Designing apps for Mac OS 10.10 has its own rules. We've noted some of them in a handy little checklist when we recently updated Tower for Yosemite.
The App Yosemitization Checklist guides you through the major areas and aspects to consider when bringing your application to OS X 10.10.
App Icon
Choose an appropriate app icon style: Graphic or Realistic
Don't reuse your iOS app icon
Strive for simplicity
Realistic App Icon
Make sure that adding realism enhances both understanding and usability.
Feel free to modify a real-world depiction if it enhances the user's understanding
Use perspective appropriately: Remember that an app icon looks like it’s sitting on a desk in front of you
Use textures appropriately: Icons that represent real objects should also look as though they are made of real materials
Setup Light appropriately: Try to avoid a frontal key light. Find a light scenario which shows the shape and weight of your icon. Think about the "scene" your icon sits in in and how different colors might bleed in. Consider using an orange light source from the left (or bottom) and a blue light source from the right (or bottom).
User Interface
Translucency & Vibrancy
Translucency and vibrancy help users focus on what’s important to them. Translucency, vibrancy and shadows help the active window stand out so that users instantly notice it.
Consider using a translucent toolbar to let the user’s content appear more prominent
Add translucency to your sidebar
Add translucency to popovers and overlays
Toolbar Icons
Decide about the toolbar icon styles and don't mix them: Toolbar controls (Buttons) vs. freestanding icons
Differentiate: Variate shapes and images to distinct icons
Harmonize: Apply the same perspective, size and visual weight
Use a straight on perspective
Use colors judiciously and in a meaningful way. Consider updating your color scheme to reflect the brighter system look
Sidebar Icons
Use black and transparency (alpha) only
Apply clearer outlines to your icons
Apply lighter fill colors or gradients
Remove unnecessary details
Empty Views
Update your empty view icons: Lighter grayscale tones and slight emphasize on outlines
Consider removing icons on empty views entirely and use only text instead
Preferences
Use brighter colors and remove details which do not contribute to a better understanding
Do not simply double the size for retina. Take retina screens as an opportunity to make your icons even more elegant and light on the eyes
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