Dev:Logos

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.

Overview
The syntax provided by Logos greatly simplifies the development of MobileSubstrate extensions ("tweaks") which can hook other methods throughout the OS. 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 its page.

Examples
Here is an example of a very simple Logos tweak generated by logify.pl

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:

%init
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
Open a hook block for the class named Classname.

Here's a trivial example:

%subclass
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
Begin a hook group (for conditional initialization or code organization) with the name Groupname. All ungrouped hooks are in the implicit "_ungrouped" group.

%class
Forward-declare a class. Outmoded by %c, but still exists. Creates a $Class variable, and initializes it with the "_ungrouped" group.

%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
Generate an anonymous constructor (of default priority).

%end
Close a hook/subclass/group block.

%config
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
Evaluates to Class at runtime. If specified the + sigil, evaluate to MetaClass instead of Class. If not specified, the sigil defaults to -, evaluate to Class.

%orig
Call the original hooked method. Doesn't function in a %new'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 self and _cmd, Logos does this for you.

%log
Dump the method arguments to syslog. Typed arguments included in %log 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.

Extension for Logos
xi or xmi file can be used Logos directives as #define macro for preprocessor.