String str = "Hello";
canvas.drawText( str , x , y , paint);

//1. 粗略计算文字宽度
Log.d(TAG, "measureText=" + paint.measureText(str));

//2. 计算文字所在矩形，可以得到宽高
Rect rect = new Rect();
paint.getTextBounds(str, 0, str.length(), rect);
int w = rect.width();
int h = rect.height();
Log.d(TAG, "w=" +w+"  h="+h);

//3. 精确计算文字宽度
int textWidth = getTextWidth(paint, str);
Log.d(TAG, "textWidth=" + textWidth);

public static int getTextWidth(Paint paint, String str) {
        int iRet = 0;
        if (str != null && str.length() > 0) {
            int len = str.length();
            float[] widths = new float[len];
            paint.getTextWidths(str, widths);
            for (int j = 0; j < len; j++) {
                iRet += (int) Math.ceil(widths[j]);
            }
        }
        return iRet;
    }