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Protocol Buffers - Google's data interchange format

Copyright 2008 Google Inc.

This directory contains the Objective C Protocol Buffers runtime library.

Requirements

The Objective C implementation requires:

  • Objective C 2.0 Runtime (32bit & 64bit iOS, 64bit OS X).
  • Xcode 13.3.1 (or later).
  • The library code does not use ARC (for performance reasons), but it all can be called from ARC code.

Installation

The distribution pulled from github includes the sources for both the compiler (protoc) and the runtime (this directory). After cloning the distribution and needed submodules (see the src directory's README), to build the compiler and run the runtime tests, you can use:

 $ objectivec/DevTools/full_mac_build.sh

This will generate the protoc binary.

Building

There are two ways to include the Runtime sources in your project:

Add objectivec/*.h, objectivec/google/protobuf/*.pbobjc.h, and objectivec/GPBProtocolBuffers.m to your project.

or

Add objectivec/*.h, objectivec/google/protobuf/*.pbobjc.h, objectivec/google/protobuf/*.pbobjc.m, and objectivec/*.m except for objectivec/GPBProtocolBuffers.m to your project.

If the target is using ARC, remember to turn off ARC (-fno-objc-arc) for the .m files.

The files generated by protoc for the *.proto files (*.pbobjc.h and *.pbobjc.m) are then also added to the target.

Usage

The objects generated for messages should work like any other Objective C object. They are mutable objects, but if you don't change them, they are safe to share between threads (similar to passing an NSMutableDictionary between threads/queues; as long as no one mutates it, things are fine).

There are a few behaviors worth calling out:

A property that is type NSString* will never return nil. If the value is unset, it will return an empty string (@""). This is inpart to align things with the Protocol Buffers spec which says the default for strings is an empty string, but also so you can always safely pass them to isEqual:/compare:, etc. and have deterministic results.

A property that is type NSData* also won't return nil, it will return an empty data ([NSData data]). The reasoning is the same as for NSString not returning nil.

A property that is another GPBMessage class also will not return nil. If the field wasn't already set, you will get a instance of the correct class. This instance will be a temporary instance unless you mutate it, at which point it will be attached to its parent object. We call this pattern autocreators. Similar to NSString and NSData properties it makes things a little safer when using them with isEqual:/etc.; but more importantly, this allows you to write code that uses Objective C's property dot notation to walk into nested objects and access and/or assign things without having to check that they are not nil and create them each step along the way. You can write this:

- (void)updateRecord:(MyMessage *)msg {
  ...
  // Note: You don't have to check subMessage and otherMessage for nil and
  // alloc/init/assign them back along the way.
  msg.subMessage.otherMessage.lastName = @"Smith";
  ...
}

If you want to check if a GPBMessage property is present, there is always as has\[NAME\] property to go with the main property to check if it is set.

A property that is of an Array or Dictionary type also provides autocreator behavior and will never return nil. This provides all the same benefits you see for the message properties. Again, you can write:

- (void)updateRecord:(MyMessage *)msg {
  ...
  // Note: Just like above, you don't have to check subMessage and otherMessage
  // for nil and alloc/init/assign them back along the way. You also don't have
  // to create the siblingsArray, you can safely just append to it.
  [msg.subMessage.otherMessage.siblingsArray addObject:@"Pat"];
  ...
}

If you are inspecting a message you got from some other place (server, disk, etc), you may want to check if the Array or Dictionary has entries without causing it to be created for you. For this, there is always a \[NAME\]_Count property also provided that can return zero or the real count, but won't trigger the creation.

For primitive type fields (ints, floats, bools, enum) in messages defined in a .proto file that use proto2 syntax there are conceptual differences between having an explicit and default value. You can always get the value of the property. In the case that it hasn't been set you will get the default. In cases where you need to know whether it was set explicitly or you are just getting the default, you can use the has\[NAME\] property. If the value has been set, and you want to clear it, you can set the has\[NAME\] to NO. proto3 syntax messages do away with this concept, thus the default values are never included when the message is encoded.

The Objective C classes/enums can be used from Swift code.

Objective C Generator Proto File Options

objc_class_prefix=<prefix> (no default)

This options allow you to provide a custom prefix for all the symbols generated from a proto file (classes (from message), enums, the Root for extension support).

