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：

image.png

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：

image.png

这是程序的输出结果：

image.png

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)