1 简介

之前已经讲过TextView的基础知识，和段落级别的Span，现在在这进一步进行讲解，这篇文字主要讲解如何给TextView设置字符级别的Span。如果一个Span想要影响段落层次的文本格式，则需要继承CharacterStyle。

2 CharacterStyle

CharacterStyle是个抽象类，字符级别的Span都需要继承这个类，这个类里面有一个抽象方法：

public abstract void updateDrawState(TextPaint tp)

通过改变TextPaint的属性就可以得到不同的展现形式。在这个抽象类里面还有一个静态方法：

public static CharacterStyle wrap(CharacterStyle cs)

一个CharacterStyle类型的Span只能给一个Spaned片段使用，如果想这个Span给多个片段使用可以使用wrap方法。wrap方法的具体代码如下：

public static CharacterStyle wrap(CharacterStyle cs) {
    if (cs instanceof MetricAffectingSpan) {
        return new MetricAffectingSpan.Passthrough((MetricAffectingSpan) cs);
    } else {
        return new Passthrough(cs);
    }
}

再看Passthrough的代码

private static class Passthrough extends CharacterStyle {
    private CharacterStyle mStyle;

    /**
     * Creates a new Passthrough of the specfied CharacterStyle.
     */
    public Passthrough(CharacterStyle cs) {
        mStyle = cs;
    }

    /**
     * Passes updateDrawState through to the underlying CharacterStyle.
     */
    @Override
    public void updateDrawState(TextPaint tp) {
        mStyle.updateDrawState(tp);
    }

    /**
     * Returns the CharacterStyle underlying this one, or the one
     * underlying it if it too is a Passthrough.
     */
    @Override
    public CharacterStyle getUnderlying() {
        return mStyle.getUnderlying();
    }
}

不难发现其实就是复制了一个CharacterStyle。

3 UpdateAppearance

如果一个Span修改字符级别的文本外观，则实现UpdateAppearance。

UpdateAppearance

上面的Span都实现了UpdateAppearance接口，上面的诸多Span都是通过updateDrawState(TextPaint ds)方法来实现相应的效果。

1. BackgroundColorSpan：ds.bgColor = mColor；
2. ForegroundColorSpan：ds.setColor(mColor)；
3. StrikethroughSpan：ds.setStrikeThruText(true)；
4. UnderlineSpan：ds.setUnderlineText(true)；
5. MaskFilterSpan：ds.setMaskFilter(mFilter)；

BackgroundColorSpan和ForegroundColorSpan

BackAndFront

UnderlineSpan和StrikethroughSpan：

UnderAndStrike

Mask

可以看一下ClickableSpan的源代码

public abstract class ClickableSpan extends CharacterStyle implements UpdateAppearance {

    /**
     * Performs the click action associated with this span.
     */
    public abstract void onClick(View widget);
   
    /**
     * Makes the text underlined and in the link color.
     */
    @Override
    public void updateDrawState(TextPaint ds) {
        ds.setColor(ds.linkColor);
        ds.setUnderlineText(true);
    }
}

点击后通过updateDrawState(TextPaint ds)方法改变字体外观，onClick(View widget)则交给子类实现相应的逻辑。
MaskFilterSpan中ds.setMaskFilter(mFilter)可以给字体设置模糊和浮雕效果。

span = new MaskFilterSpan(new BlurMaskFilter(density*2, BlurMaskFilter.Blur.NORMAL));
span = new MaskFilterSpan(new EmbossMaskFilter(new float[] { 1, 1, 1 }, 0.4f, 6, 3.5f));

4 UpdateLayout

如果一个Span修改字符级文本度量|大小，则实现UpdateLayout。在Android源码中，只有MetricAffectingSpan实现了UpdateLayout接口。

UpdateLayout

UpdateLayout

接下来看一下MetricAffectingSpan的源码。

public abstract class MetricAffectingSpan
extends CharacterStyle
implements UpdateLayout {

    public abstract void updateMeasureState(TextPaint p);

    /**
     * Returns "this" for most MetricAffectingSpans, but for 
     * MetricAffectingSpans that were generated by {@link #wrap},
     * returns the underlying MetricAffectingSpan.
     */
    @Override
    public MetricAffectingSpan getUnderlying() {
        return this;
    }

    /**
     * A Passthrough MetricAffectingSpan is one that
     * passes {@link #updateDrawState} and {@link #updateMeasureState}
     * calls through to the specified MetricAffectingSpan 
     * while still being a distinct object,
     * and is therefore able to be attached to the same Spannable
     * to which the specified MetricAffectingSpan is already attached.
     */
    /* package */ static class Passthrough extends MetricAffectingSpan {
        private MetricAffectingSpan mStyle;
        
