embark-dtwitter-workshop/4 Coding: contract.md at start

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Coding: DTwitter contract

We can start updating our /contracts/DTwitter.sol contract code and watch as Embark watches and recompiles everything for us as we go.

Functions

Let’s begin by writing some of our contract functions.

Create account

The createAccount function will be used for creating users in the contract. We need to add the new user to the user/address mapping so we can keep track of the address that created the user.

Add a user to the users mapping and populate details (creationDate, owner, username, description)
```
users[usernameHash].creationDate = now; // 'now' is not recommended for accuracy
users[usernameHash].owner = msg.sender;
users[usernameHash].username = username;
users[usernameHash].description = description;
```
Note: we are using the block timestamp now to specify a creation date for the user (and also for the tweet time as we'll see later on). This application of now is fine, as ther are no reliances on this block timestamp for randomness. Please see the Solidity documentation for more information.
Add an entry to our owners mapping so we can retrieve a user by their address. This will be useful for when we need to find out if our current address is associated with a user.
```
owners[msg.sender] = usernameHash;
```

Edit account

We will be updating the user's profile in the editAccount function, so we need to be able to update the user's description and picture in the contract.

Update the description (could be empty):

users[usernameHash].description = description;

Only update the user's picture if the hash passed in is not empty or null (essentially disallows deletions):
```
if (bytes(pictureHash).length > 0) {
   users[usernameHash].picture = pictureHash;
}
```

User exists

The userExists function is used for checking if a user exists in the users mapping stored in the contract.

In Solidity, we can see if an item in a struct exists by checking that a property of an item is not set to it's default value. For example, we can check if a specific user exists in the users mapping by see if the user's creationDate is not 0 (the default value in Solidity for the block timestamp which is a uint).
```
return users[usernameHash].creationDate != 0;
```

For the tweet function to work, we need to add the tweet to the user in the users mapping. First, we need to get the user associated with the address who initiated the contract call (the msg.sender). Then, we can store the tweet in the users mapping against that user. Finally, we can emit an event that a tweet occurred. Later, we will subscribe to this event to get tweets as they are added to the contract in real time.

Get our user from the usernameHash

User storage user = users[usernameHash];

Get our new tweet index

uint tweetIndex = user.tweets.length++;

Update the user's tweets at the tweet index
```
user.tweets[tweetIndex] = content;
```
Emit the tweet event and notify the listeners
```
emit NewTweet(usernameHash, content, now);
```

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