今天分析下經(jīng)常使用加載布局的layoutInflater
我們?cè)诩虞d布局的時(shí)候都會(huì)主動(dòng)或者被動(dòng)的用到 LayoutInflater ，比如 Activity 的setContentView方法和Fragment的onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)回調(diào)等。LayoutInflater 的作用就是把布局文件xml實(shí)例化為相應(yīng)的View組件。我們可以通過(guò)三種方法獲取 LayoutInflater：
1.Activity.getLayoutInflater()；
2.LayoutInflater.from(context)；
3.context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)；
每個(gè)方法都和 Context 相關(guān)聯(lián)，其中方法1和方法2最終都會(huì)通過(guò)方法3來(lái)實(shí)現(xiàn)。
獲取到 LayoutInflater 后，通過(guò)調(diào)用inflate方法來(lái)實(shí)例化布局。而inflate方法由很多重載，我們常用的是inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)，所有 inflate 方法最終會(huì)調(diào)用到 inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)。下面就從這個(gè)方法入手，開(kāi)始分析 LayoutInflater 的源碼。
inflate方法
先看一下inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)的三個(gè)參數(shù)：
- XmlPullParser parser：很顯然是一個(gè) XML 解析器，這個(gè)解析器就是 LayoutInflater 所要加載的 XML 布局轉(zhuǎn)化來(lái)的，通過(guò) PULL 方式解析。
- ViewGroup root：裝載要加載的 XML 布局的根容器，比如，在 Activity 的setContentView方法中就是 id 為android.R.id.content的 FrameLayout 根布局了。
- boolean attachToRoot：是否將所解析的布局添加到根容器中，同時(shí)也影響了所解析布局的寬高。
被廣泛討論的是root和attachToRoot的不同傳參對(duì)被加載的布局文件的影響，下面看代碼。
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final Context inflaterContext = mContext;
// 將parser轉(zhuǎn)成AttributeSet接口，用來(lái)讀取xml中設(shè)置的View屬性
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root; // 此方法返回的View，默認(rèn)是root
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
...
final String name = parser.getName(); // 獲取當(dāng)前的標(biāo)簽名
...
if (TAG_MERGE.equals(name)) { // 處理<merge>標(biāo)簽
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
// 遞歸處理
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
// 創(chuàng)建View對(duì)象
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
...
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs); // 獲取根View的寬高
if (!attachToRoot) { // 如果attachToRoot為false，則給根View設(shè)置寬高
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
...
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true); // 遞歸處理
...
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
// 如果root不空，且attachToRoot為true，則將根View添加到容器中
root.addView(temp, params);
}
// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) {
// 如果root空或者attachToRoot為false，則將返回結(jié)果設(shè)置為根View
result = temp;
}
}
} catch (XmlPullParserException e) {
...
} catch (Exception e) {
...
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
// 要么是root，要么是創(chuàng)建的根View
return result;
}
}
從代碼中可以看出root和attachToRoot不同傳參的影響：
- 如果root不為null，attachToRoot設(shè)為true，則會(huì)將加載的布局添加到一個(gè)父布局中，即root，并且返回root；
- 如果root不為null，attachToRoot設(shè)為false，則會(huì)對(duì)布局文件最外層的所有l(wèi)ayout屬性進(jìn)行設(shè)置，并且返回該布局的根View，當(dāng)該view被添加到父view當(dāng)中時(shí)，這些layout屬性會(huì)自動(dòng)生效；
- 如果root為null，attachToRoot將失去作用，設(shè)置任何值都沒(méi)有意義，返回的也是要加載的布局的根View；
rInflate方法
從上面的方法中可以看到處理<merge>標(biāo)簽時(shí)會(huì)調(diào)用rInflate，處理子View時(shí)會(huì)調(diào)用rInflateChildren方法。其實(shí)rInflateChildren中調(diào)用的是rInflate，而rInflate也調(diào)用了rInflateChildren，從而形成了遞歸調(diào)用，也就是遞歸處理子View。
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
// 處理<requestFocus>標(biāo)簽
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
// 處理<tag>標(biāo)簽
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
// 處理<include>標(biāo)簽
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) { // <merge>標(biāo)簽異常
throw new InflateException("<merge /> must be the root element");
} else { // 創(chuàng)建View對(duì)象
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true); // 遞歸處理孩子節(jié)點(diǎn)
viewGroup.addView(view, params); // 將View添加到父布局中
}
}
if (pendingRequestFocus) { // 父布局處理焦點(diǎn)
parent.restoreDefaultFocus();
}
if (finishInflate) { // 結(jié)束加載
parent.onFinishInflate();
}
}
該方法中會(huì)處理<requestFocus>、<tag>、<include>、<merge>和普通View標(biāo)簽。其中：
- <requestFocus>是重新定位焦點(diǎn)的，調(diào)用的consumeChildElements方法其實(shí)沒(méi)干什么事，只是簡(jiǎn)單的把該標(biāo)簽消費(fèi)結(jié)束掉。
- <tag>標(biāo)簽一般很少用，它主要用來(lái)標(biāo)記View，給View設(shè)置一個(gè)標(biāo)簽值，例如：
<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!" >
<tag android:id="@+id/tag"
android:value="hello" />
</TextView>
findViewById(R.id.tv).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// 試一下tag標(biāo)簽
Toast.makeText(MainActivity.this, (String) v.getTag(R.id.tag), Toast.LENGTH_SHORT).show();
}
});
