Go composite variant type for JSON marshal

Implement a custom Variant type that can hold either string or int while satisfying the JSON Marshaler and Unmarshaler interfaces to support correct encoding/decoding of the values.

NPM/Yarn 404 Error when Building Docker Image

Sometimes the following error appears on Ubuntu Linux when building docker images for nodejs projects and the project includes privately scoped node packages.

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An unexpected error occurred: Request failed "404 Not Found"

Initial workaround

The initial naive workaround we used was to manually change our DNS entry in /etc/resolv.conf to point to the local gateway.

/etc/resolv.conf

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nameserver	10.0.0.1
search		npm.ourdomain.com

Then manually stop the systemd-resolved service.

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sudo service systemd-resolved stop

How to build the Golang Bitcoin Node for Windows 10 (btcd)

The latest btcd release build is from 2015. There is a nice description how to build it for Linux/Mac, but for Windows it’s suggested to download the latest binaries, which are 626 commits behind master as of February 2018. Fortunately, with Golang we can easily build binaries for Windows, and we can follow the steps of the Linux build process. The only issue I faced was a problem in the Glide dependency manager and that’s how this little post was born.

Update Go version on Linux

1. Download the latest (or desired) Go version

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wget https://storage.googleapis.com/golang/go1.9.linux-amd64.tar.gz

2. Remove old installation

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sudo rm -rf /usr/local/go

3. Extract archive into /usr/local/go

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sudo tar -C /usr/local -xzf go-archive-filename.tar.gz

4. Test if everything is OK

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go version

Clean up disk space taken by Docker

Show disk space used by docker images, containers and volumes.

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$ docker system df

TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              5                   5                   1.722GB             784.5MB (45%)
Containers          5                   0                   78.12MB             78.12MB (100%)
Local Volumes       4                   4                   231.5MB             0B (0%)
Build Cache         0                   0                   0B                  0B

The docker prune command may be used with some docker objects like images, containers and volumes to free unused disk space.

Free space used by containers.

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docker container prune

Free space used by volumes.

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docker volume prune

In addition, you can use the docker system prune command to clean up multiple types of objects at once.

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docker system prune -af

Start Android Emulator From Terminal

tip

Sometimes running the emulator from Android Studio doesn’t work and hangs with black screen. It can be started from terminal to avoid the problem.

By default the emulator is installed in ~/Library/Android/sdk/tools. To list all available Android Virtual Devices (AVD), use the following command:

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./emulator -list-avds

Nexus_5X_API_28_x86
Nexus_S_API_15
Pixel_2_XL_API_28
Pixel_API_27

Run the following command to start the emulator from terminal:

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./emulator -avd Nexus_S_API_15 -gpu host

Check out the docs for more emulator commands and flags.

Android 7+ Trace Files Location on Nexus 5X

The location where android trace files are stored on Nexus 5X with Android 7.0 is

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/sdcard/Android/data/com.domain.myapp/files/filename.trace

After using the os.Debug class to generate trace files for an app

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Debug.startMethodTracing("mytracefile");
Debug.stopMethodTracing();

the file can be pulled from the stored location like

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adb pull /sdcard/Android/data/com.domain.myapp/files/mytracefile.trace

Useful Android ADB Commands

Show all events happening in the phone. The command is very useful for debugging transitions between activities, background, Home button, launcher icon start, notification icon start, etc.

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adb logcat -b events

Show current tasks and activities running on your connected device:

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adb shell dumpsys activity

Android ListView Optimizations

These tips are gathered from articles and Google IO videos. The items are not ordered by importance.

1. Reuse convertView

Always reuse convertView. This can improve the performance of ListView scrolling up to 50-60% percent, especially when ListView items are complex.

2. Use the Holder pattern if possible.

3. Remove realtime computation of the scrollbar thumb size

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android:smoothScrollbar="false"