Currently, when calling a service with its block form, a `#result`
method is automatically created on the caller object. Even if it never
clashed so far, this could happen.
This patch removes that method, and instead use a more classical way of
doing things: the result object is now provided as an argument to the
main block. This means if we need to access the result object in an
outcome block, it will be done like this from now on:
```ruby
MyService.call(params) do |result|
on_success do
# do something with the result object
do_something(result)
end
end
```
In the same vein, this patch introduces the ability to match keys from
the result object in the outcome blocks, like we already do with step
definitions in a service. For example:
```ruby
on_success do |model:, contract:|
do_something(model, contract)
end
```
Instead of
```ruby
on_success do
do_something(result.model, result.contract)
end
```
While using `OpenStruct` is nice, it’s generally not a very good idea as
it usually leads to performance problems.
The `OpenStruct` source code even says basically to avoid it.
Since the context object is crucial in our services, this patch replaces
`OpenStruct` with a custom implementation instead.
Currently in services, the `contract` step is only used to define where
the contract will be called in the execution flow. Then, a `Contract`
class has to be defined with validations in it.
This patch allows the `contract` step to take a block containing
validations, attributes, etc. directly. No need to then open a
`Contract` class later in the service.
It also has a nice side effect, as it’s now easy to define multiples
contracts inside the same service. Before, we had the `class_name:`
option, but it wasn’t really useful as you had to redefine a complete
new contract class.
Now, when using a name for the contract other than `default`, a new
contract will be created automatically using the provided name.
Example:
```ruby
contract(:user) do
attribute :user_id, :integer
validates :user_id, presence: true
end
```
This will create a `UserContract` class and use it, also putting the
resulting contract in `context[:user_contract]`.