在ListView的自定義Adapter中，應(yīng)該都有用到過(guò)View的setTag方法，即：使用ViewHolder來(lái)重復(fù)利用View。
parseViewTag方法：
private void parseViewTag(XmlPullParser parser, View view, AttributeSet attrs)
throws XmlPullParserException, IOException {
final Context context = view.getContext();
final TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.ViewTag);
// 讀取tag的id
final int key = ta.getResourceId(R.styleable.ViewTag_id, 0);
// 讀取tag的值
final CharSequence value = ta.getText(R.styleable.ViewTag_value);
// 給View設(shè)置該tag
view.setTag(key, value);
ta.recycle();
// 結(jié)束該標(biāo)簽（子View無(wú)效）
consumeChildElements(parser);
}
- <include>標(biāo)簽不能是根標(biāo)簽，parseInclude方法單獨(dú)分析。
- <merge>標(biāo)簽只能是根標(biāo)簽，這里會(huì)拋異常。
parseInclude方法
private void parseInclude(XmlPullParser parser, Context context, View parent,
AttributeSet attrs) throws XmlPullParserException, IOException {
int type;
if (parent instanceof ViewGroup) { // 必須在ViewGroup里才有效
// 處理theme屬性
...
// If the layout is pointing to a theme attribute, we have to
// massage the value to get a resource identifier out of it.
// 拿到layout指定的布局
int layout = attrs.getAttributeResourceValue(null, ATTR_LAYOUT, 0);
...
if (layout == 0) { // 必須是合法的id
final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
throw new InflateException("You must specify a valid layout "
+ "reference. The layout ID " + value + " is not valid.");
} else { // 類(lèi)似于inflate的處理
// 拿到layout的解析器
final XmlResourceParser childParser = context.getResources().getLayout(layout);
try {
final AttributeSet childAttrs = Xml.asAttributeSet(childParser);
while ((type = childParser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty.
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(childParser.getPositionDescription() +
": No start tag found!");
}
// layout的根標(biāo)簽
final String childName = childParser.getName();
if (TAG_MERGE.equals(childName)) { // 處理<merge>
// The <merge> tag doesn't support android:theme, so
// nothing special to do here.
rInflate(childParser, parent, context, childAttrs, false);
} else { // 處理View
final View view = createViewFromTag(parent, childName,
context, childAttrs, hasThemeOverride);
final ViewGroup group = (ViewGroup) parent;
final TypedArray a = context.obtainStyledAttributes(
attrs, R.styleable.Include);
// 獲取<include>里設(shè)置的id
final int id = a.getResourceId(R.styleable.Include_id, View.NO_ID);
// 獲取<include>里設(shè)置的visibility
final int visibility = a.getInt(R.styleable.Include_visibility, -1);
a.recycle();
ViewGroup.LayoutParams params = null;
try { // 獲取<include>里設(shè)置的寬高
params = group.generateLayoutParams(attrs);
} catch (RuntimeException e) {
// Ignore, just fail over to child attrs.
}
if (params == null) {
// 獲取layout里設(shè)置的寬高
params = group.generateLayoutParams(childAttrs);
}
// <include>里設(shè)置的寬高優(yōu)先于layout里設(shè)置的
view.setLayoutParams(params);
// Inflate all children.
rInflateChildren(childParser, view, childAttrs, true);
if (id != View.NO_ID) {
// include里設(shè)置的id優(yōu)先級(jí)高
view.setId(id);
}
// include里設(shè)置的visibility優(yōu)先級(jí)高
switch (visibility) {
case 0:
view.setVisibility(View.VISIBLE);
break;
case 1:
view.setVisibility(View.INVISIBLE);
break;
case 2:
view.setVisibility(View.GONE);
break;
}
group.addView(view);
}
} finally {
childParser.close();
}
}
} else {
throw new InflateException("<include /> can only be used inside of a ViewGroup");
}
LayoutInflater.consumeChildElements(parser);
}
- include里必須設(shè)置layout屬性，且layout的id必須合法；
- include里設(shè)置的id優(yōu)先級(jí)高于layout里設(shè)置的id，即：兩者同時(shí)設(shè)置時(shí)，后者會(huì)失效；
- include里設(shè)置的width和height屬性?xún)?yōu)先級(jí)高于layout里設(shè)置的寬高；
- include里設(shè)置的visibility屬性?xún)?yōu)先級(jí)高于layout設(shè)置的visibility。
createViewFromTag方法
正常View標(biāo)簽都是通過(guò)createViewFromTag來(lái)創(chuàng)建對(duì)應(yīng)的View對(duì)象的。
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
if (name.equals("view")) {
// 真正的View標(biāo)簽名存在class屬性中
name = attrs.getAttributeValue(null, "class");
}
...
try {
View view;
if (mFactory2 != null) { // 先使用Factory2
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) { // 再使用Factory
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
// 通過(guò)標(biāo)簽名中是否包含'.'來(lái)區(qū)分是否為自定義View
if (-1 == name.indexOf('.')) {
// 處理系統(tǒng)View
view = onCreateView(parent, name, attrs);
} else { // 自定義View用的是全限定類(lèi)名
// 處理自定義View
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
...
