LayoutInflater源码分析

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24K男
2018.05.03 16:17 字数 1501

在《(-)Android中的单例模式》分析中,我们分析了Android中单例模式的实现,且以LayoutInflater为实例,本博文就带大家来认识下我们常用的LayoutInflater源码。

1. setContentView

首先来看我们平时给Activity设置布局的姿势:

setContentView(R.layout.activity_main)

那么R.layout.activity_main是怎么呈现在屏幕上的呢?中间过程与LayoutInflater的有关系吗?
带着这个疑问,我们继续前行分析。

1.1 PhoneWindow.setContentView

继续上面的分析,我们往下看:

Activity.setContentView

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

    public Window getWindow() {
        return mWindow;// 这是个什么鬼
    }

我们可以看到,我们调用的Activity.setContentView实际上调用了mWindow.setContentView方法。

那么这个mWindow是何方大神呢?从Activity的attach方法中我们可以看到,mWindow实际上是一个PhoneWindow实例。


    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*/);

        // 这个就是我们使用的mWindow
        mWindow = new PhoneWindow(this, window);

        //....
    }

接下来我们接着看PhoneWindow.setContentView方法。
PhoneWindow.setContentView

    @Override
    public void setContentView(int layoutResID) {
        if (mContentParent == null) {
            // 1. 首次进入,加载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 {
            // 2.将我们设置的布局文件,添加到mContentParent
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

通过注释2处的代码:mLayoutInflater.inflate(layoutResID, mContentParent)成功将我们添加的布局文件加入到mContentParent,并显示在屏幕上。

那么到这还会产生下面的疑问:

  1. installDecor方法在做什么操作?
  2. mContentParent是什么?
  3. 为什么调用LayoutInflater.infalte就能呈现我们设置的布局文件?
    不要着急,我们一个个来分析。

1.2 PhoneWindow.installDecor

installDecor方法实在是太长,我们只挑出核心代码来分析。


    private void installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
            // 1. 生成mDecor
            mDecor = generateDecor(-1);
            //....
        } else {
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
            // 2. 生成mContentParent,我们设置的布局文件就是添加到了mContentParent
            mContentParent = generateLayout(mDecor);
            // 3. 查找ActionBar的气息地
            final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                    R.id.decor_content_parent);

            if (decorContentParent != null) {
                // 设置ActioBar相关
                mDecorContentParent = decorContentParent;
                mDecorContentParent.setWindowCallback(getCallback());
                if (mDecorContentParent.getTitle() == null) {
                    mDecorContentParent.setWindowTitle(mTitle);
                }
                //...
                invalidatePanelMenu(FEATURE_ACTION_BAR);
                }
            } else {
                // 没有ActionBar,则查找设置Title
                mTitleView = (TextView) 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);
                    }
                }
            }

            //...
           
    }

installDecor主要干了下面这几件事:

  1. 初始化DecorView(现在你可能还不知道DecorView是个什么鬼,下面就会分析)。
  2. 初始化mContentLayout(还记得么,我们设置的布局就是添加到了它里面)。

1.3 PhoneWindow.generateDecor

直接上代码:

    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());
    }

generateDecor方法的实现很简单,就是根据设置构造了一个DecorView实例。

我们来看看DecorView到底是个什么鬼?:


public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks {
    DecorView(Context context, int featureId, PhoneWindow window,
            WindowManager.LayoutParams params) {
        super(context);
        mFeatureId = featureId;

        //...

        updateAvailableWidth();

        setWindow(window);

        updateLogTag(params);

        mResizeShadowSize = context.getResources().getDimensionPixelSize(
                R.dimen.resize_shadow_size);
        initResizingPaints();
    }
}

现在你知道了,DecorView是一个FrameLayout,看起来很普通的样子。

1.4 PhoneWindow.generateLayout

直接上源码(作了精简)


protected ViewGroup generateLayout(DecorView decor) {
        // 1. 设置当前的主题,我们常用的setFlag和requestWindowFeature都要在setContentView之前调用,否则就无效了
        TypedArray a = getWindowStyle();
        mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);
        int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)
                & (~getForcedWindowFlags());
        if (mIsFloating) {
            setLayout(WRAP_CONTENT, WRAP_CONTENT);
            setFlags(0, flagsToUpdate);
        } else {
            setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);
        }
        // .... 一大堆设置feature的代码

