TextView高频度绘图下的问题
在一些场景下。比如界面上有大量的聊天并且活跃度高，内容包含了文字,emoji,图片等各种信息的复杂文本，采用TextView来展示这些内容信息。就容易观察到，聊天消息在频繁刷新的时候，性能有明显下降，GPU火焰图抖动也更加频繁。
性能瓶颈在那里？
- 1 TextView有1w行代码，除了里面包含的View的属性和方法外，还包含如下内容，Spans,MovementMethod,InputConnection, TransformationMethod, Layout, Editor(你以为这应该写在EditText里面，google说我就不)。
- 2 纯看代码还是很难看出来问题。找到问题的方法有2种。
google一下，在前人的基础上找到途径，参见链接：请自备梯子。另外，在你的聊天页里面出现textview处理的地方分析trace的时间，也能直接看到TextView的setText方法耗时很长，是一个明显的瓶颈。
TextView的setText方法分析
- 1 源代码如下：
public void setText(CharSequence text, BufferType type) {
setText(text, type, true, 0);
if (mCharWrapper != null) {
mCharWrapper.mChars = null;
}
}
private void setText(CharSequence text, BufferType type,
boolean notifyBefore, int oldlen) {
if (text == null) {
text = "";
}
// If suggestions are not enabled, remove the suggestion spans from the text
if (!isSuggestionsEnabled()) {
text = removeSuggestionSpans(text);
}
if (!mUserSetTextScaleX) mTextPaint.setTextScaleX(1.0f);
if (text instanceof Spanned &&
((Spanned) text).getSpanStart(TextUtils.TruncateAt.MARQUEE) >= 0) {
if (ViewConfiguration.get(mContext).isFadingMarqueeEnabled()) {
setHorizontalFadingEdgeEnabled(true);
mMarqueeFadeMode = MARQUEE_FADE_NORMAL;
} else {
setHorizontalFadingEdgeEnabled(false);
mMarqueeFadeMode = MARQUEE_FADE_SWITCH_SHOW_ELLIPSIS;
}
setEllipsize(TextUtils.TruncateAt.MARQUEE);
}
int n = mFilters.length;
for (int i = 0; i < n; i++) {
CharSequence out = mFilters[i].filter(text, 0, text.length(), EMPTY_SPANNED, 0, 0);
if (out != null) {
text = out;
}
}
if (notifyBefore) {
if (mText != null) {
oldlen = mText.length();
sendBeforeTextChanged(mText, 0, oldlen, text.length());
} else {
sendBeforeTextChanged("", 0, 0, text.length());
}
}
boolean needEditableForNotification = false;
if (mListeners != null && mListeners.size() != 0) {
needEditableForNotification = true;
}
if (type == BufferType.EDITABLE || getKeyListener() != null ||
needEditableForNotification) {
createEditorIfNeeded();
mEditor.forgetUndoRedo();
Editable t = mEditableFactory.newEditable(text);
text = t;
setFilters(t, mFilters);
InputMethodManager imm = InputMethodManager.peekInstance();
if (imm != null) imm.restartInput(this);
} else if (type == BufferType.SPANNABLE || mMovement != null) {
text = mSpannableFactory.newSpannable(text);
} else if (!(text instanceof CharWrapper)) {
text = TextUtils.stringOrSpannedString(text);
}
if (mAutoLinkMask != 0) {
Spannable s2;
if (type == BufferType.EDITABLE || text instanceof Spannable) {
s2 = (Spannable) text;
} else {
s2 = mSpannableFactory.newSpannable(text);
}
if (Linkify.addLinks(s2, mAutoLinkMask)) {
text = s2;
type = (type == BufferType.EDITABLE) ? BufferType.EDITABLE : BufferType.SPANNABLE;
/*
* We must go ahead and set the text before changing the
* movement method, because setMovementMethod() may call
* setText() again to try to upgrade the buffer type.
*/
mText = text;
// Do not change the movement method for text that support text selection as it
// would prevent an arbitrary cursor displacement.
if (mLinksClickable && !textCanBeSelected()) {
setMovementMethod(LinkMovementMethod.getInstance());
}
}
}
mBufferType = type;
mText = text;
if (mTransformation == null) {
mTransformed = text;
} else {
mTransformed = mTransformation.getTransformation(text, this);
}
final int textLength = text.length();
if (text instanceof Spannable && !mAllowTransformationLengthChange) {
Spannable sp = (Spannable) text;
// Remove any ChangeWatchers that might have come from other TextViews.
