Android窗口機制（二）Window，PhoneWindow，DecorView，setContentView源碼理解

時間 2019-11-12

標籤 android 窗口機制 window phonewindow decorview setcontentview 源碼理解欄目 Android 简体版

原文原文鏈接

跟你們講解Window，PhoneWindow，DecorView他們的理解以及他們之間的聯繫android

Window

咱們來看下源碼裏面的說明app

/**
 * Abstract base class for a top-level window look and behavior policy.  An
 * instance of this class should be used as the top-level view added to the
 * window manager. It provides standard UI policies such as a background, title
 * area, default key processing, etc.
 *
 * <p>The only existing implementation of this abstract class is
 * android.view.PhoneWindow, which you should instantiate when needing a
 * Window.
 */
public abstract class Window {
    ...
    @Nullable
    public View findViewById(@IdRes int id) {
        return getDecorView().findViewById(id);
    }

/**
 * Convenience for * {@link #setContentView(View, android.view.ViewGroup.LayoutParams)}
 * to set the screen content from a layout resource.  The resource will be * inflated, adding all top-level views to the screen. * * @param layoutResID Resource ID to be inflated.
 * @see #setContentView(View, android.view.ViewGroup.LayoutParams)
 */
     public abstract void setContentView(@LayoutRes int layoutResID);
     ...
}

一個頂級窗口查看和行爲的一個抽象基類。這個類的實例做爲一個頂級View添加到Window Manager。它提供了一套標準的UI方法，好比添加背景，標題等等。當你須要用到Window的時候，你應該使用它的惟一實現類PhoneWindow。能夠看到，Window是一個抽象基類，它提供了一系列窗口的方法，好比設置背景，標題等等，而它的惟一實現類則是PhoneWindowide

PhoneWindow

Window的惟一實現類佈局

public class PhoneWindow extends Window implements MenuBuilder.Callback {

    private final static String TAG = "PhoneWindow";

    ...

    // This is the top-level view of the window, containing the window decor.
    private DecorView mDecor;

    // This is the view in which the window contents are placed. It is either
    // mDecor itself, or a child of mDecor where the contents go.
    private ViewGroup mContentParent;

    private ViewGroup mContentRoot;
    ...
}

能夠看到，在PhoneWindow裏面，出現了成員變量DecorView的而這裏，DecorView則是PhoneWindow裏面的一個內部類，它是繼承與FrameLayoutpost

private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {

        /* package */int mDefaultOpacity = PixelFormat.OPAQUE;

        /** The feature ID of the panel, or -1 if this is the application's DecorView */
        private final int mFeatureId;

        private final Rect mDrawingBounds = new Rect();

        private final Rect mBackgroundPadding = new Rect();

        private final Rect mFramePadding = new Rect();

        private final Rect mFrameOffsets = new Rect();
        ....
 }

既然是FrameLayout，也就能夠加載佈局文件，也就是說，咱們那些標題欄，內容欄，頂級上看是加載在DecorView上的。而DecorView則是由PhoneWindow負責添加動畫

二者關係以及setContentView源碼流程

接下咱們就從一個常見的方法中去認知他們之間的關係，那就是activity裏面的setContentView，就是咱們日常把佈局內容顯示到界面上的一個方法。點擊activity.setContentView時ui

public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

裏面方法調用了getWindow().setContentView，而這個getWindow方法獲取的就是Activity上的Windowthis

/**
     * Retrieve the current {@link android.view.Window} for the activity.
     * This can be used to directly access parts of the Window API that
     * are not available through Activity/Screen.
     *
     * @return Window The current window, or null if the activity is not
     *         visual.
     */
    public Window getWindow() {
        return mWindow;
    }

能夠看到若是當前mWindow爲null的話，則表示當前Activity不在窗口上，這裏的mWindow.setContentView，實際上調用到的是它的實現類方法phoneWindow.setContentViewspa

@Override
    public void setContentView(int layoutResID) {
        // 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.
        if (mContentParent == null) {
            //建立DecorView，並添加到mContentParent上
            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 {
            //將要加載的資源添加到mContentParent上
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            //回調通知表示完成界面加載
            cb.onContentChanged();
        }
    }

若是當前內容還未放置到窗口，此時mContentParent==null,也就是第一次調用的時候，調用那個installDecor方法。FEATURE_CONTENT_TRANSITIONS，則是標記當前內容加載有沒有使用過分動畫，也就是轉場動畫。若是內容已經加載過，而且不須要動畫，則會調用removeAllViews。添加完Content後若有設置了FEATURE_CONTENT_TRANSITIONS則添加Scene來過分啓動。不然mLayoutInflater.inflate(layoutResID, mContentParent);將咱們的資源文件經過LayoutInflater對象轉換爲View樹，而且添加至mContentParent視圖中。既然是第一次啓動則會調用到installDecor，從字面上看能夠知道該方法用來添加DecorView，看下里面說明.net

private void installDecor() {
        if (mDecor == null) {
        //調用該方法建立new一個DecorView
            mDecor = generateDecor();
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        }
        //一開始DecorView未加載到mContentParent，因此此時mContentParent=null
        if (mContentParent == null) {
        //該方法將mDecorView添加到Window上綁定佈局
            mContentParent = generateLayout(mDecor);

            // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
            mDecor.makeOptionalFitsSystemWindows();

            final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                    R.id.decor_content_parent);
                
                ...//添加其餘資源
                ...//設置轉場動畫
        }
    }

能夠看到該方法，先經過吊桶generateDecor建立DecorView

protected DecorView generateDecor() {
        return new DecorView(getContext(), -1);
    }

建立完後再經過調用generateLayout將setContentView的內容賦值到mContentParent，這個方法有點長，咱們看下

protected ViewGroup generateLayout(DecorView decor) {
        // Apply data from current theme.
        //根據當前設置的主題來加載默認佈局
        TypedArray a = getWindowStyle();
        //若是你在theme中設置了window_windowNoTitle，則這裏會調用到，其餘方法同理，
        //這裏是根據你在theme中的設置去設置的
        if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
            requestFeature(FEATURE_NO_TITLE);
        } else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
            // Don't allow an action bar if there is no title.
            requestFeature(FEATURE_ACTION_BAR);
        }
        //是否有設置全屏
        if (a.getBoolean(R.styleable.Window_windowFullscreen, false)) {
            setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN & (~getForcedWindowFlags()));
        }
        
        ...//省略其餘加載資源
        
        // 添加布局到DecorView，前面說到，DecorView是繼承與FrameLayout，它自己也是一個ViewGroup，而咱們前面建立它的時候，只是調用了new DecorView，此時裏面並沒有什麼東西。而下面的步奏則是根據用戶設置的Feature來建立相應的默認佈局主題。舉個例子，若是我在setContentView以前調用了requestWindowFeature(Window.FEATURE_NO_TITLE)，這裏則會經過getLocalFeatures來獲取你設置的feature，進而選擇加載對應的佈局，此時則是加載沒有標題欄的主題，對應的就是R.layout.screen_simple

        int layoutResource;
        int features = getLocalFeatures();
        // System.out.println("Features: 0x" + Integer.toHexString(features));
        if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
            layoutResource = R.layout.screen_swipe_dismiss;
        } ... //省略其餘判斷方法
        } else {
            // Embedded, so no decoration is needed.
            layoutResource = R.layout.screen_simple;
            // System.out.println("Simple!");
        }

        mDecor.startChanging();
        //選擇對應佈局建立添加到DecorView中
        View in = mLayoutInflater.inflate(layoutResource, null);
        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        mContentRoot = (ViewGroup) in;
        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
        ...
        return contentParent;
    }

首先generateLayout會根據當前用戶設置的主題去設置對應的Feature，接着，根據對應的Feature來選擇加載對應的佈局文件，(Window.FEATURE_NO_TITLE)接下來經過getLocalFeatures來獲取你設置的feature，進而選擇加載對應的佈局，這也就是爲何咱們要在setContentView以前調用requesetFeature的緣由。此時則是加載沒有標題欄的主題，對應的就是R.layout.screen_simple，咱們看下里面的佈局

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    android:orientation="vertical">
    <ViewStub android:id="@+id/action_mode_bar_stub"
              android:inflatedId="@+id/action_mode_bar"
              android:layout="@layout/action_mode_bar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:theme="?attr/actionBarTheme" />
    <FrameLayout
         android:id="@android:id/content"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:foregroundInsidePadding="false"
         android:foregroundGravity="fill_horizontal|top"
         android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

能夠看到是LinearLayout裏面包含了兩個，由於設置可NoTitle，因此上面只有一個ViewStub，不然還有一個FrameLayout。也證實前面第一篇中說的，「DecorView只有一個子元素爲LinearLayout。表明整個Window界面，包含通知欄，標題欄，內容顯示欄三塊區域。」注意FrameLayout裏面的id，@android:id/content ，咱們setContentView的內容就是添加到這個FrameLayout中。

generateLayout的返回是contentParent，而它的獲取則是ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);`

public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;

正是id爲content的FrameLayout。以後咱們setContentView則是添加在mContentParent上面了。回到前面剛開始的方法

@Override
    public void setContentView(int layoutResID) {
        // 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.
        if (mContentParent == null) {
            //建立DecorView，並添加到mContentParent上
            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 {
            //將要加載的資源添加到mContentParent上
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            //回調通知表示完成界面改變
            cb.onContentChanged();
        }
    }

此時已經建立完DecorView而且獲取到mContentParent，接着就是將你setContentView的內容添加到mContentParent中，也就是

mLayoutInflater.inflate(layoutResID, mContentParent);
 或者
 mContentParent.addView(view, params);

最後調用Callback來通知界面發生改變。Callback是Window裏面的一個接口，裏面聲明瞭當界面更改觸摸時調用的各類方法。這裏的話，咱們看下onContentChanged，在PhoneWindow裏面並無看到onContentChanged的實現類，而咱們又知道Activity自己又是加載在Window上的，咱們看下Activity

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback { ... }

能夠看到Activity裏面實現了Window.Callback接口而裏面onContentChanged則是空的，也就是咱們能夠經過重寫該方法來監聽佈局內容的改變了