        /**
         * Creates a new Passthrough of the specfied MetricAffectingSpan.
         */
        public Passthrough(MetricAffectingSpan cs) {
            mStyle = cs;
        }

        /**
         * Passes updateDrawState through to the underlying MetricAffectingSpan.
         */
        @Override
        public void updateDrawState(TextPaint tp) {
            mStyle.updateDrawState(tp);
        }

        /**
         * Passes updateMeasureState through to the underlying MetricAffectingSpan.
         */
        @Override
        public void updateMeasureState(TextPaint tp) {
            mStyle.updateMeasureState(tp);
        }
    
        /**
         * Returns the MetricAffectingSpan underlying this one, or the one
         * underlying it if it too is a Passthrough.
         */
        @Override
        public MetricAffectingSpan getUnderlying() {
            return mStyle.getUnderlying();
        }
    }
}

可以看见MetricAffectingSpan同样继承了CharacterStyle，因此同样继承了抽象方法updateDrawState(TextPaint tp)，这个方法可以交给子类实现，从而实现字体外观的改变。在MetricAffectingSpan类中定义了一个抽象方法updateMeasureState(TextPaint p)，继承MetricAffectingSpan类的子类可以实现这个抽象方法，从而实现对字体大小的改变。在MetricAffectingSpan中同样也提供了一个Passthrough的类，从而完成CharacterStyle中定义的wrap方法。
接下来分别对MetricAffectingSpan的实现类进行讲述。

4.1 SubscriptSpan和SuperscriptSpan

SubscriptSpan和SuperscriptSpan实现字体的上下标展示，效果如下面的图片所示：

SubscriptSpan

SuperscriptSpan

其实这两个Span的实现特别简单，通过查看这两个类的实现，能够帮助我们对Android的字体有着更深入的理解。
SuperscriptSpan:

    @Override
    public void updateDrawState(TextPaint tp) {
        tp.baselineShift += (int) (tp.ascent() / 2);
    }

    @Override
    public void updateMeasureState(TextPaint tp) {
        tp.baselineShift += (int) (tp.ascent() / 2);
    }

SubscriptSpan:

    @Override
    public void updateDrawState(TextPaint tp) {
        tp.baselineShift -= (int) (tp.ascent() / 2);
    }

    @Override
    public void updateMeasureState(TextPaint tp) {
        tp.baselineShift -= (int) (tp.ascent() / 2);
    }

4.2 AbsoluteSizeSpan和RelativeSizeSpan

AbsoluteSizeSpan和RelativeSizeSpan用来改变相应字符的字体大小。

/**
* size: 大小
* dip: false，size单位为px，true，size单位为dip（默认为false）。
*/
//设置文字大小为24dp
span = new AbsoluteSizeSpan(24, true);

AbsoluteSizeSpan

//设置文字大小为大2倍
span = new RelativeSizeSpan(2.0f);

RelativeSizeSpan

AbsoluteSizeSpan:

    @Override
    public void updateDrawState(TextPaint ds) {
        if (mDip) {
            ds.setTextSize(mSize * ds.density);
        } else {
            ds.setTextSize(mSize);
        }
    }

    @Override
    public void updateMeasureState(TextPaint ds) {
        if (mDip) {
            ds.setTextSize(mSize * ds.density);
        } else {
            ds.setTextSize(mSize);
        }
    }

RelativeSizeSpan:

    @Override
    public void updateDrawState(TextPaint ds) {
        ds.setTextSize(ds.getTextSize() * mProportion);
    }

    @Override
    public void updateMeasureState(TextPaint ds) {
        ds.setTextSize(ds.getTextSize() * mProportion);
    }

4.3 ScaleXSpan

ScaleXSpan影响字符集的文本格式。它可以在x轴方向上缩放字符集。

//设置水平方向上放大3倍
span = new ScaleXSpan(3.0f);

ScaleXSpan

源码：

    @Override
    public void updateDrawState(TextPaint ds) {
        ds.setTextScaleX(ds.getTextScaleX() * mProportion);
    }

    @Override
    public void updateMeasureState(TextPaint ds) {
        ds.setTextScaleX(ds.getTextScaleX() * mProportion);
    }

4.4 StyleSpan、TypefaceSpan和TextAppearanceSpan

StyleSpan、TypefaceSpan和TextAppearanceSpan都可以字体的样式进行改变，StyleSpan可以对字体设置bold或者italic的字符样式，TypefaceSpan可以对字体设置其他的样式，TextAppearanceSpan通过xml文件从而对字体进行设置。

//设置bold+italic的字符样式
span = new StyleSpan(Typeface.BOLD | Typeface.ITALIC);

StyleSpan

//设置serif family
span = new TypefaceSpan("serif");

TypefaceSpan

span = new TextAppearanceSpan(this, R.style.SpecialTextAppearance);

