Manual metadata loading

Introduction

When the other two metadata systems aren’t suitable it is also possible to load metadata through code. The other two systems use the functionality described in this section to actually load the metadata.

See also

BridgeSupport XML files
Loading metadata from XML files
Compiled metadata system
Loading metadata using compiled files

Metadata dictionaries

One or the arguments for loadBundleFunctions(), loadFunctionList() and registerMetaDataForSelector() contains (or “is” for registerMetaDataForSelector()) a metadata dictionary containing information about function and method interfaces that cannot be extract from a basic type signature for that function or method.

Copies of these structures can be also retrieved at runtime using the __metadata__() method on both function and selector objects, which makes it possible to introspect the metadata information when needed.

The metadata is a Python dictionary with a particular structure (all keys are optional):

  • arguments: A dictionary containing more information on arguments. The keys of this dictionary are integers with the argument offset (for methods index 0 is the first implicit argument, index 2 is the first argument that is visible in a prototype). The values are metadata dictionaries for the arguments and are decribed later on.

    In metadata that is returned the __metadata__() method of function and selector objects the arguments value is a tuple with items for all arguments.

  • retval: A metadata dictionary with more information on the return value. The contents of this dictionary is described later on.

  • suggestion: For methods only: the method should not be called from Python, and calling it will raise and exception with the suggestion value in the exception message.

  • variadic: If present and the value is True the function or method takes a variable number of arguments. PyObjC can only call such functions when either on of the arguments is a printf_format, or the dictionary contains information on the argument array (as described by keys further on in this list).

  • c_array_delimited_by_null: If present and the value is True, and the function is a variadic function, the variable part of the function argument list is a list of values where the last item of the list is a null value. All elements of the list are the same type, that of the last type that is present in the prototype.

    In python the function is called with the additional arguments after the fixed arguments (just like in C), but without a null value at the end of the argument array.

    An example of such a function is execlp(3)

  • c_array_length_in_arg: If present and the value is an integer, and the function is a variadic function, the variable part of the function argument list is a list of values and the value for this key indicates which function argument contains the length of that list. All elements of the list are the same type, that of the last type that is present in the prototype.

    In python the function is called with the additional arguments after the fixed arguments (just like in C).

Keys not listed above will be ignored by the bridge.

Note

The bridge currently does not copy the metadata when you register it with the functions listed above. Don’t rely on that behavior, it might change in a future release and changes to metadata dictionaries may or may not affect bridge behavior (basicly depending on if the change occurs before or after the bridge first uses the metadata)

c_array_lenght_in... interpretation

The keys c_array_length_in_arg (function, argument and return value metadata) and c_array_length_in_result (argument metadata) describe the index of an argument that contains the size of a C array. This section describes how the bridge determines the value to use for the array.

For _C_INOUT argument the value to use for converting to C and back from C is calculated both before and after the function call, which means that the length of the array passed to the function can have a different length than the array returned from the function. This is used for function where a buffer is passed into the function and the function indicates the useable size of that buffer by modifiying a pass-by-reference argument.

How the size of the C array is calculated depends on the type of the argument:

  • When the type is a pointer type the value is calculated by dereferencing the pointer.
  • When the type is a integer (for example, int, unsigned long or NSInteger) the length is the value of the C argument.
  • When the type is NSRange or CFRange the length of the C array is the lenght of the range.
  • When the type is id and the Objective-C instance responds to the “-count” selector the length of the C array is the result of calling that selector.
  • In all other cases the length cannot be calculated and the bridge raises an exception.

Argument and return value metadata

The argument and return value metadata is also a dictionary with a specific structure. As with the complete metadata dictionary all keys are optional unless the description mentions otherwise.

  • type: A byte string with the type encoding for the value. The default is extracted from the type encoding for the entire prototype (for methods this is extracted from the Objective-C runtime, for functions this is passed as one of the items in the function info tuple).

    This key is always present in the metadata returned by the __metadata__() method.

