Parameters

Parameters

Hook developers may opt to use install-time parameters (called Hook Parameters) in their Hook. This allows subsequent installers of the Hook to change certain behaviours the programmer defines without recompiling or re-uploading the Hook (assuming at least one account still references the existing Hook Definition.)

Hook Parameters are a set of Key-Value pairs set during the SetHook Transaction and retrievable by the Hook during runtime. Both the ParameterName (key) and the ParameterValue are set as hex blobs, and have a maximum length of 32 bytes and 256 bytes respectively.

A SetHook Transaction may define up to 16 Hook Parameters per installed Hook.

Setting Parameters

The HookParameters array is optionally defined inside each Hook in the Hooks array as shown below:

TransactionType: "SetHook",
Hooks:
[
    {
        Hook: {
            ...,
            HookParameters:
            [
                {
                    HookParameter:
                    {
                        HookParameterName:  "ABCDEF12",
                        HookParameterValue: "12345678"
                    }
                },
                ...  // optionally up to 15 more Hook Parameters
            ]
        }
    }
],
...

Default Parameters

The first user to set a novel Hook may define Hook Parameters which then become the Default Parameters for that Hook. This means any subsequent users who references the same HookDefinition will receive these originally set Hook Parameters by default.

The subsequent user may specify their own Parameters, overriding the Default Parameters for their installation.

To erase a Parameter in a subsequent installation, specify the ParameterName key without specifying a ParameterValue key.

Using Parameters in Hooks

Parameters can be read by the Hooks they are set on using hook_param.

If more than one Hook is installed in a Hook Chain, then hook_param_set can be used in limited circumstances to modify the Hook Parameters of a Hook further down the chain on the same account.

Runtime Parameters

On Xahau and the Xahau testnet, HookParameters may also be included at the top level of any transaction type according to the foregoing rules and size limits. These parameters can be accessed inside a hook using the otxn_param API.