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Activity 是如何加载布局的?

通常我们是这样来为activity设置布局的:

public class MainActivity extends Activity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}
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如果我们继承的是Activity:

setContentView(R.layout.activity_main):

父类调用的是:

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

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接下来我们来看 getWindow()

    public Window getWindow() {
        return mWindow;
    }
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再看:mWindow

    private Window mWindow;
    
    /**
 * 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 {
    ...
}
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从注释我们看到 The only existing implementation of this abstract class is android.view.PhoneWindow PhoneWindow目前是Window的唯一实现类。 我们来看PhoneWindow的setContentView()

 @Override
    public void setContentView(int layoutResID) {
            ...
        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        
        //加载动画
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
      ...
    }

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  • 来看一下 installDecor()
    private void installDecor() {
        mForceDecorInstall = false;
        //首先创建一个Decor 装饰器
        if (mDecor == null) {
            mDecor = generateDecor(-1);
         ...
        } else {
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
        //创建一个ContentParent的布局
            mContentParent = generateLayout(mDecor);
            ...
            }
        }
    }
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这里主要首先生成一个mDecor = generateDecor(-1),mDecor 是一个DecorView

// This is the top-level view of the window, containing the window decor. private DecorView mDecor; 表示DecorView是Window的一个顶层view,或者可以理解为一个根布局。

generateDecor(-1)

  protected DecorView generateDecor(int featureId) {
        Context context;
        if (mUseDecorContext) {
            Context applicationContext = getContext().getApplicationContext();
            if (applicationContext == null) {
                context = getContext();
            } else {
                context = new DecorContext(applicationContext, getContext().getResources());
                if (mTheme != -1) {
                    context.setTheme(mTheme);
                }
            }
        } else {
            context = getContext();
        }
        return new DecorView(context, featureId, this, getAttributes());
    }
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再次我们看

mContentParent = generateLayout(mDecor)

    protected ViewGroup generateLayout(DecorView decor) {
        // Apply data from current theme.
            ...

        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 (a.getBoolean(R.styleable.Window_windowActionModeOverlay, false)) {
            requestFeature(FEATURE_ACTION_MODE_OVERLAY);
        }

        if (a.getBoolean(R.styleable.Window_windowSwipeToDismiss, false)) {
            requestFeature(FEATURE_SWIPE_TO_DISMISS);
        }
        ...
        // Inflate the window decor.

        int layoutResource;
        int features = getLocalFeatures();
       ...
        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) {
           ...
            layoutResource = R.layout.screen_progress;
            // System.out.println("Progress!");
        } else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
        
            if (mIsFloating) {
                TypedValue res = new TypedValue();
                getContext().getTheme().resolveAttribute(
                        R.attr.dialogCustomTitleDecorLayout, res, true);
                layoutResource = res.resourceId;
            } else {
                layoutResource = R.layout.screen_custom_title;
            }
            // XXX Remove this once action bar supports these features.
            removeFeature(FEATURE_ACTION_BAR);
        } else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
          
            if (mIsFloating) {
                TypedValue res = new TypedValue();
                getContext().getTheme().resolveAttribute(
                        R.attr.dialogTitleDecorLayout, res, true);
                layoutResource = res.resourceId;
            } else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
                layoutResource = a.getResourceId(
                        R.styleable.Window_windowActionBarFullscreenDecorLayout,
                        R.layout.screen_action_bar);
            } else {
                layoutResource = R.layout.screen_title;
            }
            // System.out.println("Title!");
        } else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
            layoutResource = R.layout.screen_simple_overlay_action_mode;
        } else {
            // Embedded, so no decoration is needed.
            layoutResource = R.layout.screen_simple;
            // System.out.println("Simple!");
        }
            ...

        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
        if (contentParent == null) {
            throw new RuntimeException("Window couldn't find content container view");
        }

            ...
        // Remaining setup -- of background and title -- that only applies
        // to top-level windows.
        if (getContainer() == null) {
            final Drawable background;
            if (mBackgroundResource != 0) {
                background = getContext().getDrawable(mBackgroundResource);
            } else {
                background = mBackgroundDrawable;
            }
            mDecor.setWindowBackground(background);

            final Drawable frame;
            if (mFrameResource != 0) {
                frame = getContext().getDrawable(mFrameResource);
            } else {
                frame = null;
            }
            mDecor.setWindowFrame(frame);

            mDecor.setElevation(mElevation);
            mDecor.setClipToOutline(mClipToOutline);

            if (mTitle != null) {
                setTitle(mTitle);
            }

            if (mTitleColor == 0) {
                mTitleColor = mTextColor;
            }
            setTitleColor(mTitleColor);
        }

        mDecor.finishChanging();

        return contentParent;
    }
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我们看到首先 有一大堆 requestFeature(); 这是在setContentView之前做的事情,这也是为什么我们自己要去getWindow.requestFeature时必须在setContent之前的原因。

再下来根据不同的属性,加载了一些布局: 我们看一比较常见的layoutResource = R.layout.screen_simple;

<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>
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看一幅图:

这是DecorView默认的一个渲染,然后我门自己的布局都是渲染到她的FrameLayout上的

那么在这里我门现在能够明白,installDector其实实际上是在初始化两个视图容器,然后加载系统的R资源及特征,产生了一个基本布局。

最后我们回到
mLayoutInflater.inflate(layoutResID, mContentParent);

  public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
        if (DEBUG) {
            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                    + Integer.toHexString(resource) + ")");
        }

        final XmlResourceParser parser = res.getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }
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看一下 inflate(parser, root, attachToRoot)

    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
           ...

            try {
               ...

                if (TAG_MERGE.equals(name)) {
                    if (root == null || !attachToRoot) {
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }

                    rInflate(parser, root, inflaterContext, attrs, false);
                } else {
                    // Temp is the root view that was found in the xml
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                    ViewGroup.LayoutParams params = null;

                    if (root != null) {
                       ...
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                        
                            temp.setLayoutParams(params);
                        }
                    }

                   ...
                    rInflateChildren(parser, temp, attrs, true);
                   ...
                    // We are supposed to attach all the views we found (int temp)
                    // to root. Do that now.
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }

                    // Decide whether to return the root that was passed in or the
                    // top view found in xml.
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }

            } 
            ...

            return result;
        }
    }
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看到一个关键点

  1. merge标签对传入的参数有要求
if (TAG_MERGE.equals(name)) {
                    if (root == null || !attachToRoot) {
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }

                    rInflate(parser, root, inflaterContext, attrs, false);
                } 
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如果是merge标签,就要求传入的 root!=null && attachToRoot=true

  1. inflate主要注意几点
  • rInflate(parser, root, inflaterContext, attrs, false); 循环遍历节点view
  • root.generateLayoutParams(attrs); 根据root生成params
  • rInflateChildren(parser, temp, attrs, true); 循环变量子view
  • root.addView(temp, params); 将生成的view添加到root

到此我们的所有布局遍历完成,并添加到root 布局,root布局最终又添加到DectorView的R.id.content的布局里面,加载完毕。

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Android
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