protobuf/objectivec/README.md

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](../src/README.md)),
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.

All message fields *always* have a value when accessed. For primitive type
fields (ints, floats, bools, enum) there the concept of *presence*, that allows
you to tell the difference between when the field is the *default* value because
it *was not* set and when it *was* set, but *explicitly* to the *default*
value for the field. For fields with that do support *presence*, you can test
if the value was *explicitly* set via the `has\[NAME\]` property. If the value
has been set, and you want to clear it, you can set the `has\[NAME\]` to `NO`.

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 `#import`s 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/