Supporting legacy platforms

Building ARMv7 (iOS) & armeabi v7a (Android) with Xcode10

In Xcode10, the i386 architecture is deprecated for macOS, so building the Flutter engine for armv7/armeabi-v7a fails. Specifically, libraries like CoreFoundation contain only code for the x86_64 architecture.

To address this, get the MacOS 10.13 SDK from Xcode 9.x from Apple, and extract the SDK components from the .xip file. Uncompress the SDK into /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs and name the SDK MacOSX10.13.sdk:

To check if the logic is fine, run command below:

python your-flutter-engine-path/engine/src/build/mac/find_sdk.py 10.12

When find_sdk.py return 10.13, the ninja build will succeed for gen_snapshot (i386), Flutter.framework (ARMv7) and libflutter.so (armeabi-v7a).

Build Flutter engine for 32bit iOS simulator on modern Mac(x86_64)

To build the Flutter engine for iOS simulator on a modern Mac(x86_64), the gn command will generate a target_cpu value with x64. Henceforth, the Flutter.framework and gen_snapshot will be x86_64. However, sometimes you may want to develop Flutter on a 32bit simulator(like iPhone5), you will need both Flutter.framework and gen_snapshot to be i386.

Follow instruction below to change the default behavior in gn command:

1. Edit your-flutter-engine-path/engine/src/flutter/tools/gn