        // Inflate the window decor.
        // 2. 填充window decor
        // 会根据布局和设置,加载不同的布局文件(带ActionBar的和不带Actionbar的)
        int layoutResource; // 这个就是选择的布局文件
        int features = getLocalFeatures();
        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) {
            if (mIsFloating) {
                TypedValue res = new TypedValue();
                getContext().getTheme().resolveAttribute(
                        R.attr.dialogCustomTitleDecorLayout, res, true);
                layoutResource = res.resourceId;
            } else {
                layoutResource = R.layout.screen_custom_title;
            }
            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;
            }
        } else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
            layoutResource = R.layout.screen_simple_overlay_action_mode;
        } else {
            layoutResource = R.layout.screen_simple;
        }

        // 3. 将选择的布局问添加到DecorView中
        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

        // 4. 从布局文件中查找R.id.content的ViewGroup并返回
        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
        if (contentParent == null) {
            throw new RuntimeException("Window couldn't find content container view");
        }

        // ....
        return contentParent;
    }

generateLayout主要做了下面这几件事:

  1. 根据设置和主题等选择合适的布局文件,并将布局添加到DecorVew中。
  2. 从选择布局文件中查找id为R.id.content(我们给Activity设置的布局原来添加到了它里面)的ViewGroup并返回。

将选择的布局添加到DecorView.
DecorView.onResourcesLoaded

    void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
        mStackId = getStackId();

        mDecorCaptionView = createDecorCaptionView(inflater);
        // 1. 解析布局文件
        final View root = inflater.inflate(layoutResource, null);
        if (mDecorCaptionView != null) {
            if (mDecorCaptionView.getParent() == null) {
                // 添加到DecorView中
                addView(mDecorCaptionView,
                        new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
            }
            mDecorCaptionView.addView(root,
                    new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
        } else {
            // 添加到DecorView中
            addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        }
        mContentRoot = (ViewGroup) root;
        initializeElevation();
    }

到现在我们似乎产生了新的疑问,R.id.content从哪来呢?从系统为我们选择的布局文件中加载。
在注释2处,我们看到系统经过各种判断,会为我们选择一个合适的布局文件,我们简单分析两个布局文件。

(1)R.layout.screen_title.xml

<?xml version="1.0" encoding="utf-8"?>

<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>
    // 你要的R.id.content在这里
    <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>


(2)R.layout.screen_simple


<?xml version="1.0" encoding="utf-8"?>

<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" />
    
    // 你要的R.id.content在这里
    <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>

现在你应该明白了R.id.content从何而来。

1.5 Activity视图结构

Activity
|
|---PhoneWindow
|
|---DecorView(FrameLayout)
|
|---LinearLayout(系统为我们选择的布局文件根)
|
|---Actionbar
|
|---R.id.content(FrameLaout)
|
|---R.layout.main(我们设置的布局文件)

我们使用的Activity视图结构大概就是这个结构,根据不同的主题和设置可能会有所不同。

分析到现在,我们只是了解了大概的视图结构,视图是如何解析的呢?往下接着看。

2. LayoutInflater

在setContentView方法中,我们使用mLayoutInflater.inflate(layoutResID, mContentParent)来将我们自定义的布局加载到mContentParent中,似乎视图的显示和解析都与LayoutInflater.inflate有关。

2.1 inflate

我们常用的inflate方法源码


    /**
     * Inflate a new view hierarchy from the specified xml resource. Throws
     * {@link InflateException} if there is an error.
     * 
     * @param resource ID for an XML layout resource to load (e.g.,
     *        <code>R.layout.main_page</code>)
     * @param root Optional view to be the parent of the generated hierarchy.
     * @return The root View of the inflated hierarchy. If root was supplied,
     *         this is the root View; otherwise it is the root of the inflated
     *         XML file.
     */
    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
        return inflate(resource, root, root != null);
    }

