Activity、Window、View三者之间的联系

Activity类：Android四大组件之一，是开发者最常用的一个组件

Window类：是一个抽象类，具有窗口管理的功能，实现类为PhoneWindow

View类：提供对View的操作，包括绘制测量等等

 

他们三个之间的关系便是Activity类通过Window组装View对象，然后把组装出来的对象交给系统去绘制

 

接下来我们从代码的角度分析一下这个过程

 

首先，我们在写Activity的时候，会重写系统提供的onCreate方法，然后调用setContentView来绑定页面布局

 

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.hello_word);
}

setContentView是Activity内定义的一个方法，但是主要功能，缺是由Window类来实现

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

getWindow这个函数返回来一个mWindow对象，这个mWindow对象是在attach函数中被初始化

final void attach(Context context, ActivityThread aThread,
        Instrumentation instr, IBinder token, int ident,
        Application application, Intent intent, ActivityInfo info,
        CharSequence title, Activity parent, String id,
        NonConfigurationInstances lastNonConfigurationInstances,
        Configuration config, String referrer, IVoiceInteractor voiceInteractor,
        Window window, ActivityConfigCallback activityConfigCallback) {
         //此处省略N行代码
        mWindow = new PhoneWindow(this, window, activityConfigCallback);
         //此处省略N行代码
} 

由此可知，实现类便是PhoneWindow，那么我们就可以跳转到PhoneWindow类中的setContentView

@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) {
        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);
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
    mContentParentExplicitlySet = true;
}

PhoneWindow内的setContentView会做两件事

一、安装DecorView

二、把Activity传递进来的resId加载进DecorView的content内

 

tips：

 

DecorView：是我们Android系统的屏幕所呈现的视图，包括上边状态栏，中心的内容区域，以及底部导航栏

 

Android内的DecorView是复用的，每次开启新的Activity的时候，会从上一个Activity处来获取之前的DecorView

新的Activity拿到这个DecorView后，会把content内的内容移除掉，然后补上自己的content，这样就实现了页面的切换。

 

接下来我分析一下安装DecorView的过程，由上可知，是通过installDecor函数触发的操作

private void installDecor() {
    mForceDecorInstall = false;
    if (mDecor == null) {
        mDecor = generateDecor(-1);
        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);

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

        final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                R.id.decor_content_parent);

        if (decorContentParent != null) {
            mDecorContentParent = decorContentParent;
            mDecorContentParent.setWindowCallback(getCallback());
            if (mDecorContentParent.getTitle() == null) {
                mDecorContentParent.setWindowTitle(mTitle);
            }

            final int localFeatures = getLocalFeatures();
            for (int i = 0; i < FEATURE_MAX; i++) {
                if ((localFeatures & (1 << i)) != 0) {
                    mDecorContentParent.initFeature(i);
                }
            }

            mDecorContentParent.setUiOptions(mUiOptions);

            if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) != 0 ||
                    (mIconRes != 0 && !mDecorContentParent.hasIcon())) {
                mDecorContentParent.setIcon(mIconRes);
            } else if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) == 0 &&
                    mIconRes == 0 && !mDecorContentParent.hasIcon()) {
                mDecorContentParent.setIcon(
                        getContext().getPackageManager().getDefaultActivityIcon());
                mResourcesSetFlags |= FLAG_RESOURCE_SET_ICON_FALLBACK;
            }
            if ((mResourcesSetFlags & FLAG_RESOURCE_SET_LOGO) != 0 ||
                    (mLogoRes != 0 && !mDecorContentParent.hasLogo())) {
                mDecorContentParent.setLogo(mLogoRes);
            }

            // Invalidate if the panel menu hasn't been created before this.
            // Panel menu invalidation is deferred avoiding application onCreateOptionsMenu
            // being called in the middle of onCreate or similar.
            // A pending invalidation will typically be resolved before the posted message
            // would run normally in order to satisfy instance state restoration.
            PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
            if (!isDestroyed() && (st == null || st.menu == null) && !mIsStartingWindow) {
                invalidatePanelMenu(FEATURE_ACTION_BAR);
            }
        } else {
            mTitleView = findViewById(R.id.title);
            if (mTitleView != null) {
                if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
                    final View titleContainer = findViewById(R.id.title_container);
                    if (titleContainer != null) {
                        titleContainer.setVisibility(View.GONE);
                    } else {
                        mTitleView.setVisibility(View.GONE);
                    }
                    mContentParent.setForeground(null);
                } else {
                    mTitleView.setText(mTitle);
                }
            }
        }

        if (mDecor.getBackground() == null && mBackgroundFallbackResource != 0) {
            mDecor.setBackgroundFallback(mBackgroundFallbackResource);
        }

