xml 布局文件是如何变成 View 并填入 View 树的?带着这个问题,阅读源码,居然发现了一个优化布局构建时间的方案。
布局构建耗时是优化 Activity 启动速度中不可缺少的一个环节。
欲优化,先度量。有啥办法可以精确地度量布局耗时?
作者:唐子玄
链接:https://juejin.im/post/5ea312066fb9a03c5b2fe0cc
读布局文件
以熟悉的setContentView()
为切入点,看看有没有突破口:
public class AppCompatActivity
@Override
public void setContentView(View view) {
getDelegate().setContentView(view);
}
}
点开setContentView()
源码,它的实现交给了一个代理,沿着调用链往下追查,最终的实现代码在AppCompatDelegateImpl
中:
class AppCompatDelegateImpl{
@Override
public void setContentView(int resId) {
ensureSubDecor();
//'1.从顶层视图获得content视图'
ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);
//'2.移除所有子视图'
contentParent.removeAllViews();
//'3.解析布局文件并填充到content视图中'
LayoutInflater.from(mContext).inflate(resId, contentParent);
mAppCompatWindowCallback.getWrapped().onContentChanged();
}
}
这三部中,最耗时操作应该是“解析布局文件”,点进去看看:
public abstract class LayoutInflater {
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
...
//'获取布局文件解析器'
final XmlResourceParser parser = res.getLayout(resource);
try {
//'填充布局'
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
}
先调用了getLayout()
获取了和布局文件对应的解析器,沿着调用链继续追查:
public class ResourcesImpl {
XmlResourceParser loadXmlResourceParser(@NonNull String file, @AnyRes int id, int assetCookie,@NonNull String type) throws NotFoundException {
if (id != 0) {
try {
synchronized (mCachedXmlBlocks) {
...
//'通过AssetManager获取布局文件对象'
final XmlBlock block = mAssets.openXmlBlockAsset(assetCookie, file);
if (block != null) {
final int pos = (mLastCachedXmlBlockIndex + 1) % num;
mLastCachedXmlBlockIndex = pos;
final XmlBlock oldBlock = cachedXmlBlocks[pos];
if (oldBlock != null) {
oldBlock.close();
}
cachedXmlBlockCookies[pos] = assetCookie;
cachedXmlBlockFiles[pos] = file;
cachedXmlBlocks[pos] = block;
return block.newParser();
}
}
} catch (Exception e) {
...
}
}
...
}
}
沿着调用链,最终走到了ResourcesImpl.loadXmlResourceParser()
,它通过AssetManager.openXmlBlockAsset()
将 xml 布局文件转化成 Java 对象XmlBlock
:
public final class AssetManager implements AutoCloseable {
@NonNull XmlBlock openXmlBlockAsset(int cookie, @NonNull String fileName) throws IOException {
Preconditions.checkNotNull(fileName, ”fileName“);
synchronized (this) {
ensureOpenLocked();
//'打开 xml 布局文件'
final long xmlBlock = nativeOpenXmlAsset(mObject, cookie, fileName);
if (xmlBlock == 0) {
//'若打开失败则抛文件未找到异常'
throw new FileNotFoundException(“Asset XML file: ” + fileName);
}
final XmlBlock block = new XmlBlock(this, xmlBlock);
incRefsLocked(block.hashCode());
return block;
}
}
}
通过一个 native 方法,将布局文件读取到内存。走查到这里,有一件事可以确定,即 “解析 xml 布局文件前需要进行 IO 操作,将其读取至内存中”。
解析布局文件
读原码就好像“递归”,刚才通过不断地“递”,现在通过“归”回到那个关键方法:
public abstract class LayoutInflater {
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
...
//'获取布局文件解析器'
final XmlResourceParser parser = res.getLayout(resource);
try {
//'填充布局'
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
}
通过 IO 操作将布局文件读到内存后,调用了inflate()
:
public abstract class LayoutInflater {
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
...
try {
//'根据布局文件的声明控件的标签构建 View'
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
//'构建 View 对应的布局参数'
if (root != null) {
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
...
//'将 View 填充到 View 树'
if (root != null && attachToRoot) {
root.addView(temp, params);
}
...
} catch (XmlPullParserException e) {
...
} finally {
...
}
return result;
}
}
这个方法解析布局文件并根据其中声明控件的标签构建 View实例,然后将其填充到 View 树中。解析布局文件的细节在createViewFromTag()
中:
public abstract class LayoutInflater {
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,boolean ignoreThemeAttr) {
...
try {
View view;
//'通过Factory2.onCreateView()构建 View'
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
}
...
return view;
} catch (InflateException e) {
throw e;
}
...
}
}
onCreateView()
的具体实现在AppCompatDelegateImpl
中:
class AppCompatDelegateImpl{
@Override
public final View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
return createView(parent, name, context, attrs);
}
@Override
public View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
if (mAppCompatViewInflater == null) {
TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);
String viewInflaterClassName =
a.getString(R.styleable.AppCompatTheme_viewInflaterClass);
if ((viewInflaterClassName == null){
...
} else {
try {
//'通过反射获取AppCompatViewInflater实例'
Class> viewInflaterClass = Class.forName(viewInflaterClassName);
mAppCompatViewInflater =
(AppCompatViewInflater) viewInflaterClass.getDeclaredConstructor()
.newInstance();
} catch (Throwable t) {
...
