平時(shí)項(xiàng)目里會(huì)涉及到一些文字繪制，這里記錄下自己學(xué)的的。

先來一張網(wǎng)上找的圖，簡單介紹下android是按啥規(guī)則繪制字體的：

FontMetrics

這個(gè)需要結(jié)合Paint.FontMetrics這個(gè)類來看：

    /**
     * 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;
    }

需要特別關(guān)注的就是BaseLine，像canvas.drawText()里頭的y，就是baseLine的位置，所以，有時(shí)候雖然我們繪制時(shí)參數(shù)y在居中位置，但是實(shí)際體現(xiàn)的效果確不居中，這就是因?yàn)槲淖謺r(shí)基于baseLine來繪制的。這個(gè)時(shí)候的y，就需要根據(jù)文字來特地計(jì)算：

public class SportsView extends View {

    private final static float RING_WIDTH = UiUtils.dpToPixel(20);
    private final static float RADIUS = UiUtils.dpToPixel(150);
    private Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    private Rect rect = new Rect();
    private Paint.FontMetrics fontMetrics = new Paint.FontMetrics();

    public SportsView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    {
        paint.setTextSize(UiUtils.dpToPixel(100));
        paint.setTextAlign(Paint.Align.CENTER);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // 繪制環(huán)
        paint.setStyle(Paint.Style.STROKE);
        paint.setColor(Color.GREEN);
        paint.setStrokeWidth(RING_WIDTH);
        canvas.drawCircle(getWidth() / 2, getHeight() / 2, RADIUS, paint);

        // 繪制進(jìn)度條
        paint.setColor(Color.RED);
        paint.setStrokeCap(Paint.Cap.ROUND);
        canvas.drawArc(getWidth() / 2 - RADIUS, getHeight() / 2 - RADIUS,
                getWidth() / 2 + RADIUS, getHeight() / 2 + RADIUS,
                -90, 225, false, paint);

        paint.setStyle(Paint.Style.FILL);
//        // 讀取文字的實(shí)時(shí)繪制范圍，以用于后面如何對baseline進(jìn)行偏移
//        paint.getTextBounds("123", 0, "123".length(), rect);
//        int offset = (rect.top + rect.bottom) / 2;
//        // 會(huì)在baseLine上畫，baseline是一個(gè)穩(wěn)定線，因?yàn)槲淖值膟軸難以確定
//        // 文字橫向居中很容易，使用Center即可，但是縱向居中就比較麻煩了
//        // 雖然該方法會(huì)讓文字非常居中，但是，當(dāng)文字變化是，可能會(huì)跳動(dòng)，因?yàn)槔L制范圍是根據(jù)文字內(nèi)容來計(jì)算的
//        canvas.drawText("123", getWidth() / 2, getHeight() / 2 - offset, paint);

          // 左對齊情況，如果希望文字完全貼邊，即去除文字訪問的左邊部分
//        canvas.drawText("123", - rect.left, 200, paint);

        paint.getFontMetrics(fontMetrics);
        // 這樣標(biāo)準(zhǔn)比較統(tǒng)一，不會(huì)讓文字跳動(dòng)，適用于變動(dòng)場景
        float offset = (fontMetrics.ascent + fontMetrics.descent) / 2;
        canvas.drawText("123", getWidth() / 2, getHeight() / 2 - offset, paint);
    }
}

效果圖

總結(jié)：文字繪制主要是BaseLine的應(yīng)用。