third_party/protobuf/objectivec/README.md

Protocol Buffers - Google's data interchange format
===================================================

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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 7.0 (or later).
- The library code does *not* use ARC (for performance reasons), but it all can
  be called from ARC code.

Installation
------------

The full distribution pulled from github includes the sources for both the
compiler (protoc) and the runtime (this directory). To build the compiler
and run the runtime tests, you can use:

     $ objectivec/DevTools/full_mac_build.sh

This will generate the `src/protoc` binary.

Building
--------

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

Add `objectivec/\*.h` & `objectivec/GPBProtocolBuffers.m` to your project.

*or*

Add `objectivec/\*.h` & `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 Options
-----------------------------

**objc_class_prefix=\<prefix\>** (no default)

Since Objective C uses a global namespace for all of its classes, there can
be collisions. This option provides a prefix that will be added to the Enums
and Objects (for messages) generated from the proto. Convention is to base
the prefix on the package the proto is in.

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/