LayoutInflater
開(kāi)頭先附一段LayoutInflater類的注釋簡(jiǎn)介
/**
* Instantiates a layout XML file into its corresponding {@link android.view.View}
* objects. It is never used directly. Instead, use
* {@link android.app.Activity#getLayoutInflater()} or
* {@link Context#getSystemService} to retrieve a standard LayoutInflater instance
* that is already hooked up to the current context and correctly configured
* for the device you are running on.
*
* To create a new LayoutInflater with an additional {@link Factory} for your
* own views, you can use {@link #cloneInContext} to clone an existing
* ViewFactory, and then call {@link #setFactory} on it to include your
* Factory.
*
* For performance reasons, view inflation relies heavily on pre-processing of
* XML files that is done at build time. Therefore, it is not currently possible
* to use LayoutInflater with an XmlPullParser over a plain XML file at runtime;
* it only works with an XmlPullParser returned from a compiled resource
* (R.<em>something</em> file.)
*/
這是LayoutInflater開(kāi)頭的一段介紹，我們能看到幾個(gè)重要的信息：
- LayoutInfalter的作用是把XML轉(zhuǎn)化成對(duì)應(yīng)的View對(duì)象，需要用
Activity#getLayoutInflater()或者getSystemService獲取，會(huì)綁定當(dāng)前的Context - 如果需要使用自定義的
Factory查看替換加載信息，需要用cloneInContext去克隆一個(gè)ViewFactory，然后調(diào)用setFactory設(shè)置自定義的Factory - 性能原因，inflate會(huì)在build階段進(jìn)行，無(wú)法用來(lái)加載一個(gè)外部文本XML文件，只能加載R.xxx.xxx這種處理好的資源文件。
//LayoutInflater.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
//root是否為null來(lái)決定attachToRoot是否為true。
return inflate(resource, root, root != null);
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
return inflate(parser, root, root != null);
}
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
...
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
//三個(gè)inflate方法最終都會(huì)調(diào)用到下面這個(gè)三個(gè)參數(shù)的inflate方法。
/**
* parser XML節(jié)點(diǎn)包含了View的層級(jí)描述
* root 需要attached到的根目錄，如果attachToRoot為true則root必須不為null。
* attachToRoot 加載的層級(jí)是否需要attach到rootView，
* return attachToRoot為true，就返回root，反之false就返回加載的XML文件的根節(jié)點(diǎn)View。
*/
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription() + ": No start tag found!");
}
final String name = parser.getName();
...
if (TAG_MERGE.equals(name)) {
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid " + "ViewGroup root and attachToRoot=true");
}
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
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);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
rInflateChildren(parser, temp, attrs, true);
...
// We are supposed to attach all the views we found (int temp) to root. Do that now.
if (root != null && attachToRoot) {
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) {
result = temp;
}
}
} catch (XmlPullParserException e) {
...
}
return result;
}
}
inflate方法使用XmlPullParser解析XML文件，并根據(jù)得到的標(biāo)簽名執(zhí)行不同的邏輯：
1、首先如果是merge標(biāo)簽，會(huì)走rInflate方法，方法前面帶r的說(shuō)明是recurse遞歸方法
2、如果不是merge標(biāo)簽，執(zhí)行createViewFromTag，根據(jù)傳入的name和attrs獲取到name對(duì)應(yīng)的rootView并且添加到root里面。
針對(duì)merge標(biāo)簽，如果是merge標(biāo)簽必須有root并且必須attachToRoot==true，否則直接拋異常，所以我們得知merge必須作為root標(biāo)簽使用，并且不能用在子標(biāo)簽中①，rInflate方法中也會(huì)針對(duì)merge標(biāo)簽進(jìn)行檢查，保證merge標(biāo)簽不會(huì)出現(xiàn)在子標(biāo)簽中，后面會(huì)有介紹。
檢查通過(guò)則調(diào)用rInflate(parser, root, inflaterContext, attrs, false)方法，遞歸遍歷root的層級(jí)，解析加載childrenView掛載到parentView下面，rinflate詳細(xì)解析可以看rinflate。
如果不是merge標(biāo)簽則調(diào)用createViewFromTag(root, name, inflaterContext, attrs)，這個(gè)方法的作用是加載名字為name的view，根據(jù)name反射方式創(chuàng)建對(duì)應(yīng)的View，根據(jù)傳入的attrs構(gòu)造Params設(shè)置給View，返回創(chuàng)建好的View。
當(dāng)然這只是創(chuàng)建了一個(gè)View，需要再調(diào)用rInflateChildren(parser, temp, attrs, true)，這個(gè)方法也是一個(gè)遞歸方法，它的作用是根據(jù)傳入的parser包含的層級(jí)，加載此層級(jí)的子View并掛載到temp下面。
createViewFromTag
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
// Apply a theme wrapper, if allowed and one is specified.
// 如果傳入的attr中包含theme屬性，則使用此attr中的theme。
if (!ignoreThemeAttr) {
final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
final int themeResId = ta.getResourceId(0, 0);
if (themeResId != 0) {
context = new ContextThemeWrapper(context, themeResId);
}
ta.recycle();
}
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(context, attrs);
}
try {
View view;
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
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 {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (Exception e) {
...
