LayoutInflater的工作原理，从解析XML布局文件到创建Java对象，再到构建View树

LayoutInflater在Android中是一个非常重要的组件，主要负责将XML布局文件实例化为对应的View对象。LayoutInflater是一个抽象类，不能直接通过new的方式获取其实例，需要通过Activity.getLayoutInflater()或Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)来获取与当前Context已经关联且正确配置的标准LayoutInflater。

在实际工作中，有时会根据情况在代码中自定义控件或者加载布局文件，这就需要用到LayoutInflater。它的作用是用来获得布局文件View对象的。例如，在BaseAdapter的getView方法中，LayoutInflater经常被用来获取整个View并返回。

View itemView= LayoutInflater.from(context).inflate(R.layout.layout_list_item,container,false);

通过LayoutInflater.from静态函数获得一个LayoutInflater实例，其实是个PhoneLayoutInflater对象：

public static LayoutInflater from(Context context) {
    LayoutInflater LayoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    if (LayoutInflater == null) {
        throw new AssertionError("LayoutInflater not found.");
    }
    return LayoutInflater;
}

LayoutInflater服务

LAYOUT_INFLATER_SERVICE服务跟AMS、WMS等服务不同，完全是APP虚拟的一个服务，主要作用是：在本地为调用者创建PhoneLayoutInflater对象，ContextImpl在注册这个“服务”的时候，将工作委托给PolicyManager，利用makeNewLayoutInflater构建LayoutInflater。

registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() {
    public Object createService(ContextImpl ctx) {
        return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());
    }});
    
public static LayoutInflater makeNewLayoutInflater(Context context) {
    return sPolicy.makeNewLayoutInflater(context);
}

PolicyManager进一步调用com.android.internal.policy.impl.Policy对象的makeNewLayoutInflater构建PhoneLayoutInflater。

private static final String POLICY_IMPL_CLASS_NAME =
"com.android.internal.policy.impl.Policy";

public LayoutInflater makeNewLayoutInflater(Context context) {
    return new PhoneLayoutInflater(context);
}

图片

构建View树

LayoutInflater源码中按照上面的流程来构建View，同时添加了些特殊标签的处理逻辑，比如merge、include、stubview等。

public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
    XmlResourceParser parser = getContext().getResources().getLayout(resource);
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

XmlResourceParser是包含了XML文件信息的一个对象，通过XmlResourceParser将TAG信息取出，递归创建View。

public XmlResourceParser getLayout(int id) throws NotFoundException {
    return loadXmlResourceParser(id, "layout");
}

XmlResourceParser loadXmlResourceParser(int id, String type)
        throws NotFoundException {
    synchronized (mAccessLock) {
        TypedValue value = mTmpValue;
        
        if (value == null) {
            mTmpValue = value = new TypedValue();
        }
        
        getValue(id, value, true);
        
        if (value.type == TypedValue.TYPE_STRING) {
            return loadXmlResourceParser(value.string.toString(), id,
                    value.assetCookie, type);
        }
    }
}

TypedValue是与xml定义的资源对应的值，getValue获取对应xml资源：

public void getValue(int id, TypedValue outValue, boolean resolveRefs)
        throws NotFoundException {
    boolean found = mAssets.getResourceValue(id, 0, outValue, resolveRefs);
}

mAssets是一个AssetManager对象：

final boolean getResourceValue(int ident,int density, TypedValue outValue, boolean resolveRefs) {
   
    int block = loadResourceValue(ident, (short) density, outValue, resolveRefs);
    if (block >= 0) {
        if (outValue.type != TypedValue.TYPE_STRING) {
            return true;
        }
        outValue.string = mStringBlocks[block].get(outValue.data);
        return true; 
    }  
    return false;  
}

AssetManager通过native函数加载xml文件信息：

static jint android_content_AssetManager_loadResourceValue(JNIEnv* env, jobject clazz, jint ident,jshort density,jobject outValue,jboolean resolve){
    ...
    AssetManager* am = assetManagerForJavaObject(env, clazz);
    
    const ResTable& res(am->getResources());
    Res_value value;
    ResTable_config config;
    uint32_t typeSpecFlags;
    
    ssize_t block = res.getResource(ident, &value, false, density, &typeSpecFlags, &config);
   ...
    uint32_t ref = ident;
    if (resolve) {
    
        block = res.resolveReference(&value, block, &ref, &typeSpecFlags, &config);
    ...
    }
    return block >= 0 ? copyValue(env, outValue, &res, value, ref, block, typeSpecFlags, &config) : block;
}

res.getResource并不是是每次都加载一遍，第一次加载后就能获得单例ResTable，后面用的都是这个缓存，只不过ResTable不会缓存全部资源，对于布局、图像资源等，缓存的都是引用，如果是真实资源的引用话，还需要通过res.resolveReference来解析真正的资源。

const ResTable* AssetManager::getResTable(bool required) const{

    
    ResTable* rt = mResources;
    if (rt) {  return rt;   }
   ...
    const size_t N = mAssetPaths.size();
    for (size_t i=0; i
        const asset_path& ap = mAssetPaths.itemAt(i);
        Asset* idmap = openIdmapLocked(ap);
        
