Android布局填充器--深入LayoutInflater一探究竟

文章目录

• Android源码--深入LayoutInflater一探究竟（基于[android](https://so.csdn.net/so/search?q=android&spm=1001.2101.3001.7020)-12.0.0_r34分析）

前文：

单例设计模式在Android开发实际应用场景解析–activity的管理

https://blog.csdn.net/weixin_46039528/article/details/132287718?spm=1001.2014.3001.5501

Android源码设计模式–单例模式分析，系统服务开机注册单例模式源码解析

https://blog.csdn.net/weixin_46039528/article/details/132309733?spm=1001.2014.3001.5501

LayoutInflater 是一个抽象类，找到它具体的实现类

@SystemService(Context.LAYOUT_INFLATER_SERVICE)public abstract class LayoutInflater {......}

系统开机注册

registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,                new CachedServiceFetcher<LayoutInflater>() {            @Override            public LayoutInflater createService(ContextImpl ctx) {                return new PhoneLayoutInflater(ctx.getOuterContext());}});

跟进PhoneLayoutInflater，这里看到PhoneLayoutInflater是LayoutInflater的一个实现类。

public class PhoneLayoutInflater extends LayoutInflater {    //TextView 的完整路径是 android.widget.TextView,一些view的前缀字符串    private static final String[] sClassPrefixList = {        "android.widget.",        "android.webkit.",        "android.app."    };......        @Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {    //在View名字的前面添加前缀来构造 View 的完整路径，android.widget.TextView        for (String prefix : sClassPrefixList) {            try {                View view = createView(name, prefix, attrs);                if (view != null) {                    return view;                }            } catch (ClassNotFoundException e) {                // In this case we want to let the base class take a crack                // at it.            }        }......}

核心就是onCreateView通过传进来的名字，name，prefix前缀构造出对应的View对象，比如TextView。

接下来我们康康AppCompatActivity的setContentView，它的核心就是通过LayoutInflater来加载我们的布局。AppCompatActivity 的 setContentView 方法和 Activity的 setContentView 方法是有区别的。因为我们都是继承AppCompatActivity开发较多，此处只对AppCompatActivity分析。

@Overridepublic void setContentView(@LayoutRes int layoutResID) {    initViewTreeOwners();    //getDelegate返回的是一个对象。    getDelegate().setContentView(layoutResID);}

跟进getDelegate，看到返回的是AppCompatDelegate，这是一个抽象类。

@NonNullpublic AppCompatDelegate getDelegate() {    if (mDelegate == null) {        mDelegate = AppCompatDelegate.create(this, this);    }    return mDelegate;}

我们来看 AppCompatDelegate 的 create(Activity activity, AppCompatCallback callback) 方法，

        @NonNull    public static AppCompatDelegate create(@NonNull Dialog dialog,            @Nullable AppCompatCallback callback) {        //AppCompatDelegateImpl是它的一个实现类，就是它调用了setContentView        return new AppCompatDelegateImpl(dialog, callback);    }

跟进到AppCompatDelegateImpl中的，有三个重载方法

  ......    @Override    public void setContentView(int resId) {        //初始化DecorView        ensureSubDecor();        //设置setContentView 要的父布局        ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);        //加载之前移除所有的布局        contentParent.removeAllViews();        //完成布局的加载        LayoutInflater.from(mContext).inflate(resId, contentParent);        mAppCompatWindowCallback.getWrapped().onContentChanged();    }......

我们看到会加载一个系统定义好的布局contentParent，通过 LayoutInflater 的 inflate 函数将指定布局的视图添加到mContentParent 中。

