Update single and multi asset example in docs hub (#6594)

## Description

The single and multi asset examples in the docs hub do not use the
latest versions of the SRC3 and SRC20 standard. This has been updated to
follow the specifications.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

docs/book/src/blockchain-development/native_assets.md

@@ -199,18 +199,27 @@ It implements the [SRC-20; Native Asset](https://github.com/FuelLabs/sway-standa
 // ERC20 equivalent in Sway.
 contract;
 
-use src3::SRC3;
-use src5::{SRC5, State, AccessError};
-use src20::SRC20;
+use standards::{
+    src3::SRC3,
+    src5::{
+        SRC5, 
+        State, 
+        AccessError,
+    },
+    src20::{
+        SetDecimalsEvent, 
+        SetNameEvent, 
+        SetSymbolEvent, 
+        SRC20, 
+        TotalSupplyEvent,
+    },
+};
 use std::{
     asset::{
         burn,
         mint_to,
     },
-    call_frames::{
-        contract_id,
-        msg_asset_id,
-    },
+    call_frames::msg_asset_id,
     constants::DEFAULT_SUB_ID,
     context::msg_amount,
     string::String,
@@ -282,14 +291,21 @@ impl SRC5 for Contract {
 // Mint and Burn Standard
 impl SRC3 for Contract {
     #[storage(read, write)]
-    fn mint(recipient: Identity, sub_id: SubId, amount: u64) {
-        require(sub_id == DEFAULT_SUB_ID, "incorrect-sub-id");
+    fn mint(recipient: Identity, sub_id: Option<SubId>, amount: u64) {
+        require(sub_id.is_some() && sub_id.unwrap() == DEFAULT_SUB_ID, "incorrect-sub-id");
         require_access_owner();
 
+        let new_supply = storage.total_supply.read() + amount;
         storage
             .total_supply
-            .write(amount + storage.total_supply.read());
+            .write(new_supply);
         mint_to(recipient, DEFAULT_SUB_ID, amount);
+        
+        TotalSupplyEvent::new(
+            AssetId::default(), 
+            new_supply, 
+            msg_sender().unwrap()
+        ).log();
     }
 
     #[storage(read, write)]
@@ -302,10 +318,17 @@ impl SRC3 for Contract {
         );
         require_access_owner();
 
+        let new_supply = storage.total_supply.read() - amount;
         storage
             .total_supply
-            .write(storage.total_supply.read() - amount);
+            .write(new_supply);
         burn(DEFAULT_SUB_ID, amount);
+        
+        TotalSupplyEvent::new(
+            AssetId::default(), 
+            new_supply, 
+            msg_sender().unwrap()
+        ).log();
     }
 }
 
