Android 文字顯示的研究

android自定義View繪製的時候，會有很多情況下要與字體打交道，涉及到字體寬度、高度的時候要特別注意一下幾個概念，見圖：

 

 

 

baseLine：一行文字的底線。

Ascent： 字符頂部到baseLine的距離。

Descent： 字符底部到baseLine的距離。

Leading： 字符行間距。

 

 

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("中國 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("中國 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
    }
}

 

運行效果：

 

 

這是程序的輸出結果：

 

得出結論： canvas drawText() 的startX是從左下角的baseline的底線開始繪畫的，如果我們要得到字體的高度需要關注descent - ascent (ascent線在baseline上面，所以是負數)