final ChangeWatcher[] watchers = sp.getSpans(0, sp.length(), ChangeWatcher.class);
final int count = watchers.length;
for (int i = 0; i < count; i++) {
sp.removeSpan(watchers[i]);
}
if (mChangeWatcher == null) mChangeWatcher = new ChangeWatcher();
sp.setSpan(mChangeWatcher, 0, textLength, Spanned.SPAN_INCLUSIVE_INCLUSIVE |
(CHANGE_WATCHER_PRIORITY << Spanned.SPAN_PRIORITY_SHIFT));
if (mEditor != null) mEditor.addSpanWatchers(sp);
if (mTransformation != null) {
sp.setSpan(mTransformation, 0, textLength, Spanned.SPAN_INCLUSIVE_INCLUSIVE);
}
if (mMovement != null) {
mMovement.initialize(this, (Spannable) text);
/*
* Initializing the movement method will have set the
* selection, so reset mSelectionMoved to keep that from
* interfering with the normal on-focus selection-setting.
*/
if (mEditor != null) mEditor.mSelectionMoved = false;
}
}
if (mLayout != null) {
checkForRelayout();
}
sendOnTextChanged(text, 0, oldlen, textLength);
onTextChanged(text, 0, oldlen, textLength);
notifyViewAccessibilityStateChangedIfNeeded(AccessibilityEvent.CONTENT_CHANGE_TYPE_TEXT);
if (needEditableForNotification) {
sendAfterTextChanged((Editable) text);
}
// SelectionModifierCursorController depends on textCanBeSelected, which depends on text
if (mEditor != null) mEditor.prepareCursorControllers();
}
2 从上面的源代码中我们可以看到下面几个问题：
A. 代码很长（好重要的感悟啊！！！）
B. 判断type的时候，在一些条件下会重新构造Spannable，至于构造Editable咋们就无视掉。
C.对text移除ChangeWatcher(继承TextWatcher,SpanWatcher)，重新添加ChangeWatcher3 我们需要一样什么样的TextView，在这个大批量刷新的情况下。我们要的只是一个高效率展示复杂文本的TextView，不需要Editable，也不需要ChangeWatcher。更重要的我们如果能先计算好布局，省去settext这个复杂过程，直接给View调用这样性能提升会更快。
解决方法：我们需要的是一个静态的文本布局
- 1 Android提供了文本布局的基类android.text.Layout，注意看Layout的类注释：
/**
* A base class that manages text layout in visual elements on
* the screen.
* <p>For text that will be edited, use a {@link DynamicLayout},
* which will be updated as the text changes.
* For text that will not change, use a {@link StaticLayout}.
*/
public abstract class Layout {
注释中提到：对于可编辑的文本，使用DynamicLayout，当文本变化的时候会执行update。而对于静态文本，推荐使用StaticLayout.
- 2 翻过来看TextView怎么做的了？
TextView包含了3种布局:
a. BoringLayout (单行纯文字文本)
b. Staticlayout (多行复杂文本）
c. DynamicLayout (多行可编辑复杂文本）
从TextView的设计上来说，这个设计应该能满足我们的多行静态复杂文本的展示啊（判断完逻辑采用Staticlayout啊！！！），但是阅读下源码，你会发现，TextView构造一个layout，只要文本中有Spannable的出现，TextView就会构造DynamicLayout来负责文本展示。
整个TextView选择layout的源码如下：
protected Layout makeSingleLayout(int wantWidth, BoringLayout.Metrics boring, int ellipsisWidth,
Layout.Alignment alignment, boolean shouldEllipsize, TruncateAt effectiveEllipsize,
boolean useSaved) {
Layout result = null;
if (mText instanceof Spannable) { //!!!!!!!注意看这里！！！！
result = new DynamicLayout(mText, mTransformed, mTextPaint, wantWidth,
alignment, mTextDir, mSpacingMult, mSpacingAdd, mIncludePad,
mBreakStrategy, mHyphenationFrequency,
getKeyListener() == null ? effectiveEllipsize : null, ellipsisWidth);
} else {
if (boring == UNKNOWN_BORING) {
boring = BoringLayout.isBoring(mTransformed, mTextPaint, mTextDir, mBoring);
if (boring != null) {
mBoring = boring;
}
}
if (boring != null) {
if (boring.width <= wantWidth &&
(effectiveEllipsize == null || boring.width <= ellipsisWidth)) {
if (useSaved && mSavedLayout != null) {
result = mSavedLayout.replaceOrMake(mTransformed, mTextPaint,
wantWidth, alignment, mSpacingMult, mSpacingAdd,
boring, mIncludePad);
} else {
result = BoringLayout.make(mTransformed, mTextPaint,
wantWidth, alignment, mSpacingMult, mSpacingAdd,
boring, mIncludePad);
}
if (useSaved) {
mSavedLayout = (BoringLayout) result;
}
} else if (shouldEllipsize && boring.width <= wantWidth) {
