初识StaticLayout是在一个需要计算TextView高度的时候，计算完高度后对TextView进行分页显示。为此我仔细观看了TextView中计算高度的部分，并从中找到了答案

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {int widthMode = MeasureSpec.getMode(widthMeasureSpec);int heightMode = MeasureSpec.getMode(heightMeasureSpec);int widthSize = MeasureSpec.getSize(widthMeasureSpec);int heightSize = MeasureSpec.getSize(heightMeasureSpec);int width;int height;// 篇幅太长，我们略过宽度部分，高度是由getDesiredHeight()来计算的... ...if (heightMode == MeasureSpec.EXACTLY) {// Parent has told us how big to be. So be it.height = heightSize;mDesiredHeightAtMeasure = -1;} else {int desired = getDesiredHeight();height = desired;mDesiredHeightAtMeasure = desired;if (heightMode == MeasureSpec.AT_MOST) {height = Math.min(desired, heightSize);}}... ...setMeasuredDimension(width, height);
}private int getDesiredHeight() {return Math.max(getDesiredHeight(mLayout, true),getDesiredHeight(mHintLayout, mEllipsize != null));
}private int getDesiredHeight(Layout layout, boolean cap) {if (layout == null) {return 0;}/** Don't cap the hint to a certain number of lines.* (Do cap it, though, if we have a maximum pixel height.)*/int desired = layout.getHeight(cap);final Drawables dr = mDrawables;if (dr != null) {desired = Math.max(desired, dr.mDrawableHeightLeft);desired = Math.max(desired, dr.mDrawableHeightRight);}int linecount = layout.getLineCount();final int padding = getCompoundPaddingTop() + getCompoundPaddingBottom();desired += padding;if (mMaxMode != LINES) {desired = Math.min(desired, mMaximum);} else if (cap && linecount > mMaximum && (layout instanceof DynamicLayout|| layout instanceof BoringLayout)) {desired = layout.getLineTop(mMaximum);if (dr != null) {desired = Math.max(desired, dr.mDrawableHeightLeft);desired = Math.max(desired, dr.mDrawableHeightRight);}desired += padding;linecount = mMaximum;}if (mMinMode == LINES) {if (linecount < mMinimum) {desired += getLineHeight() * (mMinimum - linecount);}} else {desired = Math.max(desired, mMinimum);}// Check against our minimum heightdesired = Math.max(desired, getSuggestedMinimumHeight());return desired;
}

从上面的代码中我们可以看到TextView中文本的高度是由Lyout.getHeight(boolean)得到的，由此可见，文字的管理是通过Layout实现的。TextView内部会根据不同的设置，创建不同的Layout，总共有三种。

• DynamicLayout：用在EditText或者TextView中设置的是Spannable类型的文字。
• BoringLayout：常用在处理单行文本。
• StaticLayout：这个是默认的TextView的Layout，用在文字不会被改变的状态下。。

在这里我们主要研究了StaticLayout，用它来计算文本高度。StaticLayout是基于Builder模式创建的

var layout = StaticLayout.Builder.obtain(source, start, end, paint, width).setAlignment(alignment).setTextDirection(textDir).setLineSpacing(spacingAdd, spacingMult).setIncludePad(includePad).setBreakStrategy(breakStrategy).setHyphenationFrequency(hyphenationFrequency).setMaxLines(maxLine).build()

主要参数

• source是文本，start和end分别是开始和结束位置。
• paint是TextPaint，用来绘制文本。
• width是文本宽度，文字到达这个宽度后就会自动换行。
• alignment是文本对齐方向，主要有ALIGN_NORMAL/ALIGN_CENTER/ALIGN_OPPOSITE。
• spacingAdd是行间距的额外增加值，默认为0。spacingmult是行间距的倍数，默认是1。
• includePad是指是否在文字上下添加额外的空间，超出ascent和descent部分。
• breakStrategy是换行策略，主要有BREAK_STRATEGY_SIMPLE/BREAK_STRATEGY_HIGH_QUALITY/BREAK_STRATEGY_BALANCED。
• maxLine是最大行数。

找到了TextView计算高度的方法后，我们自定义了一个文本显示控件，

class StaticLayoutView (context: Context, attrs: AttributeSet?, defStyleAttr: Int) :View(context, attrs, defStyleAttr) {private var mStaticLayout: StaticLayout? = nullconstructor(context: Context) : this(context, null)constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, 0)fun setLayout(layout: StaticLayout) {mStaticLayout = layout}override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {var height = mStaticLayout?.height ?: 0setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec), height)}override fun onDraw(canvas: Canvas?) {super.onDraw(canvas)mStaticLayout?.draw(canvas)}}