@@ -329,6 +352,24 @@ fn require_access_owner() {
         AccessError::NotOwner,
     );
 }
+
+abi EmitSRC20Events {
+    fn emit_src20_events();
+}
+
+impl EmitSRC20Events for Contract {
+    fn emit_src20_events() {
+        // Metadata that is stored as a configurable should only be emitted once.
+        let asset = AssetId::default();
+        let sender = msg_sender().unwrap();
+        let name = Some(String::from_ascii_str(from_str_array(NAME)));
+        let symbol = Some(String::from_ascii_str(from_str_array(SYMBOL)));
+
+        SetNameEvent::new(asset, name, sender).log();
+        SetSymbolEvent::new(asset, symbol, sender).log();
+        SetDecimalsEvent::new(asset, DECIMALS, sender).log();
+    }
+}
 ```
 
 ## Multi Native Asset Example
@@ -341,18 +382,27 @@ It implements the [SRC-20; Native Asset](https://github.com/FuelLabs/sway-standa
 // ERC1155 equivalent in Sway.
 contract;
 
-use src5::{SRC5, State, AccessError};
-use src20::SRC20;
-use src3::SRC3;
+use standards::{
+    src5::{
+        SRC5, 
+        State, 
+        AccessError
+    },
+    src20::{
+        SetDecimalsEvent, 
+        SetNameEvent, 
+        SetSymbolEvent, 
+        SRC20, 
+        TotalSupplyEvent,
+    }
+    src3::SRC3,
+};
 use std::{
     asset::{
         burn,
         mint_to,
     },
-    call_frames::{
-        contract_id,
-        msg_asset_id,
-    },
+    call_frames::msg_asset_id,
     hash::{
         Hash,
     },
@@ -401,28 +451,42 @@ impl SRC20 for Contract {
 // Mint and Burn Standard
 impl SRC3 for Contract {
     #[storage(read, write)]
-    fn mint(recipient: Identity, sub_id: SubId, amount: u64) {
+    fn mint(recipient: Identity, sub_id: Option<SubId>, amount: u64) {
+        require(sub_id.is_some(), "Error: SubId is None");
         require_access_owner();
-        let asset_id = AssetId::new(contract_id(), sub_id);
+
+        let asset_id = AssetId::new(ContractId::this(), sub_id.unwrap());
         let supply = storage.total_supply.get(asset_id).try_read();
         if supply.is_none() {
             storage.total_assets.write(storage.total_assets.try_read().unwrap_or(0) + 1);
         }
-        let current_supply = supply.unwrap_or(0);
-        storage.total_supply.insert(asset_id, current_supply + amount);
+        let new_supply = supply.unwrap_or(0) + amount;
+        storage.total_supply.insert(asset_id, new_supply);
         mint_to(recipient, sub_id, amount);
+        
+        TotalSupplyEvent::new(
+            asset_id, 
+            new_supply, 
+            msg_sender().unwrap()
+        ).log();
     }
     
     #[storage(read, write)]
     fn burn(sub_id: SubId, amount: u64) {
         require_access_owner();
-        let asset_id = AssetId::new(contract_id(), sub_id);
+        let asset_id = AssetId::new(ContractId::this(), sub_id);
         require(this_balance(asset_id) >= amount, "not-enough-coins");
         
         let supply = storage.total_supply.get(asset_id).try_read();
-        let current_supply = supply.unwrap_or(0);
-        storage.total_supply.insert(asset_id, current_supply - amount);
+        let new_supply = supply.unwrap_or(0) - amount;
+        storage.total_supply.insert(asset_id, new_supply);
         burn(sub_id, amount);
+
+        TotalSupplyEvent::new(
+            asset_id, 
+            new_supply, 
+            msg_sender().unwrap()
+        ).log();
     }
 }
 
@@ -431,10 +495,10 @@ abi MultiAsset {
     fn constructor(owner_: Identity);
     
     #[storage(read, write)]
-    fn set_name(asset: AssetId, name: String);
+    fn set_name(asset: AssetId, name: Option<String>);
 
     #[storage(read, write)]
-    fn set_symbol(asset: AssetId, symbol: String);
+    fn set_symbol(asset: AssetId, symbol: Option<String>);
 
     #[storage(read, write)]
     fn set_decimals(asset: AssetId, decimals: u8);
@@ -448,23 +512,29 @@ impl MultiAsset for Contract {
     }
     
     #[storage(read, write)]
-    fn set_name(asset: AssetId, name: String) {
+    fn set_name(asset: AssetId, name: Option<String>) {
         require_access_owner();
         storage.name.insert(asset, StorageString {});
         storage.name.get(asset).write_slice(name);
+
+        SetNameEvent::new(asset, name, msg_sender().unwrap()).log();
     }
 
     #[storage(read, write)]
-    fn set_symbol(asset: AssetId, symbol: String) {
+    fn set_symbol(asset: AssetId, symbol: Option<String>) {
         require_access_owner();
         storage.symbol.insert(asset, StorageString {});
         storage.symbol.get(asset).write_slice(symbol);
+
+        SetSymbolEvent::new(asset, symbol, msg_sender().unwrap()).log();
     }
 
     #[storage(read, write)]
     fn set_decimals(asset: AssetId, decimals: u8) {
         require_access_owner();
         storage.decimals.insert(asset, decimals);
+
+        SetDecimalsEvent::new(asset, decimals, msg_sender().unwrap()).log();
     }
 }