Ersen Osman Ersen Osman - 29 days ago 8
Android Question

Why is requestLayout being called directly after invalidate

I'm learning about custom views and wanted to learn about

invalidate()
and
requestLayout()


Please refer to this answer and its diagram:


http://stackoverflow.com/a/25846243/4243687


Invalidate tells Android that the state of the view has changed and needs to be re-drawn.

RequestLayout means the size of the view may have changed and needs to be remeasured, and then re-drawn.

invalidate()
will invoke
dispatchDraw()
,
draw()
, and
onDraw()
hence it re-renders the view.

requestLayout()
on the other hand does pretty much everything from measuring to re-rendering again.

Why do so many of the examples out there (even the
TextView
source code) call
invalidate()
and then
requestLayout()
right on the next line?

Answer

invalidate() is used specifically for redrawing the content of your view. The redraw does not happen synchronously. Instead, it flags the region of your view as invalid so that it will be redrawn during the next render cycle.

requestLayout() should be used when something within it has possibly changed its dimensions. In this case, the parent view and all other parents up the view hierarchy will need to readjust themselves via a layout pass.

If you are not doing anything to your view that would change its size, then you do not have to call requestLayout.

If you go back and look at the places in the code for TextView where requestLayout is being called, it will be on methods where the view's bounds will be affected. For example, setPadding, setTypeface, setCompoundDrawables, etc.

So, when requestLayout is called, it should be paired with a call to invalidate to ensure that the entire view is redrawn.

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