字体的单元格居中显示

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基线（baeseline），坡顶（ascenter）,坡底（descenter）
上坡度（ascent），下坡度（descent）
行间距（leading）：坡底到下一行坡顶的距离
字体的高度＝上坡度＋下坡度＋行间距

ascent是指从一个字的基线(baseline)到最顶部的距离，descent是指一个字的基线到最底部的距离

注意, ascent和top都是负数

下面代码实现高度只有0.75单元格高度的居中显示，所以要下移一定距离y。

        foreground.setStyle(Style.FILL);
        foreground.setTextSize(height * 0.75f);
        foreground.setTextScaleX(width / height);
        foreground.setTextAlign(Paint.Align.CENTER);

        // Draw the number in the center of the tile
        FontMetrics fm = foreground.getFontMetrics();
        // Centering in X: use alignment (and X at midpoint)
        float x = width / 2;
        // Centering in Y: measure ascent/descent first
        float y = height / 2 - (fm.ascent + fm.descent) / 2;

网络的例子：（引用http://www.javaeye.com/topic/474526)

package com.TextTest;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Paint.FontMetrics;
import android.os.Bundle;
import android.view.View;

public class TextTest extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.main);
        setContentView(new GraphView(this));
    }

    static public class GraphView extends View {
        public GraphView(Context context) {
            super(context);
        }

        @Override
        protected void onDraw(Canvas canvas) {
            //
            Paint textPaint = new Paint( Paint.ANTI_ALIAS_FLAG);
            textPaint.setTextSize(35);
            textPaint.setColor(Color.WHITE);

            // FontMetrics对象
            FontMetrics fontMetrics = textPaint.getFontMetrics();
            String text = "abcdefghijklmnopqrstu";

            // 计算每一个坐标
            float baseX = 0;
            float baseY = 100;
            float topY = baseY + fontMetrics.top;
            float ascentY = baseY + fontMetrics.ascent;
            float descentY = baseY + fontMetrics.descent;
            float bottomY = baseY + fontMetrics.bottom;

            // 绘制文本
            canvas.drawText( text, baseX, baseY, textPaint);

            // BaseLine描画
            Paint baseLinePaint = new Paint( Paint.ANTI_ALIAS_FLAG);
            baseLinePaint.setColor(Color.RED);
            canvas.drawLine(0, baseY, getWidth(), baseY, baseLinePaint);

            // Base描画
            canvas.drawCircle( baseX, baseY, 5, baseLinePaint);

            // TopLine描画
            Paint topLinePaint = new Paint( Paint.ANTI_ALIAS_FLAG);
            topLinePaint.setColor( Color.LTGRAY);
            canvas.drawLine(0, topY, getWidth(), topY, topLinePaint);

            // AscentLine描画
            Paint ascentLinePaint = new Paint( Paint.ANTI_ALIAS_FLAG);
            ascentLinePaint.setColor( Color.GREEN);
            canvas.drawLine(0, ascentY, getWidth(), ascentY, ascentLinePaint);

            // DescentLine描画
            Paint descentLinePaint = new Paint( Paint.ANTI_ALIAS_FLAG);
            descentLinePaint.setColor( Color.YELLOW);
            canvas.drawLine(0, descentY, getWidth(), descentY, descentLinePaint);

            // ButtomLine描画
            Paint bottomLinePaint = new Paint( Paint.ANTI_ALIAS_FLAG);
            bottomLinePaint.setColor( Color.MAGENTA);
            canvas.drawLine(0, bottomY, getWidth(), bottomY, bottomLinePaint);
        }
    }

}