如何垂直对齐文本？

Question

目标：纯 Canvas 上的 Android >= 1.6。

假设我想写一个函数来绘制一个（宽度，高度）大红色矩形，然后在里面绘制一个黑色的 Hello World 文本。我希望文本在视觉上位于矩形的中心。那么让我们试试吧：

void drawHelloRectangle(Canvas c, int topLeftX, 
        int topLeftY, int width, int height) {
    Paint mPaint = new Paint();
    // height of 'Hello World'; height*0.7 looks good
    int fontHeight = (int)(height*0.7);

    mPaint.setColor(COLOR_RED);
    mPaint.setStyle(Style.FILL);
    c.drawRect( topLeftX, topLeftY, topLeftX+width, topLeftY+height, mPaint);

    mPaint.setTextSize(fontHeight);
    mPaint.setColor(COLOR_BLACK);
    mPaint.setTextAlign(Align.CENTER);
    c.drawText( "Hello World", topLeftX+width/2, ????, mPaint);
}

现在我不知道在用???? 标记的drawText 参数中放什么，即我不知道如何垂直对齐文本。

类似

???? = topLeftY + 高度/2 + fontHeight/2 - fontHeight/8;

看起来或多或少都可以，但一定有更好的方法。

Answer 1

以cx 和cy 为中心的示例：

private final Rect textBounds = new Rect(); //don't new this up in a draw method

public void drawTextCentred(Canvas canvas, Paint paint, String text, float cx, float cy){
  paint.getTextBounds(text, 0, text.length(), textBounds);
  canvas.drawText(text, cx - textBounds.exactCenterX(), cy - textBounds.exactCenterY(), paint);
}

为什么height()/2f 不一样？

exactCentre() = (top + bottom) / 2f.

height()/2f = (bottom - top) / 2f

这些只会在top 是0 时产生相同的结果。这可能适用于所有大小的某些字体，或某些大小的其他字体，但并非所有大小的所有字体。

Answer 2

textY = topLeftY + height/2 - (mPaint.descent() + mPaint.ascent()) / 2

“基线”到“中心”的距离应该是-(mPaint.descent() + mPaint.ascent()) / 2

Answer 3

根据 steelbytes 的响应，更新后的代码如下所示：

void drawHelloRectangle(Canvas c, int topLeftX, int topLeftY, int width, int height) {
    Paint mPaint = new Paint();
    // height of 'Hello World'; height*0.7 looks good
    int fontHeight = (int)(height*0.7);

    mPaint.setColor(COLOR_RED);
    mPaint.setStyle(Style.FILL);
    c.drawRect( topLeftX, topLeftY, topLeftX+width, topLeftY+height, mPaint);

    mPaint.setTextSize(fontHeight);
    mPaint.setColor(COLOR_BLACK);
    mPaint.setTextAlign(Align.CENTER);
    String textToDraw = new String("Hello World");
    Rect bounds = new Rect();
    mPaint.getTextBounds(textToDraw, 0, textToDraw.length(), bounds);
    c.drawText(textToDraw, topLeftX+width/2, topLeftY+height/2+(bounds.bottom-bounds.top)/2, mPaint);
}

Answer 4

由于在 Y 处绘制文本意味着文本的基线最终会从原点向下 Y 像素，因此当您想要在 (width, height) 的矩形内居中文本时需要执行什么操作尺寸是：

paint.setTextAlign(Paint.Align.CENTER);  // centers horizontally
canvas.drawText(text, width / 2, (height - paint.ascent()) / 2, paint);

请记住，上升是负数（这解释了减号）。

这没有考虑下降，这通常是你想要的（上升通常是基线以上的大写高度）。

Answer 5

使用 mPaint.getTextBounds() 您可以询问绘制时文本的大小，然后使用该信息您可以计算出要绘制的位置。

Answer 6

public static PointF getTextCenterToDraw(String text, RectF region, Paint paint) {
    Rect textBounds = new Rect();
    paint.getTextBounds(text, 0, text.length(), textBounds);
    float x = region.centerX() - textBounds.width() * 0.4f;
    float y = region.centerY() + textBounds.height() * 0.4f;
    return new PointF(x, y);
}

用法：

PointF p = getTextCenterToDraw(text, rect, paint);
canvas.drawText(text, p.x, p.y, paint);

Answer 7

我在尝试解决我的问题时偶然发现了这个问题，@Weston 的回答对我来说很好。

在 Kotlin 的情况下：

private fun drawText(canvas: Canvas) {
    paint.textSize = 80f
    val text = "Hello!"
    val textBounds = Rect()
    paint.getTextBounds(text, 0, text.length, textBounds);
    canvas.drawText(text, cx- textBounds.exactCenterX(), cy - textBounds.exactCenterY(), paint);
    //in case of another Rect as a container:
    //canvas.drawText(text, containerRect.exactCenterX()- textBounds.exactCenterX(), containerRect.exactCenterY() - textBounds.exactCenterY(), paint);
}

Answer 8

这是一个 SkiaSharp C# 扩展方法，任何人都在寻找它

public static void DrawTextCenteredVertically(this SKCanvas canvas, string text, SKPaint paint, SKPoint point)
{
    var textY = point.Y + (((-paint.FontMetrics.Ascent + paint.FontMetrics.Descent) / 2) - paint.FontMetrics.Descent);
    canvas.DrawText(text, point.X, textY, paint);
}

Answer 9

private final Rect textBounds = new Rect();//No need o create again

public void drawTextCentred(Canvas canvas, Paint paint, String text, float cx, float cy, float desiredWidth) {
    final float testTextSize = 48f;
    paint.setTextSize(testTextSize);
    Rect bounds = new Rect();
    paint.getTextBounds(text, 0, text.length(), bounds);
    float desiredTextSize = testTextSize * desiredWidth / bounds.width();
    paint.setTextSize(desiredTextSize);
    paint.getTextBounds(text, 0, text.length(), textBounds);
    paint.setTextAlign(Paint.Align.CENTER);
    canvas.drawText(text, cx, cy - textBounds.exactCenterY(), paint);
}