[Android] It seems impossible to add values to a WritableArray

When I add a value to a WritableArray (for example, using pushInt(1) or pushMap(myMap)), it has no effect. The size of the array is still 0, and if I try to access the data I pushed, it triggers and out of range exception.

React Native version: 0.59.2 React Native Environment Info: System: OS: Windows 10 CPU: (8) x64 Intel® Core™ i7-7700HQ CPU @ 2.80GHz Memory: 3.22 GB / 15.86 GB Binaries: Yarn: 1.7.0 - C:\Users\Mickael-AccoPilot\AppData\Roaming\npm\yarn.CMD npm: 5.6.0 - C:\Program Files\nodejs\npm.CMD IDEs: Android Studio: Version 3.3.0.0 AI-182.5107.16.33.5264788

Steps To Reproduce

This code triggers the bug every time:

        WritableArray test_array = Arguments.createArray();
        Log.d("RNTest", "test size " + test_array.size()); // => "test size 0"
        test_array.pushInt(1);
        Log.d("RNTest", "test size " + test_array.size()); // => "test size 0"
        Log.d("RNTest", "test int " + test_array.getInt(0)); // => exception "length=0; index=0"

[dratwas]

After investigation i found what is happening here. WritableNativeArray inherits from ReadableNativeArray. When you call pushInt on WritableNativeArray instance it will set this value in the Native array in https://github.com/facebook/react-native/blob/fc91bccd5333af88e49ecb19f56c5d6bb0931b1e/ReactAndroid/src/main/jni/react/jni/WritableNativeArray.cpp#L37

But when you want to read something from this array, it takes the value from local copy in https://github.com/facebook/react-native/blob/master/ReactAndroid/src/main/java/com/facebook/react/bridge/ReadableNativeArray.java#L96 which is copied only once while creating instance.

So basically the flow looks like:

• Create WritableNativeArray - It copies empty array from native to java representation
• Push Int to WritableNativeArray - It pushes Int to native array (local one is not updated)
• Read Int from WritableNativeArray - It tries to read value from local array which is still empty because it is synchronised only once.

If you use RN 0.59.8 you could use ReadableNativeArray.setUseNativeAccessor(true) to fix it, but keep in mind that it will use the native array in every ReadableNativeArray instance.

So the working example :

ReadableNativeArray.setUseNativeAccessor(true);
WritableArray test_array = Arguments.createArray();
Log.d("RNTest", "test size " + test_array.size()); // => "test size 0"
test_array.pushInt(1);
Log.d("RNTest", "test size " + test_array.size()); // => "test size 0"
Log.d("RNTest", "test int " + test_array.getInt(0)); // => exception "length=0; index=0"

NOTE: It will not work on RN 0.60 since the setUseNativeAccessor flag is removed.

[dratwas]

I’m able to reproduce it on 0.59.8 and 0.60.0-rc.0. Will take a look on it a bit later.