If not set, the generation options package_to_prefix_mappings_path and use_package_as_prefix (documented below) controls what is used instead. Since Objective C uses a global namespace for all of its classes, there can be collisions. use_package_as_prefix=yes should avoid collisions since proto package are used to scope/name things in other languages, but this option can be used to get shorter names instead. Convention is to base the explicit prefix on the proto package.

Objective C Generator protoc Options

When generating Objective C code, protoc supports a --objc_opt argument; the argument is comma-delimited name/value pairs (key=value,key2=value2). The keys are used to change the behavior during generation. The currently supported keys are:

  • generate_for_named_framework: The value used for this key will be used when generating the #import statements in the generated code. Instead of being plain #import "some/path/file.pbobjc.h" lines, they will be framework based, i.e. - #import <VALUE/file.pbobjc.h>.

    NOTE: If this is used with named_framework_to_proto_path_mappings_path, then this is effectively the default to use for everything that wasn't mapped by the other.

  • named_framework_to_proto_path_mappings_path: The value used for this key is a path to a file containing the listing of framework names and proto files. The generator uses this to decide if another proto file referenced should use a framework style import vs. a user level import (#import <FRAMEWORK/file.pbobjc.h> vs #import "dir/file.pbobjc.h").

    The format of the file is:

    • An entry is a line of frameworkName: file.proto, dir/file2.proto.
    • Comments start with #.
    • A comment can go on a line after an entry. (i.e. - frameworkName: file.proto # comment)

    Any number of files can be listed for a framework, just separate them with commas.

    There can be multiple lines listing the same frameworkName in case it has a lot of proto files included in it; and having multiple lines makes things easier to read.

  • runtime_import_prefix: The value used for this key to be used as a prefix on #imports of runtime provided headers in the generated files. When integrating ObjC protos into a build system, this can be used to avoid having to add the runtime directory to the header search path since the generate #import will be more complete.

  • package_to_prefix_mappings_path: The value used for this key is a path to a file containing a list of proto packages and prefixes. The generator will use this to locate which ObjC class prefix to use when generating sources unless the objc_class_prefix file option is set. This option can be useful if multiple apps consume a common set of proto files but wish to use a different prefix for the generated sources between them. This option takes precedent over the use_package_as_prefix option.

    The format of the file is:

    • An entry is a line of "package=prefix".
    • Comments start with #.
    • A comment can go on a line after a expected package/prefix pair. (i.e. - "package=prefix # comment")
    • For files that do NOT have a proto package (not recommended), an entry can be made as "no_package:PATH=prefix", where PATH is the path for the .proto file.

  • use_package_as_prefix, package_as_prefix_forced_prefix and proto_package_prefix_exceptions_path: The value for use_package_as_prefix can be yes or no, and indicates if a prefix should be derived from the proto package for all the symbols for files that don't have the objc_class_prefix file option (mentioned above). This helps ensure the symbols are more unique and means there is less chance of ObjC class name collisions.

    To help in migrating code to using this support, proto_package_prefix_exceptions_path can be used to provide the path to a file that contains proto package names (one per line, comments allowed if prefixed with #). These package won't get the derived prefix, allowing migrations to the behavior one proto package at a time across a code base.

    package_as_prefix_forced_prefix can be used to provide a value that will be used before all prefixes derived from the packages to help group all of these types with a common prefix. Thus it only makes sense to use it when use_package_as_prefix is also enabled. For example, setting this to "XYZ_" and generating a file with the package "something" defining "MyMessage", would have Objective-C class be XYZ_Something_MyMessage.

    use_package_as_prefix currently defaults to no (existing behavior), but that could change in the future as it helps avoid collisions when more protos get added to the build. Note that this would be a breaking change.

  • headers_use_forward_declarations: The value for this can be yes or no, and indicates if the generated headers use forward declarations for Message and Enum types from other .proto files or if the files should be imported into the generated header instead.

    By using forward declarations, less code is likely to recompile when the files do change, but Swift generally doesn't like forward declarations and will fail to include properties when the concrete definition of the type is known at import time. If your proto usages span modules, this can be a problem.

    headers_use_forward_declarations currently defaults to yes (existing behavior), but in a future release, that default may change to provide better Swift support by default.

Contributing

Please make updates to the tests along with changes. If just changing the runtime, the Xcode projects can be used to build and run tests. If your change also requires changes to the generated code, objectivec/DevTools/full_mac_build.sh can be used to easily rebuild and test changes. Passing -h to the script will show the addition options that could be useful.

Documentation

The complete documentation for Protocol Buffers is available via the web at:

https://developers.google.com/protocol-buffers/