 public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {        return inflate(resource, root, root != null);    } public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {      ......    // 获取xm1资源解析器        XmlResourceParser parser = res.getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }    }// 参数1为xmL 解析器，参数2为要解析布局的父视图，参数3为是否将要解析的视图添加到父视图中 public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {        synchronized (mConstructorArgs) {            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");            final Context inflaterContext = mContext;            final AttributeSet attrs = Xml.asAttributeSet(parser);            Context lastContext = (Context) mConstructorArgs[0];            mConstructorArgs[0] = inflaterContext;            //存储父视图            View result = root;           ......   //解析merge 标签                if (TAG_MERGE.equals(name)) {                    if (root == null || !attachToRoot) {                        throw new InflateException(" 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                    //不是merge 标签那么直接解析布局中的视图                    //这里就是通过 xml的 tag 来解析 layout 根视图                    //name 就是要解析的视图的类名，如 RelativeLayout                    //createViewFromTag会将该元素的parent 及名字传递过来                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);                    ViewGroup.LayoutParams params = null;                    if (root != null) {                        if (DEBUG) {System.out.println("Creating params from root: " +        root);                        }                        // Create layout params that match root, if supplied                        //生成布局参数                        params = root.generateLayoutParams(attrs);                        //如果attachToRoot 为 false，给 temp 设置布局参数                        if (!attachToRoot) {// Set the layout params for temp if we are not// attaching. (If we are, we use addView, below)temp.setLayoutParams(params);                        }                    }                    ......                    // Inflate all children under temp against its context.                    //解析 temp 下的所有子view                    rInflateChildren(parser, temp, attrs, true);             ......                    // We are supposed to attach all the views we found (int temp)                    // to root. Do that now.                    //如果Root 不为空，attachToRoot 为true，将 temp 添加到父视图中                    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.                    //如果root 为空或者attachToRoot 为 false,结果就是 temp                    if (root == null || !attachToRoot) {                        result = temp;                    }                }......            return result;        }    }

大致流程：解xml中的根标签，最外层的xml布局元素，如果根标签是 merge,那么调用rInflate进行解析，rnflate会将 merge 标签下的所有子View直接添加到根标签中，如果标签是普通元素，调用createViewFromTag 对该元素进行解析，调用rInflate解析 temp 根元素下的所有子 View，并且将这些子 View 都添加到temp下。

解析单个元素的createViewFromTag

@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,            boolean ignoreThemeAttr) {        if (name.equals("view")) {            name = attrs.getAttributeValue(null, "class");        }......        try {            //跟进这个方法            View view = tryCreateView(parent, name, context, attrs);            if (view == null) {                final Object lastContext = mConstructorArgs[0];                mConstructorArgs[0] = context;                try {                    //安卓系统自带View控件的创建解析                    if (-1 == name.indexOf('.')) {                        view = onCreateView(context, parent, name, attrs);                    } else {                     //自定义控件创建解析                        view = createView(context, name, null, attrs);                    }                } finally {                    mConstructorArgs[0] = lastContext;                }            }            ......    }

跟进tryCreateView

@UnsupportedAppUsage(trackingBug = 122360734)    @Nullable    public final View tryCreateView(@Nullable View parent, @NonNull String name,        @NonNull Context context,        @NonNull AttributeSet attrs) { ......        View view;        //通过onCreateView创建View        if (mFactory2 != null) {            view = mFactory2.onCreateView(parent, name, context, attrs);        } else if (mFactory != null) {            view = mFactory.onCreateView(name, context, attrs);        } else {            view = null;        }        ......        return view;    }

最终我们的view都是通过createView方法创建的。

//通过反射机制构造 view对象@Nullablepublic final View createView(@NonNull Context viewContext, @NonNull String name,        @Nullable String prefix, @Nullable AttributeSet attrs)        throws ClassNotFoundException, InflateException {   ......    //缓存中获取构造函数    Constructor<? extends View> constructor = sConstructorMap.get(name);    if (constructor != null && !verifyClassLoader(constructor)) {        constructor = null;        sConstructorMap.remove(name);    }    Class<? extends View> clazz = null;    try {        Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);        if (constructor == null) {            // Class not found in the cache, see if it's real, and try to add it            //如果 prefix不为空，那么构造完整的 View 路径加载该类            clazz = Class.forName(prefix != null ? (prefix + name) : name, false,                    mContext.getClassLoader()).asSubclass(View.class);            ......             //从class对象中获取构造函数            constructor = clazz.getConstructor(mConstructorSignature);            constructor.setAccessible(true);            //将构造函数存入缓存中            sConstructorMap.put(name, constructor);        } else {            ......        }......        try {            //通过反射构造View            final View view = constructor.newInstance(args);  ......}

这只是单个View的xml解析，要把子View全部解析完，我们看rInflate。

void rInflate(XmlPullParser parser, View parent, Context context,            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {   //深度优先遍历算法        final int depth = parser.getDepth();        int type;        boolean pendingRequestFocus = false;        while (((type = parser.next()) != XmlPullParser.END_TAG ||                parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {......            } else {                //元素名进行解析                final View view = createViewFromTag(parent, name, context, attrs);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);            //递归调用进行解析                rInflateChildren(parser, view, attrs, true);                viewGroup.addView(view, params);            }        }......    }

来源地址：https://blog.csdn.net/weixin_46039528/article/details/132353252