android canvas drawText set font size from width?

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Here’s a much more efficient method:

/**
 * Sets the text size for a Paint object so a given string of text will be a
 * given width.
 * 
 * @param paint
 *            the Paint to set the text size for
 * @param desiredWidth
 *            the desired width
 * @param text
 *            the text that should be that width
 */
private static void setTextSizeForWidth(Paint paint, float desiredWidth,
        String text) {

    // Pick a reasonably large value for the test. Larger values produce
    // more accurate results, but may cause problems with hardware
    // acceleration. But there are workarounds for that, too; refer to
    // http://stackoverflow.com/questions/6253528/font-size-too-large-to-fit-in-cache
    final float testTextSize = 48f;

    // Get the bounds of the text, using our testTextSize.
    paint.setTextSize(testTextSize);
    Rect bounds = new Rect();
    paint.getTextBounds(text, 0, text.length(), bounds);

    // Calculate the desired size as a proportion of our testTextSize.
    float desiredTextSize = testTextSize * desiredWidth / bounds.width();

    // Set the paint for that size.
    paint.setTextSize(desiredTextSize);
}

Then, all you need to do is setTextSizeForWidth(paint, 400, str); (400 being the example width in the question).

For even greater efficiency, you can make the Rect a static class member, saving it from being instantiated each time. However, this may introduce concurrency issues, and would arguably hinder code clarity.

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