<-- style.xml -->
<style name="SpecialTextAppearance" parent="@android:style/TextAppearance">
<item name="android:textColor">@color/color1</item>
<item name="android:textColorHighlight">@color/color2</item>
<item name="android:textColorHint">@color/color3</item>
<item name="android:textColorLink">@color/color4</item>
<item name="android:textSize">28sp</item>
<item name="android:textStyle">italic</item>
</style>

TextAppearanceSpan

StyleSpan:

    @Override
    public void updateDrawState(TextPaint ds) {
        apply(ds, mStyle);
    }

    @Override
    public void updateMeasureState(TextPaint paint) {
        apply(paint, mStyle);
    }

    private static void apply(Paint paint, int style) {
        int oldStyle;

        Typeface old = paint.getTypeface();
        if (old == null) {
            oldStyle = 0;
        } else {
            oldStyle = old.getStyle();
        }

        int want = oldStyle | style;

        Typeface tf;
        if (old == null) {
            tf = Typeface.defaultFromStyle(want);
        } else {
            tf = Typeface.create(old, want);
        }

        int fake = want & ~tf.getStyle();

        if ((fake & Typeface.BOLD) != 0) {
            paint.setFakeBoldText(true);
        }

        if ((fake & Typeface.ITALIC) != 0) {
            paint.setTextSkewX(-0.25f);
        }

        paint.setTypeface(tf);
    }

TypefaceSpan:

@Override
    public void updateDrawState(TextPaint ds) {
        apply(ds, mFamily);
    }

    @Override
    public void updateMeasureState(TextPaint paint) {
        apply(paint, mFamily);
    }

    private static void apply(Paint paint, String family) {
        int oldStyle;

        Typeface old = paint.getTypeface();
        if (old == null) {
            oldStyle = 0;
        } else {
            oldStyle = old.getStyle();
        }

        Typeface tf = Typeface.create(family, oldStyle);
        int fake = oldStyle & ~tf.getStyle();

        if ((fake & Typeface.BOLD) != 0) {
            paint.setFakeBoldText(true);
        }

        if ((fake & Typeface.ITALIC) != 0) {
            paint.setTextSkewX(-0.25f);
        }

        paint.setTypeface(tf);
    }

TextAppearanceSpan:

    @Override
    public void updateDrawState(TextPaint ds) {
        updateMeasureState(ds);

        if (mTextColor != null) {
            ds.setColor(mTextColor.getColorForState(ds.drawableState, 0));
        }

        if (mTextColorLink != null) {
            ds.linkColor = mTextColorLink.getColorForState(ds.drawableState, 0);
        }
    }

    @Override
    public void updateMeasureState(TextPaint ds) {
        if (mTypeface != null || mStyle != 0) {
            Typeface tf = ds.getTypeface();
            int style = 0;

            if (tf != null) {
                style = tf.getStyle();
            }

            style |= mStyle;

            if (mTypeface != null) {
                tf = Typeface.create(mTypeface, style);
            } else if (tf == null) {
                tf = Typeface.defaultFromStyle(style);
            } else {
                tf = Typeface.create(tf, style);
            }

            int fake = style & ~tf.getStyle();

            if ((fake & Typeface.BOLD) != 0) {
                ds.setFakeBoldText(true);
            }

            if ((fake & Typeface.ITALIC) != 0) {
                ds.setTextSkewX(-0.25f);
            }

            ds.setTypeface(tf);
        }

        if (mTextSize > 0) {
            ds.setTextSize(mTextSize);
        }
    }

4.5 LocaleSpan

LocaleSpan用来对字体设置不同的地区，由于不同地区的字体会导致字体大小的变化，因此LocaleSpan也需要继承MetricAffectingSpan。

LineHeightDemo

源码：

    @Override
    public void updateDrawState(TextPaint ds) {
        apply(ds, mLocale);
    }

    @Override
    public void updateMeasureState(TextPaint paint) {
        apply(paint, mLocale);
    }

    private static void apply(Paint paint, Locale locale) {
        paint.setTextLocale(locale);
    }

5 ReplacementSpan

ReplacementSpan继承了MetricAffectingSpan，但是ReplacementSpan比较复杂因此在这单独讲解。在ReplacementSpan里新增加了两个抽象方法，ReplacementSpan源码如下：

public abstract class ReplacementSpan extends MetricAffectingSpan {

    public abstract int getSize(Paint paint, CharSequence text,
                         int start, int end,
                         Paint.FontMetricsInt fm);
    public abstract void draw(Canvas canvas, CharSequence text,
                     int start, int end, float x,
                     int top, int y, int bottom, Paint paint);

    /**
     * This method does nothing, since ReplacementSpans are measured
     * explicitly instead of affecting Paint properties.
     */
    public void updateMeasureState(TextPaint p) { }

