Golang: Gomobile app cannot generate files - go

Question

Does anyone use gomobile app before and have successfully create files in phones? I tried the following code on Galaxy S4 with Android 4.4.2:
package main
import (
"golang.org/x/mobile/app"
"golang.org/x/mobile/event/lifecycle"
"golang.org/x/mobile/event/paint"
"os"
)
func main() {
os.Create("zzz.txt")
app.Main(func(a app.App) {
for e := range a.Events() {
switch e := a.Filter(e).(type) {
case lifecycle.Event:
_=e
case paint.Event:
a.Publish()
}
}
})
}
However no file is created in the phone.
I also tried an app called "AnGoIde" which allow us to write Go and compile directly in Android, and the following code is able to create the "zzz.txt" file:
package main
import "os"
func main(){
os.Create("zzz.txt")
}
Eventually I would like to save all errors in a file so I can see what cause my apps to crash, and AnGoIde doesn't support many packages so I cannot use it for my tests. Does anyone successfully generated files with gomobile apps before?
p.s. I tried to specify the directory to "/storage/emulated/0/Go/" which is the same place I store the apk file but doesn't work.

Answer 1

To write to a file your app needs some form of permission.
If you don't have one create an AndroidManifest.xml. You can see the content of the file gomobile automatically create for you using the -v flag. (gomobile build -v)
Add the following line between <manifest> and </manifest> tags.
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
Build the app again. Make sure gomobile is using your manifest file using the -v flag. I was able to create the file by os.Create("/sdcard/zzz.txt"). I also needed this for debugging so I didn't mind writing to a specific location, namely my sdcard.
Of course in your phone the location may change.
Additionally, If you just want some logs you can install Android Debug Bridge and use adb logcat. To filter out the logs from your Go app adb logcat | grep "I/GoLog" would work.