Category Archives: Web Apps

Prompt before opening an external link in AngularJS

On a recent project of creating an Angular app which would be both a website and a cordova-packaged app, we had a number of links which opened to external websites (terms and conditions, links to some process flows which couldn’t be contained within the app, etc). However because some of the branding on the sites was very similar to the app itself some test users were getting confused about whether they were still in the app, or had been redirected into a browser.

Because of these issues the client wanted us to create a small popup for some external links that would prompt the user to see if they wanted to move off the site/app. Below is a small angular directive that does this. Usage like:

Using ImageMagick to manipulate PNGs stably

This seems to be an issue that has been talked about in a number of places, however I found it very hard to find the correct solution, which is why I have documented it here.

Often as part of the build process for a webapp you’ll want to take original images and shrink them down to be the correct dimensions (either because they require certain dimensions to be accepted, such as icons, or because you want to save space by stripping out unnecessary data). For JPGs you can do this pretty easily like

The -strip removes any EXIF header information both anonymizing the image and saving potentially a few Kb of asset size.

This process is ‘stable’ because if you repeat it (within the same version of ImageMagick), the resulting file’s data will be identical. This means that you won’t get a new version of the built image in your (git) repository each time you run this command.

However recently when trying to do the same for PNGs (because I required transparency) I noticed that each time they were being built, git was committing a new version into the repository. This is bad news because it both grows the size of the repository by storing pointless identical versions of the file, and also makes it a lot harder tracking through history to see what changed because you have loads of PNG images being committed each time you do a build.

Looking at the output of identify -verbose I could see that the part that was changing each time was below:

So it appears that PNG format wants to store the update/create time in the image’s header itself. That was what was changing each time.

Searching on the internet I found a number of suggestions about how to strip these out with the convert command, and I saw that the header changed a bit but I couldn’t find any that were also removing the ‘png:tIME’ element. Finally I managed to come up with the following flags which convert the image stably:

The identify command still outputs the date: property sections but these are now being taken from the create time (ctime) and modify time (mtime) of the file itself rather than from the header and so are not stored in version control.

You might be wondering why I don’t just create a lazy build system that only updates the asset if the mod time of the source asset is greater than that of the built asset – if I was doing this on a bigger project that would be the best way, but as this was just for a small project I wanted to do quickly I thought that doing this would be the easiest way!