    /**
     * Inflate a new view hierarchy from the specified xml resource. Throws
     * {@link InflateException} if there is an error.
     * 
     * @param resource ID for an XML layout resource to load (e.g.,
     *        <code>R.layout.main_page</code>)
     * @param root Optional view to be the parent of the generated hierarchy (if
     *        <em>attachToRoot</em> is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if <em>attachToRoot</em> is false.)
     * @param attachToRoot Whether the inflated hierarchy should be attached to
     *        the root parameter? If false, root is only used to create the
     *        correct subclass of LayoutParams for the root view in the XML.
     * @return The root View of the inflated hierarchy. If root was supplied and
     *         attachToRoot is true, this is root; otherwise it is the root of
     *         the inflated XML file.
     */
    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();
        }
    }

上面两个方法是我们经常使用的方法,我们会根据加载的需求决定参数,但这两个方法都不是最终的实现,我们继续看最终实现的代码。

2.2 inflate(parser,root, attachToRoot)



    /**
     * Inflate a new view hierarchy from the specified XML node. Throws
     * {@link InflateException} if there is an error.
     * <p>
     * <em><strong>Important</strong></em>&nbsp;&nbsp;&nbsp;For performance
     * reasons, view inflation relies heavily on pre-processing of XML files
     * that is done at build time. Therefore, it is not currently possible to
     * use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
     * 
     * @param parser XML dom node containing the description of the view
     *        hierarchy.
     * @param root Optional view to be the parent of the generated hierarchy (if
     *        <em>attachToRoot</em> is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if <em>attachToRoot</em> is false.)
     * @param attachToRoot Whether the inflated hierarchy should be attached to
     *        the root parameter? If false, root is only used to create the
     *        correct subclass of LayoutParams for the root view in the XML.
     * @return The root View of the inflated hierarchy. If root was supplied and
     *         attachToRoot is true, this is root; otherwise it is the root of
     *         the inflated XML file.
     */
    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            View result = root;

            try {
                // 1. 从布局文件中查找根节点
                int type;
                while ((type = parser.next()) != XmlPullParser.START_TAG &&
                        type != XmlPullParser.END_DOCUMENT) {
                    // Empty
                }

                // 没有找到根节点
                if (type != XmlPullParser.START_TAG) {
                    throw new InflateException(parser.getPositionDescription()
                            + ": No start tag found!");
                }

                // 获取根节点名字
                final String name = parser.getName();
                // 2. 如果根节点为merge
                if (TAG_MERGE.equals(name)) {
                    // root为null,或者attachToRoot为false,直接抛出异常
                    if (root == null || !attachToRoot) {
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }
                    // 3. 填充布局
                    rInflate(parser, root, inflaterContext, attrs, false);
                } else {
                    // 4. 根据查找的根节点信息,构造布局文件的根view
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);
                    ViewGroup.LayoutParams params = null;
                    // root参数不为空,xml布局文件中的设置参数才有效
                    if (root != null) {
                        // 在root的基础上,创建布局参数
                        params = root.generateLayoutParams(attrs);
                        // 如果我们不附加到root上,我们就为temp设置布局参数
                        if (!attachToRoot) {
                            temp.setLayoutParams(params);
                        }
                    }
                    // 5.填充xml文件根view下的所有子view
                    rInflateChildren(parser, temp, attrs, true);

                    // 6. 如果root不为空,且attachToRoot为true,则将解析的view直接添加到root中
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }

                    // 7. 如果root不为空,且attachToRoot为true,则返回结果为root,否则返回我们从xml中解析的view
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }

            } catch (XmlPullParserException e) {
                final InflateException ie = new InflateException(e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } catch (Exception e) {
                final InflateException ie = new InflateException(parser.getPositionDescription()
                        + ": " + e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } finally {
                // Don't retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;
            }
            return result;
        }
    }