  • type_override: A byte string with value _C_IN, _C_OUT or _C_INOUT to indicate that the argument is an input, output or input/output argument. Ignored unless the type is a pointer type that isn’t a CoreFoundation instance.

    The value is assumed to be a single value (a pass-by-reference argument), unless there are keys in the dictionary that say otherwise (see further on in this list).

    This key is not used for return value metadata.

  • printf_format: If present and the value is True the argument value is a printf(3) style format string for a variadic function or method. Ignored unless the function or method actually is variadic.

  • sel_of_type: A byte string that describes the expected selector prototype for arguments of the _C_SEL.

    Used by the decorator selectorFor() to calculate the signature of the decorated method.

  • already_retained: Value True indicates that the return value, or a pass-by-reference output parameter, is returned to the caller with an increased reference count. An Objective-C caller will have to call “-retain” on the value when the value is no longer used.

    Used by the bridge to correctly maintain the Objective-C reference count. Python users do not have to maintain the reference count themselfes.

  • already_cfretained: Value True indicates that the return value, or a pass-by-reference output parameter, is returned to the caller with an increased reference count. An Objective-C caller will have to call “CFRelease” on the value when the value is no longer used.

    Used by the bridge to correctly maintain the Objective-C reference count. Python users do not have to maintain the reference count themselfes.

    Note

    Use either already_retained, or already_cfretained but not both.

    The two different keys are present to be able to support Objective-C Garbage Collection: in process with GC enabled the CoreFoundation and Cocoa retain count APIs behave differently. Because GC is on the way out and PyObjC doesn’t properly support GC anyway it is better to use already_retained where approprate and not use already_cfretained.

  • c_array_delimited_by_null: When True, and the argument or return value type is a pointer type, the value is a C array with a null value at the end. Python users do not have to supply the null value on calls, and the bridge will strip the null value in return values.

    When the type_override is _C_IN or _C_INOUT the input value must be a sequence of values (list, tuple, ...). The bridge allocates a buffer of the right size, converts all values and appends the approprate null value. The value can also be a buffer (such as an array.array of the approprate structure), which then must contain a null value at the end.

    When the type_override is _C_OUT the argument must be either NULL to indicate that a NULL pointer should be passed to the Objective-C function or a buffer object of the appropriate structure, and with enough room to store the function output including the null delimiter.

  • c_array_length_in_arg: The argument or return value is a C array where the length of the array is specified in another argument. Ignored when the type is not a pointer type. The value for this key is either a single integer, or two integers (for _C_INOUT arguments).

    When the type_override is _C_IN or _C_INOUT the input value must be a sequence of values of the correct lenght (at least the length that’s expected by the function, additional items in the sequence are ignored).

    When the type_override is _C_OUT the value can be NULL (NULL pointer passed to the function) or None (PyObjC allocates a C array of the right size and writes nul bytes in the entire buffer).

    When the value of the key is a single integer this argument index for the argument that contains the expected size of the array. When the value of the key is a tuple of two integers these are the indexes for the argument that contains the size that should be used when calling the function and the argument that contains the size of the array that is useable after the call.

  • c_array_of_fixed_length: When the type is a pointer type the actual argument (or result) is an C array of a fixed length. The value for this key is an integer that is the length of the C array.

  • c_array_of_variable_length: When the type is a pointer type the actual argument (or result) is a C array, but the lenght of the array is unknown or cannot be described in metadata.

    For results the bridge will return a value of varlist.

    For arguments with type_override value _C_IN or _C_INOUT the value for the arugment must be a Python sequence and the bridge will allocate a C array that is long enough to contain all items of that sequence; alternatively the argument can be a Python buffer object (simular to _C_OUT arugments). For _C_OUT arguments the value for the argument must be either NULL or a Python buffer object that will be passed to the function.

  • c_array_length_in_result: Only valid for argument metadata. When the argument type is a pointer type and the type_override is _C_INOUT or _C_OUT the usuable length of the array is calculated from the return value.

    The size of the buffer that needs to be allocated is determined using one of the other c_array... keys in the metadata dictionary.