if (useSaved && mSavedLayout != null) {
result = mSavedLayout.replaceOrMake(mTransformed, mTextPaint,
wantWidth, alignment, mSpacingMult, mSpacingAdd,
boring, mIncludePad, effectiveEllipsize,
ellipsisWidth);
} else {
result = BoringLayout.make(mTransformed, mTextPaint,
wantWidth, alignment, mSpacingMult, mSpacingAdd,
boring, mIncludePad, effectiveEllipsize,
ellipsisWidth);
}
}
}
}
if (result == null) {
StaticLayout.Builder builder = StaticLayout.Builder.obtain(mTransformed,
0, mTransformed.length(), mTextPaint, wantWidth)
.setAlignment(alignment)
.setTextDirection(mTextDir)
.setLineSpacing(mSpacingAdd, mSpacingMult)
.setIncludePad(mIncludePad)
.setBreakStrategy(mBreakStrategy)
.setHyphenationFrequency(mHyphenationFrequency);
if (shouldEllipsize) {
builder.setEllipsize(effectiveEllipsize)
.setEllipsizedWidth(ellipsisWidth)
.setMaxLines(mMaxMode == LINES ? mMaximum : Integer.MAX_VALUE);
}
// TODO: explore always setting maxLines
result = builder.build();
}
return result;
}
StaticLayout表示不服，Spannable我也能处理，而且我简单效率高。Google为什么这么设计，可能是因为TextView作为Button, CheckedTextView, EditText, Switch,ToggleButton等的基类表示压力山大吧！有那位同学给我解析下为啥TextView要把EditText这个子类关于edit相关的活自己干！
找到了解决问题的方式，下面就是纯撸代码了
- 1 自定义StaticLayoutView
public class StaticLayoutView extends View {
private Layout layout;
private int width ;
private int height;
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
public StaticLayoutView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
super(context, attrs, defStyleAttr, defStyleRes);
}
public StaticLayoutView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
public StaticLayoutView(Context context, AttributeSet attrs) {
super(context, attrs);
}
public StaticLayoutView(Context context) {
super(context);
}
public void setLayout(Layout layout) {
this.layout = layout;
if (this.layout.getWidth() != width || this.layout.getHeight() != height) {
width = this.layout.getWidth();
height = this.layout.getHeight();
requestLayout();
}
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.save();
if (layout != null) {
layout.draw(canvas, null, null, 0);
}
canvas.restore();
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (layout != null) {
setMeasuredDimension(layout.getWidth(), layout.getHeight());
} else {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
}
- 2 StaticLayout [父类Layout]的一个构造函数
/**
* Subclasses of Layout use this constructor to set the display text,
* width, and other standard properties.
* @param text the text to render
* @param paint the default paint for the layout. Styles can override
* various attributes of the paint.
* @param width the wrapping width for the text.
* @param align whether to left, right, or center the text. Styles can
* override the alignment.
* @param spacingMult factor by which to scale the font size to get the
* default line spacing
* @param spacingAdd amount to add to the default line spacing
*
* @hide
*/
protected Layout(CharSequence text, TextPaint paint,
int width, Alignment align, TextDirectionHeuristic textDir,
float spacingMult, float spacingAdd) {
a. 构造CharSequence，平时我们对复杂文本的处理，就是通过spannerString来构造，比如某段用什么颜色，某段用什么size
b. 构造TextPaint，可以设置整个文本的字体size,color等（当然spannerString来分段控制更好），也可以来设置字体的type等
c. 输入其他参数，构造完整个StaticLayout后,调用StaticLayoutView的
setLayout方法就完成了整个绘制，这个构造的过程当然也可以放在子线程来做。
- 3 细节问题
细节永远是最麻烦的事情，比如你项目有默认设定的复杂文本的颜色，文字size，阴影等等，不像textview天生就提供textsize,textcolor,shadow这些属性，你的选择有2个。第一，自己构造这些属性，第二，围绕你的CharSequence和textpaint构造更多方便实现你业务ui效果的方法。
- 4 StaticLayout的用途
a.文中高频度大量textview刷新优化。
b.一个textview显示大量的文本，比如一些阅读app。
c. 在控件上画文本，比如一个ImageView中心画文本。
d. 一些排版效果,比如多行文本文字居中对齐等。
