declare the solidity version, so let’s go with .422 and we
can see that in the URL like it supports up to point .4.23, but 22 is good enough for our use cases. So we are gonna call this cryptomonsters and we are gonna say is
we are gonna extend the functionality of the ERC721 basic token. And it’s gonna complain
that it doesn’t know which token you are talking about. So that just mean you have to import this specific contract into your, into this cryptomonsters contract. So all we did is declared our own token called cryptomonsters that extends and basically takes the
functionality provided by this ERC721 basic token and allow us to just focus
on hour-specific games logic, instead of worrying about
the details of implementing the interface and the intricacies of that. So let me just bring this up here, so the first thing we’ll
do is start by defining a struct for our monster. And we just call this struct monster and some of the characteristics that we are gonna have for our monster and this can be anything
you like depending on the rules of your game, but let’s give our monster a name. We will also give them a level or a rank and they also have some attack power as well as some defense power. And we are gonna use this powers to make our monsters battle and try to level up our monsters and get them to be really strong. So we have defined our monsters struct and now we are just gonna keep track of all the monsters
that exist in the world. So we are just gonna define an array, which is gonna keep track of our monsters. You could also use
something like a mapping, but we are just gonna use a simple array and use the index of the array as the ID of that particular monster. So the first monster is
gonna have index zero. And another thing will do
is keep track of the owner, of the owner’s address and we’ll record this in our constructor and we’ll do this so that only the owner can create a crypto monsters. So okay we have our constructor clear public to make the warning go away. Here, what’s this other warning, it says, it’s duplicated,
use constructor, okay. So this is telling me to
use the constructor keyword, we won’t get into that right now. I am gonna leave that as function and let you dig deeper into that warning. But what we are gonna
define is the function to actually create our monsters, right. So let’s define a create monster function and this gonna be public and we only want the
owner to be calling this because as the owner of the game, you should be or you can
create any rules you want, but for this particular game only the owner will be able to create monsters. So if we just get the
new monster’s ID, right, and which is gonna be the
length of the monsters Array. So when it’s empty, the
length is gonna be zero, so the ID of the first
monster is gonna be zero and then we are just
gonna say monsters.push, this new monster and we are gonna give it some default values to begin with. So we need to give it a name, we need to give it a level, so we’ll start with level one and you can pick any values you want for the attack power or defense power, but we are just gonna stick with hundred as the initial defaults for our monster. So let’s also add a parameter
where you can specify the name of the monster you wanna create. And we are also going to specify which address we are
assigning this monster to. So this function will
basically create a new monster and give ownership of
that monster that NFT that ERC721 token to the address
specified by this variable. And so right now, we haven’t
written any codes so far in these three lines
that actually associates the monster to this address. So what we are gonna do for that is actually use one of
the functions provided by this ERC721 basic token called mint. And what mint does is actually create a new token for us and assigns it to the address that we specify. So let’s go and take a
look at this function in here little quick. Mint internal function
to mint a new token, reverts if the token ID already exists, and the two is the address
that will own the token and second parameter
is the ID of the token. And you can see that this
logic is already provided, is already written for us, so we don’t need to worry about building that logic for ourselves. Another thing to notice over here is that when we are emitting or when we are firing this transfer event we actually added this emit keyword. And this is to make it clear that we are actually firing an event because if we take out this keyword then it kind of looks like
we are calling a function. It’s indistinguishable from
this add token to function. So in recent version, we
added this a emit keyword to make it more obvious
that you are actually firing an event and not calling a function. By we are just using that
same emit function over here to say hey emit a new token and assign it to this to address and the token will have
this particular ID. And so that’s all we need
for creating monsters. So we have created a function where only the owner of the game, the owner of cryptomonsters
can issue people new monsters and give that monster a name, okay cool. Now we want to provide a way for the actual game mechanics, right. So our actual logic of the game, we want our monsters to battle. So let’s write a battle function and see what that would look like. And we are gonna make it public. And we are going to give it the monster ID to use as well as the target
ID of the other monster that is engaged in the battle Right, and we are gonna
use one of the modifiers from our parent classes here
called the only owner of and we are gonna pass it this monster ID. So only the owner, only the person who is the
owner of this monster ID can make it battle, can call this battle function
on that particular token. And so what we are gonna do, I am just gonna minimize
this terminal real quick, so what we are gonna do is get a reference to our monsters that are specified and we are gonna say monster1 equals monsters specified by this ID. Right and that’s just
the index in the array. And the second monster, monster2 is the target monster. Okay, cool, so we have got reference to our two monsters and now we need to write the logic that will actually make them battle and effect their characteristics like their level, their attack power, and the defense power. You can write whatever
logic you want, right, that’s up to your creativity,
but for this example, we are just gonna keep it really simple and we are gonna say
that if we attack power of the first monster is
greater than or equal to the other monster defense power. Then monster1 will be the winner, because it has the stronger attack power than the other monster’s defense power. So in this case what we are gonna say is monster1.level and just incremented by one so you level up because
