This would unblock #2010.
Closes#2032.
Closes#1880.
I know this comes out of the blue, but it seems to me the right change to do. This is a proposal, let me know what you think.
First, I started with the fact that we need to make `CancelWhen()` public.
Then, I realized that I don't like the `CancelWhen()` naming. `CancelAt()` is better?
But `CancelAt()` sounds like an order to do something, not as getter.
So, it should be named `Get...`. `GetCancelWhen()`? `GetCancelAt()`? Sounds strage.
`GetDeadline()` - not bad, bit it is not that clear, what the deadline is.
And I looked at the `WithDeadline()` method. And in comment, the first line is `@brief Create a context with expiration.`.
"Expiration" sounds better explanation for the essense of the "deadline".
So, I went with `GetExpiration()`. I think "Expiration" is also a better name, because should we want to add the method called "bool IsExpired()", it comes naturally using the existing terminology, sounds better than "`IsPastDeadline()`".
Next thing - if we have "`GetExpiration()`", it is strange to have a method called `Get()`. So, we have `WithExpiration()` and `WithValue()`. So, it sounds like the getter should be called `GetValue()`. I did that rename as well.
Then, I looked at "`With...`" method naming. It is a factory method. If for getters we have `Get`, then for factory methods we should have `Create`. So, I renamed `WithExpiration()` and `WithValue()` to `CreateWithExpiration()` and `CreateWithValue()`.
Then I looked at `Context::time_point` typedef. First, in general, if we can avoid public typedefs, it is better, because we don't need to document them and worry if we broke client code when we change them.
Second, it is strange that we use `Azure::DateTime` everywhere, but not in context.
So, I replaced it with `Azure::DateTime`. It is good because it allows us to drop to/from epoch conversions (#1880), and really all that extra dance we do to cast to representation and back to datetime is the ways to stay within legal type casting limits of compiler type declaration, while in reality there isn't much that is happening in the code: `DateTime` is essentially an `int64`, and with all these conversions to epoch time, then to representation and back, compiler does not generate real code, it still operates with that only `int64`.
Why we cast to `DateTime::rep` and back at all? Context stores it as `atomic`, and I am not questioning that. You can't get `std::atomic<DateTime>` today explicitly, so we "extract" representation (`int64`) and store it as `std::atomic<DateTime::rep>`, which is the same thing as `std::atomic_int64`.
-- Update: --
1. "Expiration" => "Deadline" (Jeffrey's request)
2. Added `Context::HasExpiration()` (response to Jinming) // plus, we do have `HasValue()` so why not have `HasDeadline()`.
3. `WithDeadline()`, `WithValue` => Two overloads of `CreateChildContext()` (my own initiative).
-- Update 2: --
`CreateChildContext()` => `WithDeadline()`, `WithValue`
-- Update 3: --
Removed `HasDeadline()`
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