Android開發的同學們對setContentView肯定都不陌生，但凡寫到Activity，都離不開這個函式，今天我們就來看看它內部的實現吧！
備註：本文基於Android 8.1.0版本。
1、Activity 與 AppCompatActivity的區別
當我們在老版本Android SDK開發的時候新建的Project的預設繼承的是Activity，而在5.0之後預設繼承的就是AppCompatActivity。二者的區別從AppCompatActivity的註釋中可窺一斑。
/**
* Base class for activities that use the
* <a href="{@docRoot}tools/extras/support-library.html">support library</a> action bar features.
*
* <p>You can add an {@link android.support.v7.app.ActionBar} to your activity when running on API level 7 or higher
* by extending this class for your activity and setting the activity theme to
* {@link android.support.v7.appcompat.R.style#Theme_AppCompat Theme.AppCompat} or a similar theme.
*
* <div class="special reference">
* <h3>Developer Guides</h3>
*
* <p>For information about how to use the action bar, including how to add action items, navigation
* modes and more, read the <a href="{@docRoot}guide/topics/ui/actionbar.html">Action
* Bar</a> API guide.</p>
* </div>
*/
複製程式碼翻譯過來就是AppCompatActivity是所有使用了Support包中 ActionBar特性的Activity的父類。
關係可以這麼形容：AppCompatActivity————>FragmentActivity————>Activity。
2、setContentView
AppCompatActivity中的setContentView也非常簡潔，可以看出來需要去代理類中繼續檢視程式碼。
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);
}
複製程式碼
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}
// 真正到了這裡
private static AppCompatDelegate create(Context context, Window window,
AppCompatCallback callback) {
if (Build.VERSION.SDK_INT >= 24) {
return new AppCompatDelegateImplN(context, window, callback);
} else if (Build.VERSION.SDK_INT >= 23) {
return new AppCompatDelegateImplV23(context, window, callback);
} else if (Build.VERSION.SDK_INT >= 14) {
return new AppCompatDelegateImplV14(context, window, callback);
} else if (Build.VERSION.SDK_INT >= 11) {
return new AppCompatDelegateImplV11(context, window, callback);
} else {
return new AppCompatDelegateImplV9(context, window, callback);
}
}
複製程式碼而代理類實現的setContentView是在AppCompatDelegateImplV9中實現的：
@Override
public void setContentView(View v) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
contentParent.addView(v);
mOriginalWindowCallback.onContentChanged();
}
複製程式碼3、createSubDecor
setContentView的第一步就是確保SubDecor被install，下面原始碼中有註釋
// 此處可以看出SubDecor是一個ViewGroup
private ViewGroup createSubDecor() {
TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);
if (!a.hasValue(R.styleable.AppCompatTheme_windowActionBar)) {
a.recycle();
// 這個錯大家可能遇到過，當使用了AppCompatActivity但是沒有設定一個Theme.AppCompat的主題，則會報這個Exception。
throw new IllegalStateException(
"You need to use a Theme.AppCompat theme (or descendant) with this activity.");
}
// 接下來就到了設定一些Window屬性的地方，下面會再說
if (a.getBoolean(R.styleable.AppCompatTheme_windowNoTitle, false)) {
requestWindowFeature(Window.FEATURE_NO_TITLE);// 設定無title
} else if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBar, false)) {
// Don`t allow an action bar if there is no title.
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBarOverlay, false)) {
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionModeOverlay, false)) {
requestWindowFeature(FEATURE_ACTION_MODE_OVERLAY);
}
mIsFloating = a.getBoolean(R.styleable.AppCompatTheme_android_windowIsFloating, false);
a.recycle();
// Now let`s make sure that the Window has installed its decor by retrieving it
mWindow.getDecorView();// 就是建立DecorView
final LayoutInflater inflater = LayoutInflater.from(mContext);
ViewGroup subDecor = null;
// 根據標記來決定inflate哪個layout
if (!mWindowNoTitle) {
if (mIsFloating) {
// If we`re floating, inflate the dialog title decor
subDecor = (ViewGroup) inflater.inflate(
R.layout.abc_dialog_title_material, null);
// Floating windows can never have an action bar, reset the flags
mHasActionBar = mOverlayActionBar = false;
......
} else if (mHasActionBar) {
......
}
} else {
}
if (subDecor == null) {
throw new IllegalArgumentException(
"AppCompat does not support the current theme features: { "
+ "windowActionBar: " + mHasActionBar
+ ", windowActionBarOverlay: "+ mOverlayActionBar
+ ", android:windowIsFloating: " + mIsFloating
+ ", windowActionModeOverlay: " + mOverlayActionMode
+ ", windowNoTitle: " + mWindowNoTitle
+ " }");
}
if (mDecorContentParent == null) {
mTitleView = (TextView) subDecor.findViewById(R.id.title);
}
// Make the decor optionally fit system windows, like the window`s decor
ViewUtils.makeOptionalFitsSystemWindows(subDecor);
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
R.id.action_bar_activity_content);
// 獲取PhoneWindow中的content佈局物件
final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
if (windowContentView != null) {
// There might be Views already added to the Window`s content view so we need to
// migrate them to our content view
while (windowContentView.getChildCount() > 0) {
final View child = windowContentView.getChildAt(0);
windowContentView.removeViewAt(0);
contentView.addView(child);
}
// Change our content FrameLayout to use the android.R.id.content id.
// Useful for fragments.
windowContentView.setId(View.NO_ID);
// 將contentView的id設定為android.R.id.content
contentView.setId(android.R.id.content);
// The decorContent may have a foreground drawable set (windowContentOverlay).
