Question

I have created a custom View (let’s call it MyView) which basically just draws some text on itself using the canvas. The text to be drawn is set using a global variable.

At some point during the program’s execution, I want to change the global variable, and have the MyView redraw itself to update the text. I tried findViewById() and then invalidate(), but this does nothing. I suspect that since nothing within the MyView has changed, it thinks it has no reason to call onDraw(). Is there any way to force a View to redraw itself even if it thinks it doesn’t need to?

Answer 1

If I have a member variable inside the MyView that stores the text, and create a public setter for it, then just calling that method causes the MyView to redraw itself

Setting a variable inside the View will not invoke a draw on the View. In fact, neither does the view system know nor care about internal variables.

Invoking invalidate() on a View causes it to draw itself via the View. You should check this out: http://developer.android.com/guide/topics/ui/custom-components.html.

A TextView internally invalidates itself when you invoke setText() and redraws itself with the new text set via the setText() call. You should also do something similar.