you would win this battle and monster1s attack
power would also go up as a result of this victory. And let’s say it goes up by ten. But if this condition isn’t met, right, then monster2 will win, right, so monster2.level will increment by 1. And its defense power will
increase by 10 as well. So this is, these are the changes that happens if monster1 wins and these are the changes that
happens if monster2 wins, but basically with every
battle that you win, you level up and it effects either your attack power or your defense power. Sweet, this is all the
logic you really need for a really simple ERC721 token and we have written a
very simplified version of crypto kitties called
crypto monsters where you can create monsters as the owner of the game and you can get them to battle each other. So let’s see what this would
look like if we run it. We gonna bring up our
JavaScript’s VM test block chain and we are just going to deploy our crypto monsters’ contract and there is no parameters initially so just create, right, and you will see that all the
functions that are provided there is a lot more than we
declared in our contract, right, and that’s because
we are inheriting from this ERC721 basic token all
this other functionality. But, okay let’s create a monster and so the owner is the
account that deployed and if create a monster called Pikapika and we assign it to ourselves, all right, so this is the address and we are gonna say create monster, let’s bring up our terminal
to see whether our cause are going through this
with the create monster. And it’s succeeded, right, so now if I check, you
know if I just paste that same address into here into the balance of function, we get one, right,
because I have one token that is assigned to me and we can also check the exist function so it’s the first one, so the token ID will be zero And we can check that
it exist and the owner also matches up with our
address over here awesome. Cools so now let’s create
a second monster as well and we’ll call it Dumbledore, so create monster and now
I wanna check my balance, I should have two because I have two monsters assigned to me and okay so that now I have two monsters, I can actually make them battle. The monsters don’t have to be owned by two different addresses So let’s see what it looks like if we make these monsters battle. So I am gonna take the
monster ID of zero and one Right, and let’s just check
our monster statistics before we go forward. So for Pikapika and for Dumbledore the value should be default because they haven’t been in battle so they are both level one. But let’s bring up, let’s keep a track of Pikapika’s power. So when I click battle, Pikapika is gonna battle Dumbledore and I click battle, so make sure the transaction
went through, right, it was successful and so now, so my balance will stay the same, right, because still have the
same number of tokens, but if I check the stats
over here of Pikapika, so if I refresh this I
click monsters again, I see that Pikapika actually won, so it leveled up and its
attack power went up by ten. So similarly you can battle
your different monsters and level up or and gain more power and so each of these monsters through each of their battles, they will have different levels, different attack powers,
different defense power, which will make them unique and you could also attach something like a cool image to each of these tokens. But we basically created a simple game using ERC721 interface using an implementation
provided by open zeppelin. And if you want to transfer as well, so let’s try transferring
one of our tokens. So Pikapika has the most
attack power in the game, so you know someone is interested and you know trading
something for Pikapika, so I am gonna so let’s say they gave me like five bucks in person
and now I can transfer them ownership of Pikapika, so let’s just look at
our say if transfer from, I just wanna get look at
the parameters that it has now it’s over, so from to and token ID, right. So the from is gonna be our author, right or it’s gonna be us,
who is the owner, right and let’s say we wanna
transfer to the second account and pass that and zero for the
token ID of Pikapika, right and so right now the owner of Pikapika, which is token ID zero
is the CA35 account, but after I go through
this transfer function and it succeeds or execution failed. Let’s see why execution had failed, oh okay, because I didn’t
copy the to address properly so let’s try this again. Oh, because I am actually not Oh I did from the wrong account, so let me try that again, I’ll try to fix this arguments as well. So the first argument
should be CA35, right and then second argument
should be 147 something, so this argument is wrong, so just delete that and
I just try this again, copy and paste 160C, okay
so that looks correct and transferring zero for Pikapika and we have to do it from
the owner account, right so now if I try to click say if transfer, I’ll do this again, all right and okay this time execution succeeded, so now if I check the ownership of the zero token ID again,
this should be different because I just transferred it
over to the second account. So I click it and I get back
the new owner of this token. So yeah, that’s just a very simple example of how to build a non-fungible token and how to build a game using some you know cool game mechanics that you can dream up of. So I highly encourage you to check out this ERC721 basic token further and read through the EIP to understand the use cases better and yeah also check out the code for crypto kitties a lot of it but not all of it is available on places like etherscan, so you definitely check that out and learn from that as well and see how you know they are approaching their game mechanics. But yeah, thanks for watching this simple ERC721 example and we hope to see you next time.

16 thoughts to “How To Build a Simple Ethereum Game – Live Coding”

  1. A simple way to understand the ERC-721 now and how one should be able to implement it with any application in mind. Thanks haseeb!

  2. A small technical point though, a bitcoin that has gone through a mixer may be less fungible given the taint of possible money laundering.

  3. During the crypto winter bear markets I like watching these live coding sessions to motivate me to learn more and keep spirits up. Enjoyed this video very much 😀

  4. Como se hace la imagen de lo que vamos hacer? ejemplo quiero hacer un token erc721 con perros unicos como lo haria??

  5. Hey, @blockgeeks!
    I'm looking for a skilled coder that can work on an ERC721 project with me. I'm drafting a project that's really groundbreaking!
    Someone as skilled as yourself would be amazing! Where do you recommend I look for a potential partner?

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