pattern_patch gem

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Apply and revert pattern-based patches to any string or text file.

This is a preliminary utility gem to apply and revert patches to strings (typically file contents). One of the main intended use cases for this plugin is source-code modification, e.g. when automatically integrating an SDK.

Please provide any feedback via issues in this repo.

See the full documentation for more details.

require "pattern_patch"

# Add a meta-data key to the end of the application element of an Android manifest
PatternPatch::Patch.new(
  regexp: %r{^\s*</application>},
  text: "        <meta-data android:name=\"foo\" android:value=\"bar\" />\n",
  mode: :prepend
).apply "AndroidManifest.xml"

Capture groups may be used within the text argument in any mode. Note that this works best without interpolation (single quotes or %q). If you use double quotes, the backslash must be escaped, e.g. text: "\\1\"MyOtherpod\"".

# Change the name of a pod in a podspec
PatternPatch::Patch.new(
  regexp: /(s\.name\s*=\s*)"MyPod"/,
  text: '\1"MyOtherPod"',
  mode: :replace
).apply "MyPod.podspec"

Patches in :append mode using capture groups in the text argument may be reverted. This is not currently supported in :prepend mode.

Revert patches

Revert patches by passing the optional :revert parameter:

# Revert the patch that added the metadata key to the end of the Android manifest, resulting in the original.
PatternPatch::Patch.new(
  regexp: %r{^\s*</application>},
  text: "        <meta-data android:name=\"foo\" android:value=\"bar\" />\n",
  mode: :prepend
).apply "AndroidManifest.xml"

Patches using the :replace mode cannot be reverted.

Define patches in YAML files

Load a patch defined in YAML and apply it.

PatternPatch::Patch.from_yaml("patch.yaml").apply "file.txt"

Define patch text in external files

Load the contents of a file to use for the insertion/substitution text:

PatternPatch::Patch.new(
  regexp: /\z/,
  text_file: "text_to_insert_at_end.txt",
  mode: :append
)

When loading from a YAML file, the text_file path is interpreted relative to the directory of the YAML file, e.g.:

patch.yaml:

text_file: text_to_insert_at_end.txt
PatternPatch::Patch.from_yaml("/path/to/patches/patch.yaml")

This will load the contents of /path/to/patches/text_to_insert_at_end.txt.

ERB in text and text_file

ERB is processed in the text value or the contents of a text_file:

PatternPatch::Patch.new(
  regexp: /x/,
  text: '<%= PatternPatch::VERSION %>',
  mode: :replace
).apply file_path

Optionally pass a :binding option to #apply to use a specific Binding:

replacement_text = "y"
PatternPatch::Patch.new(
  regexp: /x/,
  text: '<%= replacement_text %>',
  mode: :replace
).apply file_path, binding: binding

This is particularly useful with a text_file argument.

The #apply and #revert methods also accept :safe_level and :trim_mode options for use with ERb. These can be set at the global level using PatternPatch.safe_level and PatternPatch.trim_mode. The #safe_level and '#trim_mode' attributes in the Methods module are convenience methods to set and retrieve these global values.

PatternPatch::Patch.new(
  regexp: /x/,
  text_file: "template.erb",
  mode: :replace
).apply file_path, trim_mode: "<>"

Regular expressions with modifiers in YAML

The regexp field in a YAML file may be specified with or without slashes or a modifier:

# Results in /^x/
regexp: '^x'
# Results in /^x/i
regexp: '/^x/i'

Currently only the slash literal notation is supported in YAML.

Loading patches from a specific folder

Use PatternPatch.patch_dir and PatternPatch.patch to easily load patches by name.

PatternPatch.patch_dir = "/path/to/patches"
# Use /path/to/patches/patch_name.yml
PatternPatch.patch(:patch_name).apply file_path

Why use pattern_patch?

Modifying files from code is a common task. When modifying a file that uses a standard format, such as XML or JSON, you can use a standard library in almost any language to parse, interpret and update file contents.

If you have to patch other formatted text, particularly source code, there may not be a standard library available to parse a given format. In addition, using a library limits control over formatting. If you use REXML to modify XML, it will generate a file using single quotes for all attributes. While this is legitimate XML, it can cause problems in some cases, and it generates a diff that shows irrelevant, inconsequential changes. Some XML libraries can make other changes to the file format, such as joining multiline tags. In some cases (e.g., Android manifests) this is quite visible and annoying.

This gem offers a more general solution to the problem. A Patch is defined as an operation that can be performed on any file at all. If the file's contents do not match the #regexp attribute, no change is made, but the patch may still be applied. These operations may be externally defined in separate files or in code (or using a combination of both). Further, many patches may be reverted by recognizing the pattern that would result from application of the patch and reversing its effect.

This gem is used extensively in the branch_io_cli gem to patch source code, Podfiles and Cartfiles. A collection of patches is kept in lib/assets/patches, both YAML patch definitions and source patches using ERB. The PatchHelper class easily loads the patch assets and applies them to the relevant files. The process there is similar to rendering partial templates in a web framework like Rails. It is also used in the patch plugin for Fastlane. In fact this gem grew out of that plugin.

This idea was loosely inspired by Facebook's react-native link automation for Android.