FontMetrics 淡入淺出

摘要： Google文件中的描述： /** * Class that describes the various各種 metrics指標 for a font at a given text size. * Remember, Y values increase going down, so th...

Google文件中的描述：

/**
* Class that describes the various各種 metrics指標 for a font at a given text size.
* Remember, Y values increase going down, so those values will be positive正數,
* and values that measure distances距離 going up will be negative負數. 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 floattop;
//【字元頂部】The recommended推薦的 distance above the baseline for singled spaced text.
public floatascent;
//【字元底部】The recommended distance below the baseline for singled spaced text.
public floatdescent;
//The maximum distance below the baseline for the lowest glyph in the font at a given text size.
public floatbottom;
//【字元行間距】The recommended additional額外的 space to add between lines兩行之間 of text.
public floatleading;
}

該類描述給定文字大小的字型的各種度量。記住，Y值向下增加，所以這些值為正值，測量向上距離的值為負值。此類由GetFontMetrics（）返回。

網上找的各種模型圖

要點如下：

• 基準點是baseline
• Ascent是baseline之上至字元最高處的距離
• Descent是baseline之下至字元最低處的距離
• Leading文件說的很含糊，其實是上一行字元的descent到下一行的ascent之間的距離
• Top指的是指的是最高字元到baseline的值，即ascent的最大值
• bottom指的是最下字元到baseline的值，即descent的最大值
  為了幫助理解，我特此搜尋了不同的示意圖。對照示意圖，會很容易理解FontMetrics的引數。

Paint的ascent和descent方法--算高度

這兩個是native方法

//Return the distance above (negative) the baseline (ascent) based on the current typeface字型 and text size.
public native float ascent();
//Return the distance below (positive) the baseline (descent) based on the current typeface and text size.
public native float descent();

可以通過 mPaint.ascent()+ mPaint.descent() 獲取文字高度

Paint的measureText方法--獲取寬度

/**
 * Return the width of the text.
 *
 * @param textThe text to measure. Cannot be null.
 * @param start The index of the first character to start measuring
 * @param end1 beyond the index of the last character to measure
 * @returnThe width of the text
 */
public float measureText(String text, int start, int end) {
if (text == null) throw new IllegalArgumentException("text cannot be null");
if ((start | end | (end - start) | (text.length() - end)) < 0) throw new IndexOutOfBoundsException();
if (text.length() == 0 || start == end) return 0f;
if (!mHasCompatScaling) return (float) Math.ceil(native_measureText(text, start, end, mBidiFlags));

final float oldSize = getTextSize();
setTextSize(oldSize*mCompatScaling);
float w = native_measureText(text, start, end, mBidiFlags);
setTextSize(oldSize);
return (float) Math.ceil(w*mInvCompatScaling);
}

通過這個方法即可以輕鬆的獲取到文字的的寬度

Paint的getTextBounds方法--獲取邊界

/**
 * Return in bounds (allocated by the caller 由呼叫者分配) the smallest最小的 rectangle that
 * encloses圍起來 all of the characters, with an implied origin預設的起始位置 at (0,0).
 * @param textString to measure and return its bounds
 * @param start Index of the first char in the string to measure
 * @param end1 past the last char in the string measure
 * @param bounds Returns the unioned bounds of all the text. Must be allocated by the caller.
 */
public void getTextBounds(String text, int start, int end, Rect bounds) {
if ((start | end | (end - start) | (text.length() - end)) < 0)throw new IndexOutOfBoundsException();
if (bounds == null) throw new NullPointerException("need bounds Rect");
nativeGetStringBounds(mNativePaint, mNativeTypeface, text, start, end, mBidiFlags, bounds);
}

這個方法需要提供一個引數 Rect 矩形區域，這個方法將文字的區域傳遞到 Rect

Paint的獲取

比如對於TextView，要通過TextView的getPaint()得到Paint，而不能new一個或者拿其他不相干的Paint

/**
 * @return the base paint used for the text.Please use this only to
 * consult查閱 the Paint's properties屬性 and not to change them.
 */
public TextPaint getPaint() {
return mTextPaint;
}

如果是在自定義View 中，則可以直接通過Paint呼叫相關方法去獲取文字的寬高等資訊

測試程式碼

public class MainActivity extends Activity {

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

String text = "aA bB fF gG";

TextView tv_info = new TextView(this);

tv_info.setTextColor(Color.BLUE);

tv_info.setBackgroundColor(0x5500ff00);

tv_info.setTextSize(TypedValue.COMPLEX_UNIT_SP, 18);

tv_info.setText(text);

setContentView(tv_info);

tv_info.measure(0, 0);//必須手動呼叫測量方法，否則getMeasuredHeight獲取不到值。另外，即使測量過了getHeight也獲取不到值

Paint mPaint = tv_info.getPaint();

Log.i("bqt", "ascent()的值為" + mPaint.ascent() + "，descent()的值為" + mPaint.descent());//-33.398438， 8.7890625

Log.i("bqt", "字元的高度為" + (-mPaint.ascent() + mPaint.descent()) + "，字元的長度為" + mPaint.measureText(text));//42.1875，190.0

Log.i("bqt", "測量的高度為" + tv_info.getMeasuredHeight() + "，測量的寬度為" + tv_info.getMeasuredWidth());//49，190

Rect mRect = new Rect();

mPaint.getTextBounds(text, 0, text.length(), mRect);

Log.i("bqt", "字元的邊界為" + mRect.left + "，" + mRect.top + "，" + mRect.right + "，" + mRect.bottom);//1，-28，187，8

Log.i("bqt", "字元的高度為" + (mRect.bottom - mRect.top) + "，字元的長度為" + (mRect.right - mRect.left));//36，186

}

}

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