}
}
}
boolean inheritContext = false;
if (IS_PRE_LOLLIPOP) {
inheritContext = (attrs instanceof XmlPullParser)
// If we have a XmlPullParser, we can detect where we are in the layout
? ((XmlPullParser) attrs).getDepth() > 1
// Otherwise we have to use the old heuristic
: shouldInheritContext((ViewParent) parent);
}
//'通过createView()创建View实例'
return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,
IS_PRE_LOLLIPOP, /* Only read android:theme pre-L (L+ handles this anyway) */
true, /* Read read app:theme as a fallback at all times for legacy reasons */
VectorEnabledTintResources.shouldBeUsed() /* Only tint wrap the context if enabled */
);
}
}
AppCompatDelegateImpl
又把构建 View 委托给了 AppCompatViewInflater.createView()
:
final View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs, boolean inheritContext,
boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
final Context originalContext = context;
...
View view = null;
//'以布局文件中控件的名称分别创建对应控件实例'
switch (name) {
case "TextView":
view = createTextView(context, attrs);
verifyNotNull(view, name);
break;
case "ImageView":
view = createImageView(context, attrs);
verifyNotNull(view, name);
break;
case "Button":
view = createButton(context, attrs);
verifyNotNull(view, name);
break;
case "EditText":
view = createEditText(context, attrs);
verifyNotNull(view, name);
break;
case "Spinner":
view = createSpinner(context, attrs);
verifyNotNull(view, name);
break;
case "ImageButton":
view = createImageButton(context, attrs);
verifyNotNull(view, name);
break;
case "CheckBox":
view = createCheckBox(context, attrs);
verifyNotNull(view, name);
break;
case "RadioButton":
view = createRadioButton(context, attrs);
verifyNotNull(view, name);
break;
case "CheckedTextView":
view = createCheckedTextView(context, attrs);
verifyNotNull(view, name);
break;
case "AutoCompleteTextView":
view = createAutoCompleteTextView(context, attrs);
verifyNotNull(view, name);
break;
case "MultiAutoCompleteTextView":
view = createMultiAutoCompleteTextView(context, attrs);
verifyNotNull(view, name);
break;
case "RatingBar":
view = createRatingBar(context, attrs);
verifyNotNull(view, name);
break;
case "SeekBar":
view = createSeekBar(context, attrs);
verifyNotNull(view, name);
break;
case "ToggleButton":
view = createToggleButton(context, attrs);
verifyNotNull(view, name);
break;
default:
view = createView(context, name, attrs);
}
...
return view;
}
//'构建 AppCompatTextView 实例'
protected AppCompatTextView createTextView(Context context, AttributeSet attrs) {
return new AppCompatTextView(context, attrs);
}
...
}
没想到,最终居然是通过switch-case
的方法来 new View 实例。
而且我们没有必要手动将布局文件中的TextView
都换成AppCompatTextView
,只要使用AppCompatActivity
,它在Factory2.onCreateView()
接口中完成了控件转换。
测量构建布局耗时
通过上面的分析,可以得出两条结论:
1. Activity 构建布局时,需要先进行 IO 操作,将布局文件读取至内存中。
2. 遍历内存布局文件中每一个标签,并根据标签名 new 出对应视图实例,再把它们 addView 到 View 树中。
这两个步骤都是耗时的!到底有多耗时呢?
LayoutInflaterCompat
提供了setFactory2()
,可以拦截布局文件中每一个 View 的创建过程:
class Factory2Activity : AppCompatActivity() {
private var sum: Double = 0.0
@ExperimentalTime
override fun onCreate(savedInstanceState: Bundle?) {
LayoutInflaterCompat.setFactory2(LayoutInflater.from(this@Factory2Activity), object : LayoutInflater.Factory2 {
override fun onCreateView(parent: View?, name: String?, context: Context?, attrs: AttributeSet?): View? {
//'测量构建单个View耗时'
val (view, duration) = measureTimedValue { delegate.createView(parent, name, context!!, attrs!!) }
//'累加构建视图耗时'
sum += duration.inMilliseconds
Log.v(“test”, “view=${view?.let { it::class.simpleName }} duration=${duration} sum=${sum}”)
return view
}
//'该方法用于兼容Factory,直接返回null就好'
override fun onCreateView(name: String?, context: Context?, attrs: AttributeSet?): View? {
return null
}
})
super.onCreate(savedInstanceState)
setContentView(R.layout.factory2_activity2)
}
}
在super.onCreate(savedInstanceState)
之前,将自定义的Factory2
接口注入到LayoutInflaterCompat
中。
调用delegate.createView(parent, name, context!!, attrs!!)
,就是手动触发源码中构建布局的逻辑。
measureTimedValue()
是 Kotlin 提供的库方法,用于测量一个方法的耗时,定义如下:
public inline fun measureTimedValue(block: () -> T): TimedValue {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
//'委托给MonoClock'
return MonoClock.measureTimedValue(block)
}
public inline fun Clock.measureTimedValue(block: () -> T): TimedValue {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
val mark = markNow()
//'执行原方法'
val result = block()
return TimedValue(result, mark.elapsedNow())
}
public data class TimedValue(val value: T, val duration: Duration)
方法返回一个TimedValue
对象,其第一个属性是原方法的返回值,第二个是执行原方法的耗时。测试代码中通过解构声明
分别将返回值和耗时赋值给view
和duration
。然后把构建每个视图的耗时累加打印。
了解了构建布局的过程,就有了对症下药优化的方向。
有了测量构建布局耗时的方法,就有了对比优化效果的工具。
限于篇幅,构建布局耗时缩短 20 倍的方法只能放到下一篇了。
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