Activity中setContentView浅析

大家好，又见面了，我是你们的朋友全栈君。

protected void onCreate(final Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

setContentView()方法会将我们的视图设置到哪儿去了？

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

Activity会调用当前Activity的Window的setContentView()方法，而Window类是一个抽象类，唯一实现类PhoneWindow。

PhoneWindwo对象在Activity中的attach()方法中创建。

 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) {
    attachBaseContext(context);

    mFragments.attachHost(null /*parent*/);

    mWindow = new PhoneWindow(this, window);
    mWindow.setWindowControllerCallback(this);
    mWindow.setCallback(this);
    mWindow.setOnWindowDismissedCallback(this);
    mWindow.getLayoutInflater().setPrivateFactory(this);
    //......
}

那我们接着看PhoneWindow的setContentView()方法。

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

当我们第一此setContentView()时，mContentParent为空，会调用installDecor()方法做些初始化工作。然后再将我们setContentView里的View加载到mContentParent上面去。

mLayoutInflater.inflate(layoutResID, mContentParent)

来看看installDecor()做了什么伟大的事情:

private DecorView mDecor;

private ViewGroup mContentParent;

private ViewGroup mContentRoot;

private void installDecor() {
    if (mDecor == null) {
        mDecor = generateDecor();
        mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
        mDecor.setIsRootNamespace(true);
        if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
            mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
        }
    }
    if (mContentParent == null) {
        mContentParent = generateLayout(mDecor);

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

mDecor是DecorView类对象，而DecorView继承自FrameLayout，generateDecor()方法就是初始化创建一个空的FrameLayout。来看generateLayout()方法是怎样初始化我们需要的mContentParent的：

protected ViewGroup generateLayout(DecorView decor) {   
    TypedArray a = getWindowStyle();
    if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
        requestFeature(FEATURE_NO_TITLE);
    } else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
        requestFeature(FEATURE_ACTION_BAR);
    }

    if (a.getBoolean(R.styleable.Window_windowActionBarOverlay, false)) {
        requestFeature(FEATURE_ACTION_BAR_OVERLAY);
    }
    //....
    if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
        layoutResource = R.layout.screen_swipe_dismiss;
    } else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
        if (mIsFloating) {
            TypedValue res = new TypedValue();
            getContext().getTheme().resolveAttribute(
                    R.attr.dialogTitleIconsDecorLayout, res, true);
            layoutResource = res.resourceId;
        } else {
            layoutResource = R.layout.screen_title_icons;
        }
        removeFeature(FEATURE_ACTION_BAR);
    } else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
            && (features & (1 << FEATURE_ACTION_BAR)) == 0) {
        layoutResource = R.layout.screen_progress;

    } else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {}
    //.....
    int layoutResource;
    //.....
    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;
 }

generateLayout()方法会根据我们Acivity主题样式，选择加载不同的系统布局资源，并将该视图添加到DecorView中去。

View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
mContentRoot = (ViewGroup) in;

无论选择哪一个布局资源里面都一个id名为content的FrameLayout。再从该布局中找到我们需要的contentParent。

(ViewGroup)findViewById(ID_ANDROID_CONTENT);

最后返回contentParent，将setContentView()中的视图加载到mContentParent上去。