// Remove this as we handle it ourselves
if (windowContentView instanceof FrameLayout) {
((FrameLayout) windowContentView).setForeground(null);
}
}
// Now set the Window`s content view with the decor
mWindow.setContentView(subDecor);
contentView.setAttachListener(new ContentFrameLayout.OnAttachListener() {
@Override
public void onAttachedFromWindow() {}
@Override
public void onDetachedFromWindow() {
dismissPopups();
}
});
return subDecor;
}
複製程式碼3.1 requestWindowFeature
@Override
public boolean requestWindowFeature(int featureId) {
......
switch (featureId) {
case FEATURE_SUPPORT_ACTION_BAR:
throwFeatureRequestIfSubDecorInstalled();
mHasActionBar = true;// 僅僅是對變數賦值
return true;
......
}
return mWindow.requestFeature(featureId);
}
// 這個又解釋了一個原因，我們如果在setContentView之後再次去設定requestWindowFeature，會丟擲Exception。
private void throwFeatureRequestIfSubDecorInstalled() {
if (mSubDecorInstalled) {
throw new AndroidRuntimeException(
"Window feature must be requested before adding content");
}
}
複製程式碼3.2 mWindow.getDecorView()
各位小夥伴應該都知道Android裡的Window這個類的實現子類其實是PhoneWindow，所以我們直接取PhoneWindow中去查getDecorView這個函式。最終會走到這裡，注意下面兩個標註了重點的地方
private void installDecor() {
mForceDecorInstall = false;
if (mDecor == null) {
mDecor = generateDecor(-1);// 重點
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
} else {
mDecor.setWindow(this);
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);// 重點
......
}
}
// generateDecor最後只是new了一個DecorView
protected DecorView generateDecor(int featureId) {
......
return new DecorView(context, featureId, this, getAttributes());
}
// 看一下DecorView的定義可以看出它是一個FrameLayout
/** @hide */
public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks {
}
複製程式碼generateLayout函式過多，此處不貼出程式碼，值只分析下過程：
- 設定一些Window的屬性；
- 根據Window屬性選擇一個layoutResource，這些layoutResource有一個共性是都有一個@android:id/content的佈局，因為在AppCompatDelegateImplV9的createSubDecor函式裡會用到這個content；
- 選出layoutResource之後會進入一句關鍵的程式碼：mDecor.onResourcesLoaded(mLayoutInflater, layoutResource)；layoutResource就被inflate出來並且新增到DecorView中了。備註，新增View的時候使用的LayoutParams是MATCH_PARENT；
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
mStackId = getStackId();
if (mBackdropFrameRenderer != null) {
loadBackgroundDrawablesIfNeeded();
mBackdropFrameRenderer.onResourcesLoaded(
this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
getCurrentColor(mNavigationColorViewState));
}
mDecorCaptionView = createDecorCaptionView(inflater);
final View root = inflater.inflate(layoutResource, null);// inflate出View
if (mDecorCaptionView != null) {
if (mDecorCaptionView.getParent() == null) {
addView(mDecorCaptionView,
new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
mDecorCaptionView.addView(root,
new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
} else {
// Put it below the color views.
addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
mContentRoot = (ViewGroup) root;
initializeElevation();
}
複製程式碼3.3 再回到createSubDecor
此時就開始建立真正的subDecor了，也有四個可選的layout，根據之前設定的屬性來選擇，然後去inflate出來。
// SubDecor中也一定有這個id
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
R.id.action_bar_activity_content);
// 這裡的content就是是PhoneWindow中的content，上面提到過
final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
if (windowContentView != null) {
// There might be Views already added to the Window`s content view so we need to
// migrate them to our content view
// 合併PhoneWindow中的view到SubDecor中的content中
while (windowContentView.getChildCount() > 0) {
final View child = windowContentView.getChildAt(0);
windowContentView.removeViewAt(0);
contentView.addView(child);
}
// Change our content FrameLayout to use the android.R.id.content id.
// Useful for fragments.
// id在這裡發生了變化
windowContentView.setId(View.NO_ID);
contentView.setId(android.R.id.content);
// The decorContent may have a foreground drawable set (windowContentOverlay).
// Remove this as we handle it ourselves
if (windowContentView instanceof FrameLayout) {
((FrameLayout) windowContentView).setForeground(null);
}
}
複製程式碼3.4 mWindow.setContentView
開始設定PhoneWindow的contentView，再把程式碼切到PhoneWindow中
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
// mContentParent是不是看起來有點熟悉呢？generateLayout函式的返回值就是它
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
view.setLayoutParams(params);
final Scene newScene = new Scene(mContentParent, view);
transitionTo(newScene);// 有過渡動畫的情況下用到了Scene
} else {
// 備註，mContentParent之前是@android:id/content，現在是View.NO_ID；
// 現在的@android:id/content是SubDecor中的action_bar_activity_content
mContentParent.addView(view, params);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
複製程式碼備註：到了這裡，SubDecor 已經被新增到了PhoneWindow中，並且@android:id/content是SubDecor中的action_bar_activity_content。接下來別的操作是關於細節的設定。
4. 再回到setContentView
@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();
}
複製程式碼此時我們可以看出setContentView中最複雜的程式碼就是ensureSubDecor，接下來的程式碼就只是使用SubDecor中的content，將我們傳入的layout inflate出來然後加進去。
5、總結
setContentView的過程就是通過PhoneWindow建立DecorView，然後建立SubDecor，最終將傳遞進來的佈局add進來。
這樣大家也更容易明白為什麼通過一些效能分析工具檢視佈局層次及數量的時候總是比我們自己寫的Layout多，也更容易明白對Activity設定View的函式被命名為setContentView。
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