Add Apple's swift-format as a linter

test/command_callback/test_swift_swiftformat_command_callbacks.vader

@@ -0,0 +1,25 @@
+Before:
+  call ale#assert#SetUpLinterTest('swift', 'swiftformat')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(Should use default command when not in a swift package):
+  call ale#test#SetFilename('../swift-test-files/non-swift-package-project/src/folder/dummy.swift')
+
+  AssertLinter 'swift-format',
+  \ ale#Escape('swift-format') . ' --mode lint %t'
+
+Execute(Should use swift run when in a swift package):
+  call ale#test#SetFilename('../swift-test-files/swift-package-project/src/folder/dummy.swift')
+
+  AssertLinter 'swift',
+  \ ale#Escape('swift') . ' run swift-format --mode lint %t'
+
+Execute(Should let users configure a global executable and override local paths):
+  call ale#test#SetFilename('../swift-test-files/swift-package-project/src/folder/dummy.swift')
+
+  let g:ale_swift_swiftformat_executable = '/path/to/custom/swift-format'
+
+  AssertLinter '/path/to/custom/swift-format',
+  \ ale#Escape('/path/to/custom/swift-format') . ' --mode lint %t'

test/handler/test_swiftformat_handler.vader

@@ -0,0 +1,28 @@
+Before:
+  runtime ale_linters/swift/swiftformat.vim
+
+After:
+  call ale#linter#Reset()
+
+Execute(The swiftformat handler should parse lines correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 4,
+  \     'col': 21,
+  \     'type': 'W',
+  \     'code': 'DoNotUseSemicolons',
+  \     'text': 'remove '';'' and move the next statement to the new line',
+  \   },
+  \   {
+  \     'lnum': 3,
+  \     'col': 12,
+  \     'type': 'W',
+  \     'code': 'Spacing',
+  \     'text': 'remove 1 space'
+  \   },
+  \ ],
+  \ ale_linters#swift#swiftformat#Handle(bufnr(''), [
+  \   'Sources/main.swift:4:21: warning: [DoNotUseSemicolons]: remove '';'' and move the next statement to the new line',
+  \   'Sources/main.swift:3:12: warning: [Spacing]: remove 1 space',
+  \ ])