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# core

Clients use this module and renders state struct managed by this.
Actions are modeled as function returning `void` and clients listen to changes to the state via event callbacks.

```c
// Sample definition:

// This dispatches an action without waiting for a result.
void plac_play_song(plac_app*, plac_song*);

// First argument is event payload and second argument is context/user data.
typedef void (*plac_cb_playback_change)(plac_song*, void*);

// The last argument is context/user data, which will be passed to the callback.
void plac_on_playback_change(plac_app*, plac_cb_playback_change, void*);

// Events without payload have the following signature.
typedef void (*plac_cb_foo)(void*);
```

## Code Styles

This section describes module internal rules for writing code.

### Struct creation function

A function that creates `struct Foo`,

- should be named `init`
- should return a value (`Foo`), rather than a pointer of `Foo` (`*Foo`)
- can return an error union

This is what many Zig projects, including `std`, uses.
Let the caller decide where to store.

An exported function (C API) should wrap `init` function and assign the result to an allocated space.

```zig
const std = @import("std");

extern struct Foo {
	bar: i32,

	pub fn init() Foo {
		return .{ bar = 0 };
	}

	pub fn make() callconv(.C) ?*Foo {
		const dst = std.heap.c_allocator.create(Foo) catch return null;
		dst.* = init();
		return dst;
	}
}

comptime {
	@export(&Foo.make, .{ .name = "plac_foo_make" });
}
```

### Struct destroy function

A function that releases resources owned by `struct Foo`,

- should be named `deinit`
- should return `void`
- should not return an error union
- should not destroy a pointer to itself

An exported function (C API) should wrap `deinit` function and free the pointer to itself.

```zig
const std = @import("std");

extern struct Foo {
	pub fn deinit(self: *const Foo) void {
		// ...
	}

	pub fn free(self_ptr: ?*const Foo) callconv(.C) void {
		const self = self_ptr orelse return;
		self.deinit();
		std.heap.c_allocator.destroy(self);
	}
}

comptime {
	@export(&Foo.free, .{ .name = "plac_foo_free" });
}
```

### Function parameters

Zig team has a plan to drop automatic pass-by-reference conversion for function parameters.
To minimize the cost of future migration, do not annotate a parameter with value type if pass-by-value is not desirable.
For this reason, struct methods should use pointer type for `self` parameter.

```zig
struct Foo {
	// OK
	pub fn bar(self: *Foo): void {}

	// OK
	pub fn bar(self: *const Foo): void {}

	// NG
	pub fn bar(self: Foo): void {}
}
```

### Exported functions

When an exported function takes a pointer, it should be an optional pointer rather than regular pointer, to guard against accidentally passing NULL pointer.

```zig
extern struct Foo {
	pub fn bar(self: *const Foo) void {
		// ...
	}
}

export fn do_something(foo_ptr: ?*const Foo) void {
	const foo = foo_ptr orelse return;
	foo.bar();
}
```

If the function returns result code, define a code for receiving a NULL pointer.
If the function returns a pointer, return a NULL pointer and document about it in header file.