最近繪制自定義view時(shí)赫舒，用到畫(huà)筆繪制文本，針對(duì)drawText的繪制做一些總結(jié)唐全。
Canvas.drawText的方法定義如下：
/**
* 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);
}
其中埃跷，x坐標(biāo)比較好理解，是文本起始繪制位置的x坐標(biāo)芦瘾。但是y是指文本繪制的baseline的y坐標(biāo)捌蚊。
具體測(cè)試代碼可參考Android 圖解Canvas drawText文字居中的那些事
要理解上圖中的繪制效果近弟，讓我們?cè)僬J(rèn)識(shí)下FontMetrics類缅糟，該類是Paint的內(nèi)部類。
/**
* Class that describes the various metrics for a font at a given text size.
* Remember, Y values increase going down, so those values will be positive,
* and values that measure distances going up will be negative. This class
* is returned by getFontMetrics().
*/
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;
}
在設(shè)置好字體尺寸后祷愉，通過(guò)Paint.getFontMetricsInt()方法來(lái)獲得一個(gè)FontMetrics對(duì)象窗宦。FontMetrics定義了幾個(gè)繪制時(shí)要用到的關(guān)鍵坐標(biāo)位置，各位置含義和圖示如下二鳄。注意赴涵，這四個(gè)字段的值是相對(duì)于baseline的相對(duì)值。
- top值: 可繪制的最高高度y坐標(biāo) - baseline線y坐標(biāo)订讼，為負(fù)值
- bottom值：可繪制的最低高度y坐標(biāo) - baseline線y坐標(biāo)髓窜，為正值
- ascent值：系統(tǒng)建議的字符繪制最高高度y坐標(biāo) - baseline線y坐標(biāo)，為負(fù)值
-
descent值：系統(tǒng)建議的字符繪制最低高度y坐標(biāo) - baseline線y坐標(biāo)欺殿，為正值
FontMetrics標(biāo)線.png
baseline計(jì)算
場(chǎng)景1:給定文字繪制最高點(diǎn)的y值
即已知Top值寄纵，則baseline = top - fontMetrics.top.
場(chǎng)景2:給定文字繪制的中間線y值
1、已知a段（bottom和baseline之間距離）高度：a = fontMetrics.bottom
2脖苏、計(jì)算b段（bottom和中線的距離）高度：b = (fontMetrics.bottom - fontMetrics.top)/2
3程拭、計(jì)算c段（baseline和中線的距離）高度：c=b-a
4、計(jì)算baseline = centerY + c = centerY + b -a = centerY - (fontMetrics.bottom + fontMetrics.top) /2
實(shí)際計(jì)算時(shí)棍潘，也可以用decent和asent恃鞋，即baseline = centerY - (fontMetrics.ascent + fontMetrics.descent) /2
場(chǎng)景3:指定文字繪制的Rect
原理和場(chǎng)景2一致崖媚，此時(shí)centerY=（rect.bottom+rect.top）/2。
最終baseLine = （rect.bottom+rect.top）/2 - (fontMetrics.top + fontMetrics.bottom) /2
參考文檔：
Android 圖解Canvas drawText文字居中的那些事
drawText方法的baseline計(jì)算
Android Canvas drawText實(shí)現(xiàn)中文垂直居中
