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Sveglia app is composed by two main model classes: Timer and TimerTemplate, this post will explain some implementations details of Timer class.

Timer class models a countdown timer. Countdown timers can be created, paused, resumed and destroyed.

To create a timer the minimal information needed is countdown target time: the precise time at which timer will end. This information is stored in target_at attribute.

A mandatory requirement: timers must accumulate pause time durations to track the total pause time duration. Here are a couple ways to handle this task.

paused_for and paused_at

Using an accumulator: paused_for. Its initial value is set to nil and it’s updated at every resume event adding the result of Time.now - paused_at to current value.

origin_target_at

The origin_target_at is set at creation time with original target_at attribute’s value. At the end of timer total pause time duration is the result of target_at - origin_target_at.

This way to handle pause time durations accumulation is more efficient because it doesn’t involve the n operations to add each i^th pause duration used by paused_for accumulation.

Life cycle

To summarize a timer’s life cycle below are described steps involving in each timer event: creation, pause, resume and end.

When you create a new timer:

• target_at and origin_target_at are set to timer’s end time,
• created_at is set to Time.now,
• paused_at is set to nil.

When you pause a running timer:

• paused_at attribute is updated with Time.now value.

When you resume a paused timer:

• paused_at attribute is set to nil,
• target_at attribute is updated with result of Time.now - paused_at.

When a timer ends:

• total pause time duration dynamic attribute is the result of target_at - origin_target_at.