View 源码分析——setContentView

原创

CatEatFish

修改于 2020-07-09 06:21:20

5440

文章被收录于专栏：干活分享干活分享

分析一下 android 中布局的加载流程，每次新建 activity 时都要在 onCreate 中调用 setContentView(R.layout.activity_main);调用这个方法具体做了哪些事情呢？

分析onCreate中的setContentView：

// mainActivity中
public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        }

跳转setContentView()方法,

需要关注的地方有两个：

1、 installDecor(); 用来初始化DecorView

2、 mLayoutInflater.inflate(layoutResID, mContentParent); 将自己的布局添加到mContentParent（mContentParent是什么？等会解释）

// PhoneWindow.java 中
  public void setContentView(int layoutResID) {
       // 刚初始化 mContentParent ==null
        if (mContentParent == null) {
            // 初始化DecorView
            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 {
            // 此处inflate自己布局并且添加到mContentParent布局中
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
      //  .....省略不重要的
    }

看看installDecor()方法中做了什么

1、创建 DecorView

2、构建 mContentParent

  // PhoneWindow中
private void installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
            // 1、创建 DecorView
            mDecor = generateDecor(-1);
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        } else {
          //如果创建了 则与当前的 window 对象绑定
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
            //  2、构建 mContentParent 
            mContentParent = generateLayout(mDecor);
         }
    }

具体是如何创建 mContentParent 呢？

protected ViewGroup generateLayout(DecorView decor) {

        // 系统资源id
        int layoutResource;
        int features = getLocalFeatures();

        // 这些 if else 判断是为了选择加载系统哪个布局的（只截取了一部分）
        if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
            layoutResource = R.layout.screen_swipe_dismiss;
            setCloseOnSwipeEnabled(true);
        } 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;
            }
            // XXX Remove this once action bar supports these features.
            removeFeature(FEATURE_ACTION_BAR);
            // System.out.println("Title Icons!");
        } else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
                && (features & (1 << FEATURE_ACTION_BAR)) == 0) {
            // Special case for a window with only a progress bar (and title).
            // XXX Need to have a no-title version of embedded windows.
            layoutResource = R.layout.screen_progress;
            // System.out.println("Progress!");
        } else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
            // Special case for a window with a custom title.
            // If the window is floating, we need a dialog layout
            if (mIsFloating) { .......}

        // mDecor要改变的标记位
        mDecor.startChanging();
        // 它将 layoutResource 布局文件解析成 View 添加到了 DecorView 之中
        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
        // findViewById 找到添加的布局作为父布局contentParent 
       ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
       ......
      // 将此 contentParent 返回 
     return contentParent;
  }

下面看几个R.layout.screen_simple的布局

//layoutResource = R.layout.screen_title_icons;

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:fitsSystemWindows="true"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <!-- Popout bar for action modes -->
    <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"/>
    <RelativeLayout android:id="@android:id/title_container"
        style="?android:attr/windowTitleBackgroundStyle"
        android:layout_width="match_parent"
        android:layout_height="?android:attr/windowTitleSize">
        <!-- The title background has 9px left padding. -->
        <ImageView android:id="@android:id/left_icon"
            android:visibility="gone"
            android:layout_marginEnd="9dip"
            android:layout_width="16dip"
            android:layout_height="16dip"
            android:scaleType="fitCenter"
            android:layout_alignParentStart="true"
            android:layout_centerVertical="true" />
        <ProgressBar android:id="@+id/progress_circular"
            style="?android:attr/progressBarStyleSmallTitle"
            android:visibility="gone"
            android:max="10000"
            android:layout_centerVertical="true"
            android:layout_alignParentEnd="true"
            android:layout_marginStart="6dip"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
        <!-- There are 6dip between this and the circular progress on the right, we
             also make 6dip (with the -3dip margin_left) to the icon on the left or
             the screen left edge if no icon. This also places our left edge 3dip to
             the left of the title text left edge. -->
        <ProgressBar android:id="@+id/progress_horizontal"
            style="?android:attr/progressBarStyleHorizontal"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginStart="-3dip"
            android:layout_toStartOf="@android:id/progress_circular"
            android:layout_toEndOf="@android:id/left_icon"
            android:layout_centerVertical="true"
            android:visibility="gone"
            android:max="10000" />
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="horizontal"
            android:layout_toStartOf="@id/progress_circular"
            android:layout_toEndOf="@android:id/left_icon"
            >
            <TextView android:id="@android:id/title"
                style="?android:attr/windowTitleStyle"
                android:layout_width="0dip"
                android:layout_height="match_parent"
                android:layout_weight="1"
                android:background="@null"
                android:fadingEdge="horizontal"
                android:scrollHorizontally="true"
                android:gravity="center_vertical"
                android:layout_marginEnd="2dip"
                />
            <!-- 2dip between the icon and the title text, if icon is present. -->
            <ImageView android:id="@android:id/right_icon"
                android:visibility="gone"
                android:layout_width="16dip"
                android:layout_height="16dip"
                android:layout_weight="0"
                android:layout_gravity="center_vertical"
                android:scaleType="fitCenter"
                />
            </LinearLayout>
    </RelativeLayout>
    <FrameLayout android:id="@android:id/content"
        android:layout_width="match_parent"
        android:layout_height="0dip"
        android:layout_weight="1"
        android:foregroundGravity="fill_horizontal|top"
        android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>


// layoutResource = R.layout.screen_title;
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:fitsSystemWindows="true">
    <!-- Popout bar for action modes -->
    <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:layout_width="match_parent" 
        android:layout_height="?android:attr/windowTitleSize"
        style="?android:attr/windowTitleBackgroundStyle">
        <TextView android:id="@android:id/title" 
            style="?android:attr/windowTitleStyle"
            android:background="@null"
            android:fadingEdge="horizontal"
            android:gravity="center_vertical"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </FrameLayout>
    <FrameLayout android:id="@android:id/content"
        android:layout_width="match_parent" 
        android:layout_height="0dip"
        android:layout_weight="1"
        android:foregroundGravity="fill_horizontal|top"
        android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

// 可以看出每个布局中都有一个被LinearLayout 包裹的 id为 id="@android:id/content" 的FrameLayout 控件