    /**
     * This method does nothing, since ReplacementSpans are drawn
     * explicitly instead of affecting Paint properties.
     */
    public void updateDrawState(TextPaint ds) { }
}

抽象方法getSize(Paint paint, CharSequence text, int start, int end, Paint.FontMetricsInt fm)返回所占的宽度。其实根据getSize方法的参数我们能够计算原本那些字符所占用的宽度，计算方法如下：

    @Override
    public int getSize(Paint paint, CharSequence text, int start, int end, Paint.FontMetricsInt fm) {
        //return text with relative to the Paint
        mWidth = (int) paint.measureText(text, start, end);
        return mWidth;
    }

通过这个宽度我们可以给文字制作相应的效果。
抽象方法draw，可以让我们在合适的区域绘制相应的图形，start和end分别为span作用的起始和结束字符的index，x为起始横坐标，y为baseline对应的坐标，top为起始高度，bottom为结束高度。
在Android提供的源码里面提供了一个抽象类DynamicDrawableSpan来继承ReplacementSpan，而DynamicDrawableSpan又有一个子类ImageSpan。

5.1 DynamicDrawableSpan

DynamicDrawableSpan是一个抽象类，DynamicDrawableSpan可以做到使用Drawable替代相对应的字符序列，展现效果如下所示：

ImageSpan

下面我们来分析一下DynamicDrawableSpan的源码。

public abstract class DynamicDrawableSpan extends ReplacementSpan {
    private static final String TAG = "DynamicDrawableSpan";
    
    /**
     * A constant indicating that the bottom of this span should be aligned
     * with the bottom of the surrounding text, i.e., at the same level as the
     * lowest descender in the text.
     */
    public static final int ALIGN_BOTTOM = 0;
    
    /**
     * A constant indicating that the bottom of this span should be aligned
     * with the baseline of the surrounding text.
     */
    public static final int ALIGN_BASELINE = 1;
    
    protected final int mVerticalAlignment;
    
    public DynamicDrawableSpan() {
        mVerticalAlignment = ALIGN_BOTTOM;
    }

    /**
     * @param verticalAlignment one of {@link #ALIGN_BOTTOM} or {@link #ALIGN_BASELINE}.
     */
    protected DynamicDrawableSpan(int verticalAlignment) {
        mVerticalAlignment = verticalAlignment;
    }

    /**
     * Returns the vertical alignment of this span, one of {@link #ALIGN_BOTTOM} or
     * {@link #ALIGN_BASELINE}.
     */
    public int getVerticalAlignment() {
        return mVerticalAlignment;
    }

    /**
     * Your subclass must implement this method to provide the bitmap   
     * to be drawn.  The dimensions of the bitmap must be the same
     * from each call to the next.
     */
    public abstract Drawable getDrawable();

    @Override
    public int getSize(Paint paint, CharSequence text,
                         int start, int end,
                         Paint.FontMetricsInt fm) {
        Drawable d = getCachedDrawable();
        Rect rect = d.getBounds();
        if (fm != null) {
            fm.ascent = -rect.bottom; 
            fm.descent = 0; 

            fm.top = fm.ascent;
            fm.bottom = 0;
        }
        return rect.right;
    }

    @Override
    public void draw(Canvas canvas, CharSequence text,
                     int start, int end, float x, 
                     int top, int y, int bottom, Paint paint) {
        Drawable b = getCachedDrawable();
        canvas.save();
        int transY = bottom - b.getBounds().bottom;
        if (mVerticalAlignment == ALIGN_BASELINE) {
            transY -= paint.getFontMetricsInt().descent;
        }
        canvas.translate(x, transY);
        b.draw(canvas);
        canvas.restore();
    }

    private Drawable getCachedDrawable() {
        WeakReference<Drawable> wr = mDrawableRef;
        Drawable d = null;
        if (wr != null)
            d = wr.get();
        if (d == null) {
            d = getDrawable();
            mDrawableRef = new WeakReference<Drawable>(d);
        }
        return d;
    }

    private WeakReference<Drawable> mDrawableRef;
}

1. 抽象方法getDrawable()告诉子类需要提供一个Drawable用来绘制；
2. getSize方法中，通过设置FontMetricsInt，从而使得替代字符序列的baseline和图片的尾部对齐，而替代字符序列的垂直高度就为图片的高度；
3. draw方法中，需要绘制图片的其实x坐标很明确就是x，y坐标可以通过多种方式获取，在baseline对齐的情况下可以等于top，也可以等于y-b.getBounds().bottom，还可以等于bottom-b.getBounds().bottom-descent，各种方法都可以。

在Android系统中，提供了一个ImageSpan继承了DynamicDrawableSpan，实现了通过多种方式生成Drawable。

6 相关链接

Textview图文基础
段落级span
字符级span
自定义span