  • null_accepted: If True and the argument is a pointer it is safe to pass a NULL as the value. Defaults to True.

    This key is not used in return value metadata.

    Note

    The metadata that is currently shipped with PyObjC does not contain null_accepted data. This means that the bridge won’t check if it safe to pass NULL as a value for pointer arguments, read the Cocoa documentation to check if passing NULL is safe.

  • callable: When type argument or return value has type “^?” or “@?” the method or function takes a function or block as the argument. In Python an arbitrary callable can be passed (but see callable_retained for some limitations).

    The value of this attribute contains the metadata describing the callable. It is a metadata structure as described in this section, with some additional limitations: the arguments key of the dictionary must describe all arguments of the callable (that is all keys in range(len(arguments)) must be present), the type key of the argument and return value metadata must be present (although it is allowed to leave out the return value metadata when the function has return type void).

    For blocks the argument array must include the first implicit argument at index 0 or the arguments array, and with type b”^v”.

  • callable_retained: Then True and callable is present and the argument type is b”^?” the callable argument will be retained by the Objective-C funtion or method beyond the call.

    This key is not used in return value metadata.

    When this value is True the argument must be a global object that is annotated with the decorator callbackFor(). That decorator ensures that the C representation of the function is always present to ensure that it is safe to store a reference on the Objective-C side of the bridge.

API description

Loading frameworks and other bundles

objc.loadBundle(module_name, module_globals[, bundle_path[, bundle_identifier[, scan_classes]]])

Load the bundle specified by bundle_path or bundle_identifier and add the classes in the bundle to module_globals. The classes are not added to the module_globals when scan_classes is False (it defaults to True).

If both a bundle_path and bundle_identifier are specified the function first tries to locate the bundle using the identifier and then using the path.

When bundle_identifier is specified the bundle is located using [NSBundle +bundleWithIdentifier:], and when bundle_path is specified the bundle is located using [NSBundle +bundleWithPath:].

Note

bundle_path must be an absolute path.

Note

The current implementation loads all Objective-C classes into module_globals, as testing if a class is located in a specific bundle is fairly expensive and slowed down application initialization too much.

Creating and registering types

objc.registerCFSignature(name, encoding, typeId[, tollfreeName])

Register a CoreFoundation based type with the bridge. If tollfreeName is specified the type is tollfree bridged to that Objective-C class.

The value of typeId is None for tollfree bridged types, and the result of the “GetTypeID” function for the type for other types.

Returns the class object for the registerd type.

objc.createOpaquePointerType(name, typestr, doc)

Return a wrapper type for opaque pointers (“handles”) of a given type. The type will be registered with the bridge and will be used to wrap values with the given type signature.

objc.createStructType(name, typestr, fieldnames, doc[, pack])

Create a type to wrap structs with a given name and type signature, this type will be used by the bridge to convert values of this structure to Python.

This also adds a class method named name to objc.ivar. This class method creates a new instance variable with the struct type as its type.

  • name is a string with the name of the structure, for example “NSPoint”.
  • typestr is the encoded type of the structure and can optionally contain embedded field names
  • fieldnames is a list with the field names, the value can be None when the typestr contains embedded field names.
  • doc is the value of __doc__ for the new type
  • pack can be used to specify the value of “#pragma pack” for the structure (default is to use the default platform packing for structures).

The created type behaves itself simular to a mutable namedtuple, that is items can be accessed both using attribute access and using the sequence interface.

An example:

Point = objc.createStructType("Point", b"{Point=dd}", ["x", "y"])

p = Point(3.0, 4.0)

# Set the X field in two ways:
p.x = 5
p[0] = 6

The generated type als has a number of methods:

  • _asdict(): Returns a dict that maps from field names to attribute values
  • _replace(**kwds): Return a copy of the struct and replace attribute values with values from the keyword arguments
  • copy(): Return a copy of the struct. If an attribute is another struct that attribute gets copied as well, other attributes are not copied. That is, struct types are deep copied other types are shallow copied.

