Tasks and the Back Stack

In Sprout, the user opens the plant list, taps a Monstera, then taps Edit. Now they press Back twice. Where do they land each time? Android answers this with one simple data structure: a stack.

The rule

Start pushes, Back pops; a task is the stack.

A task is the collection of activities a user works with to do one job, kept as a back stack:

• Starting an activity pushes it on top. It becomes the one the user sees.
• Pressing Back pops the top activity. It is finished and destroyed, and the one underneath resumes.
• Calling finish() in code pops the current activity, just like Back.

Each app launched from the home screen usually gets its own task, which is what you see as a card in the recents screen.

You can change the default "always push a new copy" behaviour when needed:

• standard (default) — every start creates a new instance, even if one is already on the stack.
• singleTop — if the activity is already on top, reuse it (it receives onNewIntent) instead of stacking a copy.
• singleTask — keep at most one instance in its task; starting it again clears the activities above it.

The intent flag FLAG_ACTIVITY_CLEAR_TOP does a similar clearing for one particular start.

The shortcut

Draw the stack as a column of names, newest on top. Every action is a push or a pop.

The trap: after a form saves, forgetting to call finish(). The user presses Back and the form reappears, looking unsaved.

Drills

Answer fast — trust the rule.

Remember

• Start pushes, Back pops; a task is the stack.
• Draw the column of names and apply each push or pop.
• Call finish() when a screen's job is done; use launch modes only when the default stacking is wrong.