Difference between revisions of "Logos"

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Logos is a component of the [[Theos]] development suite that allows method hooking code to be written easily and clearly, using a set of special preprocessor directives.
{{soft redirect|https://theos.dev/docs/logos}}
== Overview ==
This article is obsolete, and has been replaced by the [https://theos.dev/docs/logos theos.dev website].
The syntax provided by Logos greatly simplifies the development of MobileSubstrate extensions ("tweaks") which can hook other methods throughout the OS.
If you need historical installation instructions, you can find the last version of this article [https://iphonedev.wiki/index.php?title=Logos&oldid=5734 here].
In this context, "method hooking" refers to a technique used to replace or modify methods of classes found in other applications on the phone.
== Getting Logos ==
Logos is distributed with [[Theos]], and you can use Logos' syntax in any Theos-built project without any extra setup.  For more information about Theos, visit [[Theos|its page]].
== Using Logos ==
=== Examples ===
Here is an example of a very simple Logos tweak generated by logify.pl
<source lang="logos">
%hook SSDownloadAsset
- (NSString *)finalizedPath { %log; NSString * r = %orig; NSLog(@" = %@", r); return r; }
- (NSString *)downloadPath { %log; NSString * r = %orig; NSLog(@" = %@", r); return r; }
- (NSString *)downloadFileName { %log; NSString * r = %orig; NSLog(@" = %@", r); return r; }
+ (id)assetWithURL:(id)url type:(int)type { %log; id r = %orig; NSLog(@" = %@", r); return r; }
- (id)initWithURLRequest:(id)urlrequest type:(int)type { %log; id r = %orig; NSLog(@" = %@", r); return r; }
- (id)initWithURLRequest:(id)urlrequest { %log; id r = %orig; NSLog(@" = %@", r); return r; }
- (id)_initWithDownloadMetadata:(id)downloadMetadata type:(id)type { %log; id r = %orig; NSLog(@" = %@", r); return r; }
You can use logify.pl to create a Logos source file from a header file that will log all of the functions of that header file. You can find logify.pl at $THEOS/bin/logify.pl and you would use it as so:
<source lang="bash">
$THEOS/bin/logify.pl ./SSDownloadAsset.h
=== List of Logos Directives ===
==== Initialization ====
===== %init =====
<source lang="logos">
%init([<class>=<expr>, …])
%init(Group[, [+|-]<class>=<expr>, …])
Initialize a group (or the default group). Passing no group name will initialize "_ungrouped", and passing class=expr arguments will substitute the given expressions for those classes at initialization time. The + sigil (as in class methods in Objective-C) can be prepended to the classname to substitute an expression for the metaclass. If not specified, the sigil defaults to -, to substitute the class itself. If not specified, the metaclass is derived from the class.
==== Block-level ====
The directives in this category open a block of code which must be closed by an %end directive (shown below).
===== %hook =====
<source lang="logos">%hook Classname</source>
Open a hook block for the class named ''Classname''.
Here's a trivial example:
<source lang="logos">
%hook SBApplicationController
-(void)uninstallApplication:(SBApplication *)application {
    NSLog(@"Hey, we're hooking uninstallApplication:!");
    %orig; // Call the original implementation of this method
===== %subclass =====
<source lang="logos">%subclass Classname: Superclass <Protocol, Protocol></source>
Subclass block - the class is created at runtime and populated with methods. ivars are not yet supported.
The %new specifier is needed for a method that doesn't exist in the superclass.
To instantiate an object of the new class, you can use the %c operator.
===== %group =====
<source lang="logos">%group Groupname</source>
Begin a hook group (for conditional initialization or code organization) with the name ''Groupname''. All ungrouped hooks are in the implicit "_ungrouped" group.
===== %class =====
<source lang="logos">%class Class</source>
{{warning|<tt>%class</tt> is deprecated. Do not use it in new code.}}
Forward-declare a class. Outmoded by %c, but still exists. Creates a $Class variable, and initializes it with the "_ungrouped" group.
===== %new =====
<source lang="logos">%new
Add a new method to a hooked class or subclass. ''signature'' is the Objective-C type encoding for the new method; if it is omitted, one will be generated.
===== %ctor =====
<source lang="logos">%ctor { … }</source>
Generate an anonymous constructor (of default priority).
===== %end =====
<source lang="logos">%end</source>
Close a hook/subclass/group block.
==== Inline ====
===== %config =====
<source lang="logos">%config(X=Y);</source>
Set a logos configuration flag.
======Configuration Flags======
* generator
**; MobileSubstrate
**: generate code that uses [[MobileSubstrate]] for hooking.
**; internal
**: generate code that uses only internal Objective-C runtime methods for hooking.
* warnings
**; none
**: suppress all warnings
**; default
**: non-fatal warnings
**; error
**: make all warnings fatal
* dump
**; yaml
**: dump the internal parse tree in YAML format
**; perl
**: dump the internal parse tree in a format suitable for evaluation as perl source.
===== %c =====
<source lang="logos">%c([+|-]Class)</source>
Evaluates to <tt>Class</tt> at runtime. If specified the + sigil, evaluate to MetaClass instead of Class. If not specified, the sigil defaults to -, evaluate to Class.
===== %orig =====
<source lang="logos">
Call the original hooked method. Doesn't function in a <tt>%new</tt>'d method. Works in subclasses, strangely enough, because MobileSubstrate will generate a supercall closure at hook time. (If the hooked
method doesn't exist in the class we're hooking, it creates a stub that just calls the superclass implementation.) args is passed to the original function - don't include <tt>self</tt> and <tt>_cmd</tt>, Logos does this for you.
===== %log =====
<source lang="logos">
%log([(<type>)<expr>, …])
Dump the method arguments to syslog. Typed arguments included in <tt>%log</tt> will be logged as well.
=== Splitting Logos Hooking Code Across Multiple Files ===
By default, the Logos pre-processor will only process one .xm file at build time. However, it is possible to split the Logos hooking code into multiple files.
First, the main file has to be renamed to an .xmi file. Then, other .xm files can be included in it using the #include directive. The Logos pre-processor will add those files to the main file before processing it.
.xmi files will be preprocessed first, then Logos will process the result, and then it will be compiled. 
.xm will be processed by Logos and then preprocessed & compiled.
[[Category:Development Tools]]

Latest revision as of 18:12, 19 February 2022

Soft redirect to:https://theos.dev/docs/logos

This article is obsolete, and has been replaced by the theos.dev website.

If you need historical installation instructions, you can find the last version of this article here.