And the following attributes are present:

  • _fields: A list of field names
  • __typestr__: The Objective-C type encoding for the struct (without embedded field names)

Changed in version 2.5: The function creates a class method on objc.ivar.

Changed in version 2.5: The type now implements the “_asdict” and “_replace” methods that are also present on collections.namedtuple() types. The attribute “_fields” was added as well.

objc.registerStructAlias(typestr, structType)

Tell the brige that structures with encoding typestr should also be coverted to Python using structType (a type created using createStructType()).

Deprecated since version 2.5: Use createStructAlias() instead.

objc.createStructAlias(name, typestr, structType)

Tell the brige that structures with encoding typestr should also be coverted to Python using structType (a type created using createStructType()).

This also adds a class method named name to objc.ivar. This class method creates a new instance variable with the struct type as its type.

Loading variable/constants

objc.loadBundleVariables(bundle, module_globals, variableInfo[, skip_undefined])

Loads a list of global variables (constants) from a bundle and adds proxy objects for them to the module_globals dictionary. If skip_undefined is True (the default) the function will skip entries that don’t refer to existing variables, otherwise it raises an error exception for these variables.

variableInfo is a sequence of variable descriptions. Every description is a tuple of two elements: the variable name (a string) and the type encoding for the variable (a byte string).

objc.loadSpecialVar(bundle, module_globals, typeid, name[, skip_undefined])

This function loads a global variable from a bundle and adds it to the module_globals dictionary. The variable should be a CoreFoundation based type, with a value that is not a valid pointer.

If skip_undefined is True (the default) the function won’t raise and exception when the variable is not present. Otherwise the function will raise an error exception.

Loading functions

objc.loadBundleFunctions(bundle, module_globals, functionInfo[, skip_undefined])

Loads a list of functions from a bundle and adds proxy objects for them to the module_globals dictionary. If skip_undefined is True (the default) the function will skip entries that don’t refer to existing functions, otherwise it raises an error exception for these functions.

bundle is either an NSBundle instance, or None. When a bundle is specified the function is looked up in that bundle, otherwise the function is looked up in any bundle (including the main program and Python extensions).

functionInfo is a sequence of function descriptions. Every description is a tuple of two or four elements: the function name (a string) and signature (a byte string) and optionally a value for the “__doc__” attribute and a metadata dictionary.

The structure of the metadata dictionary is descripted in the section Metadata dictionaries.

objc.loadFunctionList(list, module_globals, functionInfo[, skip_undefined])

Simular to loadBundleFunctions(), but loads the functions from list instead of a bundle.

List should be a capsule object with tag “objc.__inline__” and the value should be a pointer to an array of structs with the following definition:

struct function {
    char*  name;
    void   (*function)(void);
};

The last item in the array must have a NULL pointer in the name field.

Metadata for Objective-C methods and classes

objc.registerMetaDataForSelector(class_, selector, metadata)

Register a metadata structure for the given selector. The metadata is a dictionary, and the structure of that dictionary is described in the section Metadata dictionaries.

objc.registerListType(type)

Register type as a list-like type that will be bridged to Objective-C as an NSArray subclass.

objc.registerMappingType(type)

Register type as a dict-like type that will be bridged to Objective-C as an NSDictionary subclass.

objc.addConvenienceForSelector(selector, methods)

Add a list of method to every class that has selector as a selector. These additional methods are not added to the Objective-C class, but are only visibile in Python code.

The methods argument is a list of tuples (methodname, function).

Deprecated since version 2.5.

objc.addConvenienceForClass(classname, method)

Add a list of method the named class when that class is initialized, the class need not be loaded at the time of this call. These additional methods are not added to the Objective-C class, but are only visibile in Python code.

The methods argument is a list of tuples (methodname, function).

objc.setSignatureForSelector(class_name, selector, signature)

Deprecated since version 2.3.

Use the metadata system instead

Register a replacement signature for a specific selector. This can be used to provide a more exact signature for a method.