从上面的分析中,我们得出如下内容:

  1. 我们返回的view受参数root和attachToRoot控制,root不为空且attachToRoot为true的情况下,返回值为root,否则就返回我们从xml解析的view。
  2. xml中根view布局属性是否有效受参数root,root为空的情况下属性无效,否则有效。

上面的源码中涉及到几个方法:

  1. rInflate:xml根为merge调用。
  2. createViewFromTag:根据view名称构造view.
  3. rInflateChildren:加载所有的子View。

2.3 rInflate


    void rInflate(XmlPullParser parser, View parent, Context context,
        AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

        final int depth = parser.getDepth();
        int type;

        // 解析merge标签下的子view信息
        while (((type = parser.next()) != XmlPullParser.END_TAG ||
                parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {

            if (type != XmlPullParser.START_TAG) {
                continue;
            }

            final String name = parser.getName();
            
            if (TAG_REQUEST_FOCUS.equals(name)) {
                // view.requestFocus()
                parseRequestFocus(parser, parent);
            } else if (TAG_TAG.equals(name)) {
                // view.setTag(key, value);
                parseViewTag(parser, parent, attrs);
            } else if (TAG_INCLUDE.equals(name)) {
                // include不能作为根元素
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, context, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                // merge必须为根标签
                throw new InflateException("<merge /> must be the root element");
            } else {
                // 这段代码是不是似曾相识,我们后面会分析createViewFromTag与rInflateChildren方法
                final View view = createViewFromTag(parent, name, context, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflateChildren(parser, view, attrs, true);
                // 将解析出的view添加到parent中(仔细看代码,这个parent就是inflate方法中的root)
                viewGroup.addView(view, params);
            }
        }

        if (finishInflate) {
            parent.onFinishInflate();
        }
    }

rInflate方法是针对merge方法设计的,实现也比较简单,在此不做赘述。

2.4 createViewFromTag


    private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs){
        return createViewFromTag(parent, name, context, attrs, false);
    }

    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
        if (name.equals("view")) {
            name = attrs.getAttributeValue(null, "class");
        }

        // ....

        //TAG_1995 = "blink";
        if (name.equals(TAG_1995)) {
            // Let's party like it's 1995!
            return new BlinkLayout(context, attrs);
        }

        try {
            View view;
            // mFactory2和mFactory都为空,所以这段代码获取到的view为null
            // 为什么为空?因为构造函数中压根没初始化,也没外部set
            if (mFactory2 != null) {
                view = mFactory2.onCreateView(parent, name, context, attrs);
            } else if (mFactory != null) {
                view = mFactory.onCreateView(name, context, attrs);
            } else {
                view = null;
            }

            if (view == null && mPrivateFactory != null) {
                view = mPrivateFactory.onCreateView(parent, name, context, attrs);
            }

            if (view == null) {
                // 获取LayoutInflater持有的context
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    // 判断名字中是否包含.,包含.证明其为自定义view,不包含.证明其为系统自带view
                    if (-1 == name.indexOf('.')) {
                        // 1. 调用方法构造内置view
                        view = onCreateView(parent, name, attrs);
                    } else {
                        // 2. 调用方法构造自定义view
                        view = createView(name, null, attrs);
                    }
                } finally {
                    mConstructorArgs[0] = lastContext;
                }
            }

            return view;
        } catch (InflateException e) {
            throw e;
        } catch (ClassNotFoundException e) {
            final InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;

        } catch (Exception e) {
            final InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        }
    }

2.4.1 onCreateView

onCreateView(View parent, String name, AttributeSet attrs)方法用于构造系统内置view。

LayoutInflater.onCreateView

    protected View onCreateView(View parent, String name, AttributeSet attrs)
            throws ClassNotFoundException {
        return onCreateView(name, attrs);
    }

