Force a View to redraw itself

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?

I answered the same question here: https://stackoverflow.com/a/56140050/6884103 — Momen Zaqout, May 14 '19 at 23:53

Vikram Bodicherla · Accepted Answer · 2014-06-17T05:24:32.507

60

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.

edited Jun 17 '14 at 05:24

answered Aug 12 '11 at 14:12

Vikram Bodicherla

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Sorry, you are right, I did indeed call `invalidate()`. I meant to say that calling `invalidate()` didn't work because the variable I was changing was external to the `View` (it was global). So I created an internal variable and changed that before invalidating. – David John Welsh Aug 22 '11 at 02:09

score 2 · Answer 2 · answered Aug 12 '11 at 11:35

2

Okay so I figured it out. 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. Simple!

answered Aug 12 '11 at 11:35

David John Welsh

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zh liu · Answer 3 · 2022-09-15T07:09:07.977

Example:

customView.set(...)
customView.requestLayout();
customView.invalidate();

Reference: android.widget.TextView#onConfigurationChanged

@Override
protected void onConfigurationChanged(Configuration newConfig) {
    super.onConfigurationChanged(newConfig);
    if (!mLocalesChanged) {
        mTextPaint.setTextLocales(LocaleList.getDefault());
        if (mLayout != null) {
            nullLayouts();
            requestLayout();
            invalidate();
        }
    }
    if (mFontWeightAdjustment != newConfig.fontWeightAdjustment) {
        mFontWeightAdjustment = newConfig.fontWeightAdjustment;
        setTypeface(getTypeface());
    }
}

Force a View to redraw itself

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