[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)