Android Canvas drawText文字居中的一些事（图解）

@Override
public void draw(Canvas canvas) {
 super.draw(canvas);
 // 将坐标原点移到控件中心
 canvas.translate(getWidth() / 2, getHeight() / 2);
 // x轴
 canvas.drawLine(-getWidth() / 2, 0, getWidth() / 2, 0, paint);
 // y轴
 canvas.drawLine(0, -getHeight() / 2, 0, getHeight() / 2, paint);

 // 绘制文字
 paint.setTextSize(sp2px(50));
 canvas.drawText("YangLe", 0, 0, paint);
}

public static class FontMetrics {
 /**
 * The maximum distance above the baseline for the tallest glyph in
 * the font at a given text size.
 */
 public float top;
 /**
 * The recommended distance above the baseline for singled spaced text.
 */
 public float ascent;
 /**
 * The recommended distance below the baseline for singled spaced text.
 */
 public float descent;
 /**
 * The maximum distance below the baseline for the lowest glyph in
 * the font at a given text size.
 */
 public float bottom;
 /**
 * The recommended additional space to add between lines of text.
 */
 public float leading;
}

/**
 * Draw the text, with origin at (x,y), using the specified paint. The origin is interpreted
 * based on the Align setting in the paint.
 *
 * @param text The text to be drawn
 * @param x The x-coordinate of the origin of the text being drawn
 * @param y The y-coordinate of the baseline of the text being drawn
 * @param paint The paint used for the text (e.g. color, size, style)
 */
public void drawText(@NonNull String text, float x, float y, @NonNull Paint paint) {
 super.drawText(text, x, y, paint);
}

float baseLineY
= (fontMetrics.descent - fontMetrics.ascent) / 2 - fontMetrics.descent;
= -fontMetrics.ascent / 2 - fontMetrics.descent / 2;
= -(fontMetrics.ascent + fontMetrics.descent) / 2;
= Math.abs(fontMetrics.ascent + fontMetrics.descent) / 2;

@Override
public void draw(Canvas canvas) {
 super.draw(canvas);
 // 将坐标原点移到控件中心
 canvas.translate(getWidth() / 2, getHeight() / 2);
 // x轴
 canvas.drawLine(-getWidth() / 2, 0, getWidth() / 2, 0, paint);
 // y轴
 canvas.drawLine(0, -getHeight() / 2, 0, getHeight() / 2, paint);

 // 绘制居中文字
 paint.setTextSize(sp2px(50));
 paint.setColor(Color.GRAY);
 // 文字宽
 float textWidth = paint.measureText("YangLe'Blog");
 // 文字baseline在y轴方向的位置
 float baseLineY = Math.abs(paint.ascent() + paint.descent()) / 2;
 canvas.drawText("YangLe'Blog", -textWidth / 2, baseLineY, paint);
}

/**
 * 绘制多行居中文本（方式1）
 *
 * @param canvas 画布
 */
private void drawCenterMultiText1(Canvas canvas) {
 String text = "ABC";

 // 画笔
 TextPaint textPaint = new TextPaint();
 textPaint.setAntiAlias(true);
 textPaint.setColor(Color.GRAY);

 // 设置宽度超过50dp时换行
 StaticLayout staticLayout = new StaticLayout(text, textPaint, dp2px(50),
  Layout.Alignment.ALIGN_CENTER, 1f, 0f, false);
 canvas.save();
 // StaticLayout默认从（0，0）点开始绘制
 // 如果需要调整位置，只能在绘制之前移动Canvas的起始坐标
 canvas.translate(-staticLayout.getWidth() / 2, -staticLayout.getHeight() / 2);
 staticLayout.draw(canvas);
 canvas.restore();
}

public StaticLayout(CharSequence source, TextPaint paint, int width, Alignment align, 
   float spacingmult, float spacingadd, boolean includepad) {
 this(source, 0, source.length(), paint, width, align, spacingmult, spacingadd, includepad);
}

/**
 * 绘制多行居中文本（方式2）
 *
 * @param canvas 画布
 */
private void drawCenterMultiText2(Canvas canvas) {
 String[] texts = {"A", "B", "C"};

 Paint.FontMetrics fontMetrics = paint.getFontMetrics();
 // top绝对值
 float top = Math.abs(fontMetrics.top);
 // ascent绝对值
 float ascent = Math.abs(fontMetrics.ascent);
 // descent，正值
 float descent = fontMetrics.descent;
 // bottom，正值
 float bottom = fontMetrics.bottom;
 // 行数
 int textLines = texts.length;
 // 文本高度
 float textHeight = top + bottom;
 // 文本总高度
 float textTotalHeight = textHeight * textLines;
 // 基数
 float basePosition = (textLines - 1) / 2f;

 for (int i = 0; i < textLines; i++) {
  // 文本宽度
  float textWidth = paint.measureText(texts[i]);
  // 文本baseline在y轴方向的位置
  float baselineY;

  if (i < basePosition) {
   // x轴上，值为负
   // 总高度的/2 - 已绘制的文本高度 - 文本的top值（绝对值）
   baselineY = -(textTotalHeight / 2 - textHeight * i - top);

  } else if (i > basePosition) {
   // x轴下，值为正
   // 总高度的/2 - 未绘制的文本高度 - 文本的bottom值（绝对值）
   baselineY = textTotalHeight / 2 - textHeight * (textLines - i - 1) - bottom;

  } else {
   // x轴中，值为正
   // 计算公式请参考单行文本居中公式
   baselineY = (ascent - descent) / 2;
  }

  canvas.drawText(texts[i], -textWidth / 2, baselineY, paint);
 }
}