        // Only inflate or create a new TransitionManager if the caller hasn't
        // already set a custom one.
        if (hasFeature(FEATURE_ACTIVITY_TRANSITIONS)) {
            if (mTransitionManager == null) {
                final int transitionRes = getWindowStyle().getResourceId(
                        R.styleable.Window_windowContentTransitionManager,
                        0);
                if (transitionRes != 0) {
                    final TransitionInflater inflater = TransitionInflater.from(getContext());
                    mTransitionManager = inflater.inflateTransitionManager(transitionRes,
                            mContentParent);
                } else {
                    mTransitionManager = new TransitionManager();
                }
            }

            mEnterTransition = getTransition(mEnterTransition, null,
                    R.styleable.Window_windowEnterTransition);
            mReturnTransition = getTransition(mReturnTransition, USE_DEFAULT_TRANSITION,
                    R.styleable.Window_windowReturnTransition);
            mExitTransition = getTransition(mExitTransition, null,
                    R.styleable.Window_windowExitTransition);
            mReenterTransition = getTransition(mReenterTransition, USE_DEFAULT_TRANSITION,
                    R.styleable.Window_windowReenterTransition);
            mSharedElementEnterTransition = getTransition(mSharedElementEnterTransition, null,
                    R.styleable.Window_windowSharedElementEnterTransition);
            mSharedElementReturnTransition = getTransition(mSharedElementReturnTransition,
                    USE_DEFAULT_TRANSITION,
                    R.styleable.Window_windowSharedElementReturnTransition);
            mSharedElementExitTransition = getTransition(mSharedElementExitTransition, null,
                    R.styleable.Window_windowSharedElementExitTransition);
            mSharedElementReenterTransition = getTransition(mSharedElementReenterTransition,
                    USE_DEFAULT_TRANSITION,
                    R.styleable.Window_windowSharedElementReenterTransition);
            if (mAllowEnterTransitionOverlap == null) {
                mAllowEnterTransitionOverlap = getWindowStyle().getBoolean(
                        R.styleable.Window_windowAllowEnterTransitionOverlap, true);
            }
            if (mAllowReturnTransitionOverlap == null) {
                mAllowReturnTransitionOverlap = getWindowStyle().getBoolean(
                        R.styleable.Window_windowAllowReturnTransitionOverlap, true);
            }
            if (mBackgroundFadeDurationMillis < 0) {
                mBackgroundFadeDurationMillis = getWindowStyle().getInteger(
                        R.styleable.Window_windowTransitionBackgroundFadeDuration,
                        DEFAULT_BACKGROUND_FADE_DURATION_MS);
            }
            if (mSharedElementsUseOverlay == null) {
                mSharedElementsUseOverlay = getWindowStyle().getBoolean(
                        R.styleable.Window_windowSharedElementsUseOverlay, true);
            }
        }
    }
}

这个函数很长，但是并不复杂

1.先通过generateDecor函数生成了一个mDecor对象，设置一些基础参数后便是初始化完成

2.在通过generateLayout函数，根据mDecor生成了mContentParent这个ViewGroup，就是所谓的内容区域，换句话说就是显示我们Activity内布局的地方

3.最后在根据系统的预装样式做一些修饰，便完成了DecorView的初始化

 

好了，我们思路继续回到setContentView下

@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) {
        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);
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
    mContentParentExplicitlySet = true;
}

咱们通常程序内的Activity在启动的时候，DecorView是从LauncherActivity下传递过来的，所以就不用再执行install过程了，而是执行内容区域的清空view操作，然后通过inflate函数把Activity的resId加载进mContentParent对象内。

然后触发requestApplyInsets函数进行重绘刷新屏幕

最后触发onContentChanged监听，这个Callback接口在Activity内有实现

 

最后在总结一下本篇的内容

Activity内包含了Window对象，很多功能也是借助Window对象得以实现

Window对象下维护了一个DecorView

如果有错误或者疑问，请在评论区指出，我会即使更正，共同学习，共同进步，谢谢支持！