Enforce clang-format for c-like sources on Travis (flutter#4089)

* Enforce clang-format for c-like sources on Travis

* Edit CONTRIBUTING,md

* review feedback:

* ++

* ++

* ++

.travis.yml

@@ -6,3 +6,4 @@ before_script:
 script:
   - ./travis/build.sh
   - ./travis/test.sh
+  - ./travis/format.sh

CONTRIBUTING.md

@@ -201,11 +201,13 @@ To start working on a patch:
  * Make sure you are in the `engine/src/flutter` directory.
  * `git fetch upstream`
  * `git checkout upstream/master -b name_of_your_branch`
- * Hack away. Please peruse our
-   [style guides](https://flutter.io/style-guide/) and
-   [design principles](https://flutter.io/design-principles/) before
-   working on anything non-trivial. These guidelines are intended to
-   keep the code consistent and avoid common pitfalls.
+ * Hack away.
+   * Please peruse our [style guides](https://flutter.io/style-guide/) and
+     [design principles](https://flutter.io/design-principles/) before
+     working on anything non-trivial. These guidelines are intended to
+     keep the code consistent and avoid common pitfalls.
+   * C, C++, and Objective-C code should be formatted with `clang-format` before
+     submission (use `buildtools/<OS>/clang/bin/clang-format --style=file -i`).
  * `git commit -a -m "<your brief but informative commit message>"`
  * `git push origin name_of_your_branch`
 

travis/format.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+set -e
+echo "Checking formatting..."
+cd ..
+
+case "$(uname -s)" in
+  Darwin)
+    OS="mac-x64"
+    ;;
+  Linux)
+    OS="linux-x64"
+    ;;
+  *)
+    echo "Unknown operating system."
+    exit -1
+    ;;
+esac
+
+CLANG_FORMAT="buildtools/$OS/clang/bin/clang-format"
+$CLANG_FORMAT --version
+
+FILES="$(find flutter/ -name '*.cpp' -or -name '*.h' -or -name '*.c' -or -name '*.cc' -or -name '*.m' -or -name '*.mm')"
+FAILED_CHECKS=0
+
+for FILE in $FILES; do
+  set +e
+  RESULT="$(diff -u "$FILE" <($CLANG_FORMAT --style=file "$FILE"))"
+  set -e
+  if ! [ -z "$RESULT" ]; then
+    echo "$RESULT"
+    FAILED_CHECKS=$(($counter+1))
+  fi
+done
+
+if [ $FAILED_CHECKS -ne 0 ]; then
+  echo "Some files are formatted incorrectly. To fix, apply diffs from above."
+fi
+
+exit $FAILED_CHECKS