前言
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
通常情況下,我們創建一個Activity時,會通過setContentView來引入佈局,將界面視圖展現給用戶看見。那麼,爲什麼通過setContentView()就能將佈局文件加載到界面中呢?
一、setContentView()源碼分析
由於版本不同,有繼承Activity的頁面和繼承AppCompatActivity,但原理都一樣,基本都離不開LayoutInflater.from(mContext).inflate(resId, contentParent); 的方式,將佈局解析到DecorView根佈局中。
繼承Activity
SetContentView源碼如下:
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
getWindow()返回的是一個Window實例,調用Window的setContentView方法。我們知道,Window是一個抽象對象,它的具體實現類就是PhoneWindow。那麼,這裏PhoneWindow中的setContentView方法實現如下:
@Override
public void setContentView(int layoutResID) {
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);//解析佈局
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
繼承AppCompatActivity
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);
}
@Override
public void setContentView(View view) {
getDelegate().setContentView(view);
}
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
getDelegate().setContentView(view, params);
}
實際上是通過調用getDelegate().setContentView()。getDelegate()源碼實現如下:
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}
AppCompatDelegate.create()源碼:
private static AppCompatDelegate create(Context context, Window window,
AppCompatCallback callback) {
final int sdk = Build.VERSION.SDK_INT;
if (BuildCompat.isAtLeastN()) {
return new AppCompatDelegateImplN(context, window, callback);
} else if (sdk >= 23) {
return new AppCompatDelegateImplV23(context, window, callback);
} else if (sdk >= 14) {
return new AppCompatDelegateImplV14(context, window, callback);
} else if (sdk >= 11) {
return new AppCompatDelegateImplV11(context, window, callback);
} else {
return new AppCompatDelegateImplV9(context, window, callback);
}
}
AppCompatDelegate是個什麼鬼? 其實這裏利用設計模式中的代理模式,AppCompatDelegate是Activity的委託類,這樣就無法直接繼承和訪問Activity中的方法,提高了Activity的安全性和擴展性。
AppCompatDelegate是個抽象類,裏面定義了抽象方法和普通方法。例如,setContentView()、findViewByid(int)、onCreate()~onDestory()生命週期、onConfigurationChanged()屏幕翻轉、onSaveInstanceState(Bundle)保存實例等。
這裏,我們還得重點看 setContentView() 實現代碼。
@Override
public void setContentView(View v) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
contentParent.addView(v);
mOriginalWindowCallback.onContentChanged();
}
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
@Override
public void setContentView(View v, ViewGroup.LayoutParams lp) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
contentParent.addView(v, lp);
mOriginalWindowCallback.onContentChanged();
}
說到底,主要還是用到了LayoutInflater.inflate() 佈局加載解析方法將Activity中的佈局添加到父佈局中。
這個 LayoutInflater.inflate() 我們並不陌生的,比如RecycleView做列表適配器的時候都會用它加載item的view啊。
二、LayoutInflater.inflate()
inflater.inflate(layoutId, null);
inflater.inflate(layoutId, root,false);
inflater.inflate(layoutId, root,true);
/**
* parser xml解析器
* root 父容器
* attackToRoot 是否加入到父容器中
*/
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
// Context對象
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
// 父視圖
View result = root;
try {
int type;
// 找到root元素
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
}
final String name = parser.getName();
// 解析merge標籤
if (TAG_MERGE.equals(name)) {
// 如果是merge標籤調用新方法,將merge標籤內的元素全部加載到父視圖中
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 通過xml的tag來解析根視圖
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
// 不是merge標籤,直接解析佈局中的視圖
ViewGroup.LayoutParams params = null;
if (root != null) {
// 生成佈局參數
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
temp.setLayoutParams(params);
}
}
// 解析temp視圖下的所有view
rInflateChildren(parser, temp, attrs, true);
// 如果root不爲空並且attachToRoot爲true,將temp加入到父視圖中
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果root爲空 或者 attachToRoot爲false,返回的結果就是temp
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (Exception e) {
throw ie;
} finally {
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
return result;
}
}
上面的inflate方法所做的操作主要有以下幾步:
- 解析xml的根標籤
- 如果根標籤是merge,那麼調用rInflate解析,將merge標籤下的所有子View直接添加到根標籤中
- 如果不是merge,調用createViewFromTag解析該元素
- 調用rInflate解析temp中的子View,並將這些子View添加到temp中
- 通過attachToRoot,返回對應解析的根視圖
我們先看createViewFromTag方法:
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
try {
View view;
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
// 通過.來判斷是自定義View還是內置View
if (-1 == name.indexOf('.')) {//是內置
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
throw e;
} catch (ClassNotFoundException e) {
throw ie;
}
}
三、View如何創建?
上面代碼片段可見,解析View的時候是通過“
.
”來判斷是內置的View還是自定義的View的。所以,在寫佈局文件中使用自定義的View需要完整路徑。可以參考LayoutInflater通過PhoneLayoutInflater創建出來的onCreateView()。
PhoneLayoutInflater類:
public class PhoneLayoutInflater extends LayoutInflater {
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
public PhoneLayoutInflater(Context context) {
super(context);
}
protected PhoneLayoutInflater(LayoutInflater original, Context newContext) {
super(original, newContext);
}
@Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {
for (String prefix : sClassPrefixList) {
try {
View view = createView(name, prefix, attrs);
if (view != null) {
return view;
}
} catch (ClassNotFoundException e) {
}
}
return super.onCreateView(name, attrs);
}
public LayoutInflater cloneInContext(Context newContext) {
return new PhoneLayoutInflater(this, newContext);
}
}
onCreateView
方法。該方法通過將傳遞過來的View前面加上"android.widget.","android.webkit.","android.app."
用來得到該內置View對象的完整路徑,最後根據路徑來創建出對應的View。接下來看createView(name,prefix,attrs)。
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
// 從緩存中獲取view的構造函數
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
// 如果沒有緩存
if (constructor == null) {
// 如果前綴不爲空構造完整的View路徑並加載該類
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
// 獲取該類的構造函數
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
// 將構造函數加入緩存中
sConstructorMap.put(name, constructor);
} else {
}
Object[] args = mConstructorArgs;
args[1] = attrs;
// 通過反射構建View
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;
}
}
createView相對簡單,通過判斷前綴,來構建View的完整路徑,並將該類加載到虛擬機中,獲取構造函數並緩存,再通過構造函數創建該View對象,並返回。這個時候我們就獲得了根視圖。
接着調用rInflateChildren方法解析子View。最終調用的是rInflate方法,代碼如下:
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
// 獲取樹的深度,通過深度優先遍歷
final int depth = parser.getDepth();
int type;
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)) {
parseRequestFocus(parser, parent);
} else if (TAG_TAG.equals(name)) {// 解析tag標籤
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {// 解析include標籤
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {// 解析到merge標籤,並報錯
throw new InflateException("<merge /> must be the root element");
} else {
// 解析到普通的子View,並調用createViewFromTag獲得View對象
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
// 遞歸解析
rInflateChildren(parser, view, attrs, true);
// 將View加入到父視圖中
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}
rInflate方法通過深度優先遍歷的方式來構造視圖樹,當解析到一個View的時候就再次調用rInflate方法,直到將路徑下的最後一個元素,並最終將View加入到父視圖中。