}
}
先看當(dāng)前標(biāo)簽的attr屬性里面是否設(shè)置了theme，如果設(shè)置了就用當(dāng)前標(biāo)簽的theme屬性，綁定到context上面。
這里很有意思的是特殊判斷了一個(gè)TAG_1995，也就是blink，一個(gè)將包裹的內(nèi)容每隔500ms顯示隱藏的一個(gè)標(biāo)簽，怎么看都像個(gè)彩蛋~
然后調(diào)用mFactory2的onCreateView,如果沒(méi)有設(shè)置mFactory2就嘗試mFactory，否則調(diào)用mPrivateFactory,mFactory2和mFactory后面再說(shuō)，這里先往后走。
如果還是沒(méi)有加載到view，先判斷name，看名字里是不是有.，如果沒(méi)有就表明是Android原生的View，最終都會(huì)調(diào)用到createView方法，onCreateView最終會(huì)調(diào)用到createView(name, "android.view.", attrs);，會(huì)在View名字天面添加"android.view."前綴。
下面是默認(rèn)的createView的實(shí)現(xiàn)：
@Nullable
public final View createView(@NonNull Context viewContext, @NonNull String name,
@Nullable String prefix, @Nullable AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Objects.requireNonNull(viewContext);
Objects.requireNonNull(name);
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) {
// Class not found in the cache, see if it's real, and try to add it
clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
mContext.getClassLoader()).asSubclass(View.class);
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, viewContext, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else {
// If we have a filter, apply it to cached constructor
if (mFilter != null) {
// Have we seen this name before?
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
// New class -- remember whether it is allowed
clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
mContext.getClassLoader()).asSubclass(View.class);
boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
mFilterMap.put(name, allowed);
if (!allowed) {
failNotAllowed(name, prefix, viewContext, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, viewContext, attrs);
}
}
}
Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
Object[] args = mConstructorArgs;
args[1] = attrs;
try {
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
} finally {
mConstructorArgs[0] = lastContext;
}
} catch (Exception e) {
...
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
這個(gè)方法可以看到View是怎么創(chuàng)建出來(lái)的，用類的全限定名拿到class信息，有一個(gè)sConstructorMap緩存類的constructor，如果能拿到有效的構(gòu)造器就不再重復(fù)創(chuàng)建來(lái)提升效率，如果沒(méi)有緩存的構(gòu)造器，就反射得到構(gòu)造器并添加到sConstructorMap中以便后面使用。這里有個(gè)mFilter來(lái)提供自定義選項(xiàng)，用戶可以自定義哪些類不允許構(gòu)造。
拿到構(gòu)造器之后，實(shí)際上newInstance是調(diào)用了兩View個(gè)參數(shù)的構(gòu)造方法。第一個(gè)參數(shù)是Context，第二個(gè)參數(shù)是attrs，這樣我們就得到了需要加載的View。
這里可以結(jié)合LayoutInflater.Factory2一起來(lái)看，Activity實(shí)際上是實(shí)現(xiàn)了LayoutInflater.Factory2接口的：
//Activity.java
public View onCreateView(@NonNull String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
return null;
}
所以我們可以直接在Activity里面重寫onCreateView方法，這樣就可以根據(jù)View的名字來(lái)實(shí)現(xiàn)我們的一些操作，比如換膚的操作，比如定義一個(gè)名字來(lái)表示某種自定義View。可以看這樣一個(gè)用法：
<PlaceHolder
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
android:text="include LinearLayout"
android:textColor="#fff"
android:textSize="15sp" />
然后我們?cè)谥貙懙?code>onCreateView里面判斷name：
@Override
public View onCreateView(String name, Context context, AttributeSet attrs) {
if ("PlaceHolder".equals(name)) {
return new TextView(this, attrs);
}
return super.onCreateView(name, context, attrs);
}
這樣其實(shí)就不拘泥于名字可以自己創(chuàng)建對(duì)應(yīng)的View，這樣其實(shí)可以用在多個(gè)module依賴的時(shí)候，如果在moduleA中得不到moduleB的某個(gè)自定義View，可以使用一個(gè)這樣的方式來(lái)在moudleA中暫時(shí)的用來(lái)做一個(gè)占位標(biāo)記，在moduleB中做一個(gè)判斷。
同樣的，通過(guò)上面的代碼我們知道LayoutInflater是通過(guò)反射拿到構(gòu)造方法來(lái)創(chuàng)建View的，那眾所周知反射是有性能損耗的，那么我們可以在onCreateView方法中判斷名字直接new出來(lái)，當(dāng)然也可以跟AppcompatActivity里面做的一樣，做一些兼容的操作來(lái)替換成不同版本的View:
public final View createView(View parent, final String name, @NonNull Context context,
View view = null;
switch (name) {
case "TextView":
view = new AppCompatTextView(context, attrs);
break;
case "ImageView":
view = new AppCompatImageView(context, attrs);
break;
case "Button":
view = new AppCompatButton(context, attrs);
break;
case "EditText":
view = new AppCompatEditText(context, attrs);
break;
case "Spinner":
view = new AppCompatSpinner(context, attrs);
break;
case "ImageButton":
view = new AppCompatImageButton(context, attrs);
break;
...
}
...
return view;
}
還沒(méi)有展開(kāi)說(shuō)rinflate，篇幅限制，放到另外一篇文章中去分析，rinflate源碼分析。
流程圖如下：
總結(jié)：
- LayoutInfalter的作用是把XML轉(zhuǎn)化成對(duì)應(yīng)的View對(duì)象，需要用
Activity#getLayoutInflater()或者getSystemService獲取 - 加載時(shí)先判斷是否是merge標(biāo)簽，merge標(biāo)簽走遞歸方法rinflate，否則走createViewFromTag
- createViewFromTag作用是根據(jù)xml標(biāo)簽的名字去加載對(duì)應(yīng)的View，使用的是反射的方法
- LayoutInflater.Factory2是設(shè)計(jì)出來(lái)靈活構(gòu)造View的接口，可以用來(lái)實(shí)現(xiàn)換膚或者替換View的功能，同時(shí)也是AppcompatActivity用來(lái)做兼容和版本替換的接口
