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?

Ardene

I answered the same question here: https://mcmap.net/q/388106/-force-a-view-to-redraw-itself – Counterblow

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.

Domineering

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. – Ardene

Answer 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!

Ardene

Answer 3

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());
    }
}

Caudal