app_hooks.h

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/* Copyright (C) 2022 Wildfire Games.
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/*
 * hooks to allow customization / app-specific behavior.
 */

/*

Background
----------

This codebase is shared between several projects, each with differing needs.
Some of them have e.g. complicated i18n/translation facilities and require
all text output to go through it; others strive to minimize size and
therefore do not want to include that.
Since commenting things out isn't an option with shared source and
conditional compilation is ugly, we bridge the differences via "hooks".
These are functions whose behavior is expected to differ between projects;
they are defined by the app, registered here and called from lib code via
our trampolines.


Introduction
------------

This module provides a clean interface for other code to call hooks
and allows the app to register them. It also defines default stub
implementations.


Usage
-----

In the simplest case, the stubs are already acceptable. Otherwise,
you need to implement a new version of some hooks, fill an
AppHooks struct with pointers to those functions (zero the rest),
and call app_hooks_update.


Adding New Functions
--------------------

Several steps are needed (see below for rationale):
0) HOOKNAME is the name of the desired procedure (e.g. "bundle_logs")
1) add a 'trampoline' (user visible function) declaration to this header
   (typically named ah_HOOKNAME)
2) add the corresponding implementation, i.e. call to ah.HOOKNAME
3) add a default implementation of the new functionality
   (typically named def_HOOKNAME)
4) add HOOKNAME member to struct AppHooks declaration
5) set HOOKNAME member to def_HOOKNAME in initialization of
   'struct AppHooks ah'
6) add HOOKNAME to list in app_hooks_update code


Rationale
---------

While X-Macros would reduce the amount of work needed when adding new
functions, they confuse static code analysis and VisualAssist X
(the function definitions are not visible to them).
We prefer convenience during usage instead of in the rare cases
where new app hook functions are defined.

note: an X-Macro would define the app hook as such:
extern const wchar_t*, translate, (const wchar_t* text), (text), return)
.. and in its various invocations perform the above steps automatically.

*/

#ifndef INCLUDED_APP_HOOKS
#define INCLUDED_APP_HOOKS

#include "lib/os_path.h"

// trampolines for user code to call the hooks. they encapsulate
// the details of how exactly to do this.

/**
 * return path to directory into which crash dumps should be written.
 *
 * must be callable at any time - in particular, before VFS init.
 * paths are typically relative to sys_ExecutablePathname.
 *
 * @return path ending with directory separator (e.g. '/').
 **/
extern const OsPath& ah_get_log_dir();

/**
 * gather all app-related logs/information and write it to file.
 *
 * used when writing a crash log so that all relevant info is in one file.
 *
 * the default implementation attempts to gather 0ad data, but is
 * fail-safe (doesn't complain if file not found).
 *
 * @param f file into which to write.
 **/
extern void ah_bundle_logs(FILE* f);

/**
 * display an error dialog, thus overriding sys_display_error.
 *
 * @param text error message.
 * @param flags see DebugDisplayErrorFlags.
 * @return ErrorReactionInternal.
 *
 * the default implementation just returns ERI_NOT_IMPLEMENTED, which
 * causes the normal sys_display_error to be used.
 **/
extern ErrorReactionInternal ah_display_error(const wchar_t* text, size_t flags);


/**
 * holds a function pointer (allowed to be NULL) for each hook.
 * passed to app_hooks_update.
 **/
struct AppHooks
{
    const OsPath& (*get_log_dir)();
    void (*bundle_logs)(FILE* f);
    ErrorReactionInternal (*display_error)(const wchar_t* text, size_t flags);
};

/**
 * update the app hook function pointers.
 *
 * @param ah AppHooks struct. any of its function pointers that are non-zero
 * override the previous function pointer value
 * (these default to the stub hooks which are functional but basic).
 **/
void app_hooks_update(AppHooks* ah);

/**
 * was the app hook changed via app_hooks_update from its default value?
 *
 * @param offset_in_struct byte offset within AppHooks (determined via
 * offsetof) of the app hook function pointer.
 **/
extern bool app_hook_was_redefined(size_t offset_in_struct);
// name is identifier of the function pointer within AppHooks to test.
#define AH_IS_DEFINED(name) app_hook_was_redefined(offsetof(AppHooks, name))

#endif  // INCLUDED_APP_HOOKS