    @Override 
    protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {
        private static final String[] sClassPrefixList = {
        "android.widget.",
        "android.webkit.",
        "android.app."
        };

        for (String prefix : sClassPrefixList) {
            try {
                View view = createView(name, prefix, attrs);
                if (view != null) {
                    return view;
                }
            } catch (ClassNotFoundException e) {
            }
        }
        return super.onCreateView(name, attrs);
    }

    protected View onCreateView(String name, AttributeSet attrs)
            throws ClassNotFoundException {
        return createView(name, "android.view.", attrs);
    }

代码最终还是调用了createView来创建view。
在创建view时,会尝试在view名称前添加前缀(其实为包名,这样就能通过反射来创建实例了)来构造view。

2.4.2 createView

public final View createView(String name, String prefix, AttributeSet attrs)
            throws ClassNotFoundException, InflateException {
        // 关于参数prefix的说明:
        // 自定义view解析时prefix为空,内置view解析时prefix不为空
        
        // 1. 查找缓存的构造方法
        Constructor<? extends View> constructor = sConstructorMap.get(name);
        if (constructor != null && !verifyClassLoader(constructor)) {
            constructor = null;
            sConstructorMap.remove(name);
        }

         // 2. 反射获取构造方法
        Class<? extends View> clazz = null;
        try {
            if (constructor == null) {
                // 检查其是不是真的view(是否继承了view)
                clazz = mContext.getClassLoader().loadClass(
                        prefix != null ? (prefix + name) : name).asSubclass(View.class);
                
                if (mFilter != null && clazz != null) {
                    boolean allowed = mFilter.onLoadClass(clazz);
                    if (!allowed) {
                        failNotAllowed(name, prefix, attrs);
                    }
                }
                constructor = clazz.getConstructor(mConstructorSignature);
                constructor.setAccessible(true);
                sConstructorMap.put(name, constructor);
            } else {
                if (mFilter != null) {
                    Boolean allowedState = mFilterMap.get(name);
                    if (allowedState == null) {
                         // 检查其是不是真的view(是否继承了view)
                        clazz = mContext.getClassLoader().loadClass(
                                prefix != null ? (prefix + name) : name).asSubclass(View.class);
                        
                        boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                        mFilterMap.put(name, allowed);
                        if (!allowed) {
                            failNotAllowed(name, prefix, attrs);
                        }
                    } else if (allowedState.equals(Boolean.FALSE)) {
                        failNotAllowed(name, prefix, attrs);
                    }
                }
            }

            Object[] args = mConstructorArgs;
            args[1] = attrs;
            // 3. 反射构造view实例
            final View view = constructor.newInstance(args);
            if (view instanceof ViewStub) {
                final ViewStub viewStub = (ViewStub) view;
                viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
            }
            // 4. 返回view
            return view;
        } catch (NoSuchMethodException e) {
            final InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;

        } catch (ClassCastException e) {
            // If loaded class is not a View subclass
            final InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } catch (ClassNotFoundException e) {
            // If loadClass fails, we should propagate the exception.
            throw e;
        } catch (Exception e) {
            final InflateException ie = new InflateException(
                    attrs.getPositionDescription() + ": Error inflating class "
                            + (clazz == null ? "<unknown>" : clazz.getName()), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }


代码的实现比较简单,核心内容如下:

  1. 根据类名和前缀构造类的完整路径,验证类是否继承了View。
  2. 反射获取类的构造方法。
  3. 反射构造类实例并返回。

分析到现在,你应该已经明白view是如何从xml被构造出来的了。

2.5 rInflateChildren(parser, temp, attrs, true);

我们回到inflate方法,分析下根view下的子view是如何解析的。


    final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
            boolean finishInflate) throws XmlPullParserException, IOException {
        rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
    }


rInflateChildren又调用了我们前面分析过的rInflate方法,一步步解析子view并添加到根view中。

分析到这里,现在整个布局的解析你应该了然于胸了,LayoutInflater的代码结构是比较清晰明确的。

3. 总结

LayoutInflater在我们日常开中,还是比较常见的,特别是对于inflate方法中几个参数的使用和理解,笔者曾经就在面试中被问到inflate的使用方法有几种,通过源码的分析,相信我们对LayoutInflater会有一个更深入的理解。

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