Environmental Variables

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Every method execution has a environment associated with information such as:

  1. Who called the method
  2. How much money is attached to the call
  3. How many computational resources are available
  4. The current timestamp

Environment Variables









Who is Calling? Who am I?

The environment gives you access to 3 important users: the current_account, the predecessor, and the signer.

Current Account

The current_account contains the address in which your contract is deployed. This is very useful to implement ownership, e.g. making a public method only callable by the contract itself.

Predecessor and Signer

The predecessor is the account that called the method in the contract. Meanwhile, the signer is the account that signed the initial transaction.

During a simple transaction (no cross-contract calls) the predecessor is the same as the signer. For example, if alice.near calls contract.near, from the contract’s perspective, alice.near is both the signer and the predecessor. However, if contract.near creates a cross-contract call, then the predecessor changes down the line. In the example bellow, when pool.near executes, it would see contract.near as the predecessor and alice.near as the signer.

img
You can access information about the users interacting with your smart contract

::tip
In most scenarios you will only need to know the predecessor. However, there are situations in which the signer is very useful. For example, when adding NFTs into this marketplace, the contract checks that the signer, i.e. the person who generated the transaction chain, is the NFT owner.

::

Balances and Attached NEAR

The environment gives you access to 3 token-related parameters, all expressed in yoctoNEAR (1 Ⓝ = 1024yⓃ):

Attached Deposit

attached_deposit represents the amount of yoctoNEAR the predecessor attached to the call.

This amount is already deposited in your contract’s account, and is automatically returned to the predecessor if your method panics.

::warning
If you make a cross-contract call and it panics, the funds are sent back to your contract. See how to handle this situation in the callback section

::

Account Balance

account_balance represents the balance of your contract (current_account).

It includes the attached_deposit, since it was deposited when the method execution started.

If the contract has any locked $NEAR, it will appear in account_locked_balance.


Storage Used

storage_used represents the amount of storage that is currently being used by your contract.

::tip
If you want to know how much storage a structure uses, print the storage before and after storing it.

::

Telling the Time

The environment exposes three different ways to tell the pass of time, each representing a different dimension of the underlying blockchain.

Timestamp

The timestamp attribute represents the approximated UNIX timestamp at which this call was executed. It quantifies time passing in a human way, enabling to check if a specific date has passed or not.

Current Epoch

The NEAR blockchain groups blocks in Epochs. The current_epoch attribute measures how many epochs have passed so far. It is very useful to coordinate with other contracts that measure time in epochs, such as the validators.

Block Index

The block_index represents the index of the block in which this transaction will be added to the blockchain.


Gas

Your contract has a limited number of computational resources to use on each call. Such resources are measured in Gas.

Gas can be thought of as wall time, where 1 PetaGas (1_000 TGas) is ~1 second of compute time.

Each code instruction costs a certain amount of Gas, and if you run out of it the execution halts with the error message Exceeded the prepaid gas.

The environment gives you access to two gas-related arguments: prepaid_gas and used_gas.

Prepaid Gas

prepaid_gas represents the amount of Gas the predecessor attached to this call. It cannot exceed the limit 300TGas (300 * 1012 Gas).

Used Gas

used_gas contains the amount of Gas that has been used so far. It is useful to estimate the Gas cost of running a method.

::warning
During cross-contract calls always make sure the callback has enough Gas to fully execute.

::

::tip
If you already estimated the Gas a method needs, you can ensure it never runs out of Gas by using assert


“`rust
const REQUIRED_GAS: Gas = Gas(20_000_000_000_000); // 20 TGas
assert!(env::prepaid_gas() >= REQUIRED_GAS, “Please attach at least 20 TGas”);
“`

“`ts
const TGas: u64 = 1000000000000;
assert(context.prepaidGas >= 20*TGas, “Please attach at least 20 TGas”);
“`

:::

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