} catch (ClassNotFoundException e) {
...
} catch (Exception e) {
...
}
}
- 優(yōu)先通過(guò)Factory2和Factory來(lái)創(chuàng)建View，這兩個(gè)Factory等會(huì)再說(shuō)；
- 通過(guò)標(biāo)簽名中是否包含'.'來(lái)區(qū)分待創(chuàng)建的View是自定義View還是系統(tǒng)View；
- 系統(tǒng)View會(huì)在onCreateView方法中添加android.view.前綴，然后交由createView處理。
createView方法
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
// 有緩存
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
try {
if (constructor == null) { // 第一次則通過(guò)反射創(chuàng)建constructor
// Class not found in the cache, see if it's real, and try to add it
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
// 使用的是包含Context, AttributeSet這兩個(gè)參數(shù)的構(gòu)造函數(shù)
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor); // 添加到緩存中
} else { // 命中緩存
// If we have a filter, apply it to cached constructor
if (mFilter != null) { // 先過(guò)濾
// Have we seen this name before?
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
// New class -- remember whether it is allowed
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
mFilterMap.put(name, allowed);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, attrs);
}
}
}
Object lastContext = mConstructorArgs[0];
if (mConstructorArgs[0] == null) {
// Fill in the context if not already within inflation.
mConstructorArgs[0] = mContext;
}
Object[] args = mConstructorArgs;
args[1] = attrs;
// 反射創(chuàng)建View實(shí)例對(duì)象
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// 如果是ViewStub則懶加載
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
mConstructorArgs[0] = lastContext;
return view;
}
...
}
通過(guò)反射待創(chuàng)建View的構(gòu)造函數(shù)（兩個(gè)參數(shù)：Context和AttributeSet的構(gòu)造函數(shù)）來(lái)實(shí)例化View對(duì)象，如果是ViewStub對(duì)象還會(huì)進(jìn)行懶加載。
LayoutInflater.Factory/Factory2
通過(guò)以上流程，使用LayoutInflater的infalte方法加載布局文件的整體流程就分析完了。但出現(xiàn)了Factory2和Factory類(lèi)，它們會(huì)優(yōu)先創(chuàng)建View，我們來(lái)看看著兩個(gè)類(lèi)到底是什么！
它們都是LayoutInflater的內(nèi)部類(lèi)——兩個(gè)接口：
public interface Factory {
public View onCreateView(String name, Context context, AttributeSet attrs);
}
public interface Factory2 extends Factory {
public View onCreateView(View parent, String name, Context context, AttributeSet attrs);
}
Factory2繼承了Factory，增加了一個(gè)帶View parent參數(shù)的onCreateView重載方法。它們是在createViewFromTag中被調(diào)用的，默認(rèn)為null，說(shuō)明開(kāi)發(fā)人員可以自定義這兩個(gè)Factory，則通過(guò)它們可以改造待加載XML布局中的View標(biāo)簽，來(lái)使用自定義規(guī)則創(chuàng)建View。
來(lái)看一下它們的設(shè)置方法：
public void setFactory(Factory factory) {
if (mFactorySet) {
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
// 和setFactory2類(lèi)似
...
}
}
public void setFactory2(Factory2 factory) {
if (mFactorySet) { // 只能設(shè)置一次
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;
if (mFactory == null) {
mFactory = mFactory2 = factory;
} else { // 合并原有的Factory
mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
}
}
可以看到Factory和Factory2只能設(shè)置一次，否則會(huì)拋異常。
這兩個(gè)Factory的區(qū)別是什么？
Factory2 是API 11 被加進(jìn)來(lái)的；
Factory2 繼承自 Factory，也就說(shuō)現(xiàn)在直接使用Factory2即可；
Factory2 可以對(duì)創(chuàng)建 View 的 Parent 進(jìn)行操作；
總結(jié)
LayoutInflater的相關(guān)分析就這么多，文章有點(diǎn)長(zhǎng)，慢慢看吧！
- LayoutInflater的inflate的過(guò)程的核心方法是：createViewFromTag 和 createView 方法；
- LayoutInflater通過(guò)PULL解析器來(lái)解析XML布局文件，通過(guò)反射來(lái)創(chuàng)建View對(duì)象；
- LayoutInflater.Factory只能設(shè)置一次，可以用來(lái)替換View；