        if (ap.type != kFileTypeDirectory) {   
                if (i == 0) {
                  
                sharedRes = const_castmZipSet.getZipResourceTable(ap.path);
            }
            if (sharedRes == NULL) {
                ass = const_cast(this)->mZipSet.getZipResourceTableAsset(ap.path);
                if (ass == NULL) {
                
                    ass = const_cast(this)->openNonAssetInPathLocked("resources.arsc",  Asset::ACCESS_BUFFER,  ap);
                    if (ass != NULL && ass != kExcludedAsset) {
                        ass = const_cast(this)->mZipSet.setZipResourceTableAsset(ap.path, ass);
                    }}
                if (i == 0 && ass != NULL) {
                    
                    sharedRes = new ResTable();
                    sharedRes->add(ass, (void*)(i+1), false, idmap);
                    sharedRes = const_cast(this)->mZipSet.setZipResourceTable(ap.path, sharedRes);
                } } } 
        ...         
        if ((ass != NULL || sharedRes != NULL) && ass != kExcludedAsset) {
            if (rt == NULL) {
                mResources = rt = new ResTable();
                updateResourceParamsLocked();
            }
            if (sharedRes != NULL) {
                rt->add(sharedRes);
            } else {
                rt->add(ass, (void*)(i+1), !shared, idmap);
            }  }  .. }
    return rt;
}

通过上面的操作，完成了resources.arsc文件的解析，获得了一个ResTable对象，该对象包含了应用程序的全部资源信息（动态加载的先不考虑），之后就可以通过ResTable的getResource来获得指定资源，而对于xml布局文件，这里获得的就是一个引用，需要res.resolveReference二次解析，之后就得到了id对应的资源项。xml布局文件对应的资源项的值是一个字符串，其实是一个布局文件路径，指向一个经过编译的二进制格式保存的xml资源文件。有了这个Xml资源文件的路径之后，会再次通过loadXmlResourceParser来对该Xml资源文件进行解析，从而得到布局文件解析对象XmlResourceParser。

XmlResourceParser loadXmlResourceParser(String file, int id,
    int assetCookie, String type) throws NotFoundException {
    if (id != 0) {
        try {...
                  
            XmlBlock block = mAssets.openXmlBlockAsset(assetCookie, file);
            if (block != null) {
                int pos = mLastCachedXmlBlockIndex+1;
                if (pos >= num) pos = 0;
                mLastCachedXmlBlockIndex = pos;
                XmlBlock oldBlock = mCachedXmlBlocks[pos];
                if (oldBlock != null) {
                    oldBlock.close();
                }
                
                mCachedXmlBlockIds[pos] = id;
                mCachedXmlBlocks[pos] = block;
                
                return block.newParser();
         ...

返回XmlResourceParser对象，进而来实例化各种View：

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
        final AttributeSet attrs = Xml.asAttributeSet(parser);
        Context lastContext = (Context)mConstructorArgs[0];
        mConstructorArgs[0] = mContext;
        View result = root;
        try {
            int type;
            final String name = parser.getName();
            
            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, attrs, false);
            } else {
                View temp;
                if (TAG_1995.equals(name)) {
                    temp = new BlinkLayout(mContext, attrs);
                } else {
                
                    temp = createViewFromTag(root, name, attrs);
                }
                ViewGroup.LayoutParams params = null;
                if (root != null) {
                    
                    params = root.generateLayoutParams(attrs);
                    if (!attachToRoot) {
                        temp.setLayoutParams(params);
                    }
                }
                
                rInflate(parser, temp, attrs, true);
                    
                if (root != null && attachToRoot) {
                    root.addView(temp, params);
                }
                
                if (root == null || !attachToRoot) {
                    result = temp;
                }
            }

        } 
        return result; 
    }
}

inflate的主要作用是生成layout的根布局文件，并且根据参数看看是否需要添加container容器中，之后根据需要调用rInflate递归生成子View。

void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
    final int depth = parser.getDepth();
    int type;
    
    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)) {
            parseRequestFocus(parser, parent);
        } else if (TAG_INCLUDE.equals(name)) {
            // inclue标签，不能用在getDepth() == 0
            if (parser.getDepth() == 0) {
                throw new InflateException(" cannot be the root element");
            }
            parseInclude(parser, parent, attrs);
        } else if (TAG_MERGE.equals(name)) {
            
            throw new InflateException(" must be the root element");
        } else if (TAG_1995.equals(name)) {
            final View view = new BlinkLayout(mContext, attrs);
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            rInflate(parser, view, attrs, true);
            viewGroup.addView(view, params);                
        } else {
            
            final View view = createViewFromTag(parent, name, attrs);
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            rInflate(parser, view, attrs, true);
            
            viewGroup.addView(view, params);
        }
    }
    if (finishInflate) parent.onFinishInflate();
}

rInflate主要作用是开启递归遍历，生成View树，createViewFromTag的主要作用是利用反射生成View对象，最终将View数显示到屏幕上。