Cover labeling of the modifier key and the individual keys: show the correct mode for virtual keypads, never caps labels when in lower-case, put the shifted label above it and gray if needed, as on a hardware keypad, etc. Make sure to keep consistent: avoid having one lock key and the rest use press-twice, but if you do then make sure there's a reason like access to the fn number pad is needed sometimes; never switch modes between virtual and hardware keyboards, or when you switch between keypad and keyboard, etc.
Problem
Provide access to additional and alternative controls without taking up more hardware or screen space through the use of mode switching.
Solution
Modes
etc.
The same principles may also be applied to switching modes in any interface. However, Tabs are the most common implementation outside of character entry. The types of modifiers discussed here only truly become a pattern when used with Keyboards & Keypads, Pen Input, and related functions such as the Dialer.
Variations
Variations can be discussed best by categorizing first using machine-era analogies. Consider the Mode Switch to be a replacement for a hard-wired electrical switch.
Single-throw - An individual button activates an alternative modes. When inactive, the input panel returns to a neutral state. This concept is important, because multiple single-throw switches may be added to the panel, such as shift, caps-lock and symbol keys for a keyboard. This type of entry is subdivided
Multi-throw - An individual button switches sequentially between multiple modes. The number is pre-set and will not vary arbitrarily, but certain modes may be skipped when locked out. Switching is "circular" -- once the last item has been reached, the next switch activation will return to the neutral state. This switch type is very commonly used on keypads with text entry. One key will be a mode switch, moving between multi-tap, predictive text and numeric-only entry, for example. In this manner the key works much like a triple-tap entry key, but as it does not enter data itself, it is a Mode Switch instead.
Both of these variations perform equally well on both virtual and hardware keyboards and keypads, and can be used as the mode selector for pen input methods as well.
Interaction Details
Single throw Mode Switches operate in two modes. "Shift" keys and the like are activated with a single-press and are "sticky" in the accessibility sense. When the Mode Switch key is pressed, it remains active for a single character entry keystroke, after which it deactivates and the entire entry panel returns to the default state.
The other mode is for locked modes, such as "Caps Lock." When activated, these will remain active until the panel is dismissed or entry is no longer occurring. In some cases, these will be a dedicated key, and will activate as soon as selected or pressed. The other option is to make the lock mode an alternative function of the key. To avoid a mode switch for the switch, a secondary activation method using only the mode switch key itself must be used. The most typical are:
Press and hold - The mode will be entered immediately upon being selected or clicked. If the key is pressed for a short time the mode will switch from sticky single activation to locked.
Press-twice - The mode will be entered immediately upon being selected or clicked. If the key is pressed again in quick succession, the mode will switch from sticky single activation to locked.
Which method to use should be determined by following which practice is most common (if any) in the enclosing OS. The speed of double clicks or time for press-and-hold should be determined by best practice of the OS.
A single selection of the same mode switch (or any selection of another mode switch) will disable the mode lock.
The last Mode Switch activated takes priority, and disables any other switches that may have been activated before. When deactivated, the input panel will return to it's neutral position, not to the previous activated alternative mode.
When re-activated, input panels governed by single-throw switches will always open to their neutral mode and disregard the last condition.
For multi-throw switches,
THIS ONLY FOR MULTI-THROW:
WHAT ABOUT UNAVAILABLE MODES FOR MULTI-THROW???
Mode switching will be disabled for any required entry modes. For example, if a full keyboard is provided, but only numeric input can be accepted, for example, then it should be locked to numeric input. A virtual keyboard under the same conditions should display a numeric keypad instead.
Presentation Details
Indicate locked. Especially for keys where it can work as a sticky single select OR a lock key. Remember the sticky part means that a single click key will appear active until the key is used, so simply highlighting or illuminating doesn't work. Need a different indicator, like a symbol or mode indicator (Abc to ABC).
For virtual, change the symbol AND change the keycaps to reflect it... For hardware, generally on-screen, see the mode indicator pattern
Antipatterns
Do not implement a "Shift Lock." Older typewriters had no concept of a caps lock, but allowed the shift key to be locked. This also would type the alternative special characters for symbol and number keys. It is not the expected behavior today, and should be avoided.
Avoid using multi-key combinations for routine controls. Despite their prevalence on desktop computing, they are poorly understood, and the smaller keypads of mobile devices (even with a sticky key paradigm) does not support their use well. This does not preclude their use for obscure functions. In fact, the lack of any reason for users to routinely use multi-key combinations means they are a good way to support highly specialized or technical functions, such as for system resets.