[Android]Android字体高度的研究

android自定义View绘制的时候，会有很多情况下要与字体打交道，涉及到字体宽度、高度的时候要特别注意一下几个概念，见图：

 

 

 

baseLine：一行文字的底线。

Ascent： 字符顶部到baseLine的距离。

Descent： 字符底部到baseLine的距离。

Leading： 字符行间距。

 

 

 public class TestOnDraw extends Activity {

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         // TODO Auto-generated method stub
         super.onCreate(savedInstanceState);
         MyView v = new MyView(this);
         this.setContentView(v);
     }
 }

 class MyView extends View
 {

     public MyView(Context context) {
         super(context);
     }
     @Override
     protected void onDraw(Canvas canvas) {
         super.onDraw(canvas);

         Paint p = new Paint();
         p.setColor(Color.WHITE);
         p.setTextSize(50);
         p.setAntiAlias(true);
         FontMetrics fm = p.getFontMetrics();

         System.out.println("top = "+ fm.top);
         System.out.println("ascent = "+ fm.ascent);
         System.out.println("descent = "+ fm.descent);
         System.out.println("bottom = "+ fm.bottom);
         System.out.println("leading = "+ fm.leading);

         int textHeight = (int) (Math.ceil(fm.descent - fm.ascent) + 2);
         System.out.println("textHeight = "  + textHeight);

         float width =500;
         float baseline = 100f;
         float offsetAscent = baseline + fm.ascent;
         float offsetDescent = baseline +fm.descent;
         float offsetTop = baseline + fm.top;
         float offsetBottom = baseline + fm.bottom;

         canvas.drawText("中国 bp Android", 0, baseline, p);

         canvas.drawLine(0, baseline, width, baseline, p);//baseline
         canvas.drawLine(0, offsetAscent, width, offsetAscent, p);//ascent
         canvas.drawLine(0, offsetDescent, width, offsetDescent, p);//descent
         canvas.drawLine(0, offsetTop, width, offsetTop, p);//top
         canvas.drawLine(0, offsetBotton, width, offsetBottom, p);//bottom
     }
 }

 public class TestOnDraw extends Activity {

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         // TODO Auto-generated method stub
         super.onCreate(savedInstanceState);
         MyView v = new MyView(this);
         this.setContentView(v);
     }
 }

 class MyView extends View
 {

     public MyView(Context context) {
         super(context);
     }
     @Override
     protected void onDraw(Canvas canvas) {
         super.onDraw(canvas);

         Paint p = new Paint();
         p.setColor(Color.WHITE);
         p.setTextSize(50);
         p.setAntiAlias(true);
         FontMetrics fm = p.getFontMetrics();

         System.out.println("top = "+ fm.top);
         System.out.println("ascent = "+ fm.ascent);
         System.out.println("descent = "+ fm.descent);
         System.out.println("bottom = "+ fm.bottom);
         System.out.println("leading = "+ fm.leading);

         int textHeight = (int) (Math.ceil(fm.descent - fm.ascent) + 2);
         System.out.println("textHeight = "  + textHeight);

         float width =500;
         float baseline = 100f;
         float offsetAscent = baseline + fm.ascent;
         float offsetDescent = baseline +fm.descent;
         float offsetTop = baseline + fm.top;
         float offsetBottom = baseline + fm.bottom;

         canvas.drawText("中国 bp Android", 0, baseline, p);

         canvas.drawLine(0, baseline, width, baseline, p);//baseline
         canvas.drawLine(0, offsetAscent, width, offsetAscent, p);//ascent
         canvas.drawLine(0, offsetDescent, width, offsetDescent, p);//descent
         canvas.drawLine(0, offsetTop, width, offsetTop, p);//top
         canvas.drawLine(0, offsetBotton, width, offsetBottom, p);//bottom
     }
 }

 

运行效果：

 

 

这是程序的输出结果：

 

得出结论： canvas drawText() 的startX是从左下角的baseline的底线开始绘画的，如果我们要得到字体的高度需要关注descent - ascent (ascent线在baseline上面，所以是负数)