Android xml解析到View的过程

川长思鸟来 2022-05-10 00:36 185阅读 0赞

### 文章目录 ###

*   *  分析 View 系列
     *  源码分析
     *   *  前言，一些必须知道的知识
         *  进入主题
         *  查看 View的创建流程
         *  总结

## 分析 View 系列 ##

[https://blog.csdn.net/qq\_30889373/article/details/80538211\#Android\_View\_11][https_blog.csdn.net_qq_30889373_article_details_80538211_Android_View_11]

## 源码分析 ##

### 前言，一些必须知道的知识 ###

API 版本 27

我们先看看 AppCompatActivity 的跟这次主题相关的  
重要方法

public class  AppCompatActivity ...{
    
        @Override
        protected void onCreate(@Nullable Bundle savedInstanceState) {
            final AppCompatDelegate delegate = getDelegate();
            // 这个方法我们稍微记一下，待会要
            delegate.installViewFactory();
            // ....
            super.onCreate(savedInstanceState);
        }
        
        @NonNull
        public AppCompatDelegate getDelegate() {
            if (mDelegate == null) {
                mDelegate = AppCompatDelegate.create(this, this);
            }
            return mDelegate;
        }
    
        @Override
        public void setContentView(@LayoutRes int layoutResID) {
            getDelegate().setContentView(layoutResID);
        }
    }

AppCompatDelegate 是什么呢，我们进入继续观看

public abstract class AppCompatDelegate {
    	....  
        public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
            return create(activity, activity.getWindow(), callback);
        }
    
    	// 根据不同的系统版本 创建不同的 Imp
        private static AppCompatDelegate create(Context context, Window window,
                AppCompatCallback callback) {
            if (Build.VERSION.SDK_INT >= 24) {
                return new AppCompatDelegateImplN(context, window, callback);
            } else if (Build.VERSION.SDK_INT >= 23) {
                return new AppCompatDelegateImplV23(context, window, callback);
            } else {
                return new AppCompatDelegateImplV14(context, window, callback);
            }
        }
    }

我们 查看一下继承关系

![在这里插入图片描述][70]

**对应的类点击查看发现**

public class  AppCompatActivity ...{
        @Override
        protected void onCreate(@Nullable Bundle savedInstanceState) {
            final AppCompatDelegate delegate = getDelegate();
            // 这个方法我们稍微记一下，待会要进入主题
            delegate.installViewFactory();
    }
    
    // 重点关注下  LayoutInflater.Factory2 
    class AppCompatDelegateImplV9 extends AppCompatDelegateImplBase
            implements MenuBuilder.Callback, LayoutInflater.Factory2 {
            
          
        @Override
        public void installViewFactory() {
            LayoutInflater layoutInflater = LayoutInflater.from(mContext);
            if (layoutInflater.getFactory() == null) {
            	// 重点方法，第二个参数是 LayoutInflater.Factory2  也就是自己实现了
                LayoutInflaterCompat.setFactory2(layoutInflater, this);
            } else {
                if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImplV9)) {
                    Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
                            + " so we can not install AppCompat's");
                }
            }
        }
    }
    
    // 而 Factory2 接口就只有一个方法
        public interface Factory2 extends Factory {
        	
            public View onCreateView(View parent, String name, Context context, AttributeSet attrs);
        }

这里可以自行跟进去看看，我们这里只讲一个重点知识，因为类似  
[https://github.com/ximsfei/Android-skin-support][https_github.com_ximsfei_Android-skin-support]  
通过替换 Factory2 来实现换肤的方案，需要知道这个知识点

public abstract class LayoutInflater {
    
        public void setFactory2(Factory2 factory) {
        	// 可以看到只能设置一次，而上面说明的开源库，是通过反射修改这个值
        	// 替换自己的 Factory2
            if (mFactorySet) {
                throw new IllegalStateException("A factory has already been set on this LayoutInflater");
            }
            if (factory == null) {
                throw new NullPointerException("Given factory can not be null");
            }
            mFactorySet = true;
            if (mFactory == null) {
                mFactory = mFactory2 = factory;
            } else {
                mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
            }
        }
    }

### 进入主题 ###

> 那么上面讲了这么多跟 xml解析有什么关系呢，我们进入主题后，你就明白前面的意思了  
> ···

// 1
    public class  AppCompatActivity ...{
    
        @Override
        public void setContentView(@LayoutRes int layoutResID) {
            getDelegate().setContentView(layoutResID);
        }
    }
    
    // 2
    class AppCompatDelegateImplV9 {
        @Override
        public void setContentView(int resId) {
            ensureSubDecor();
            ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
            contentParent.removeAllViews();
            // 通过 LayoutInflater 解析，我们跟进去看下
            LayoutInflater.from(mContext).inflate(resId, contentParent);
            mOriginalWindowCallback.onContentChanged();
        }
    }
    
    // 3
    public abstract class LayoutInflater {
    
        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) {
    	     final Resources res = getContext().getResources();
    	     // 通过 资源ID 获取 XmlResourceParser
    	     final XmlResourceParser parser = res.getLayout(resource);
    	     try {
    	     	// 进入查看
    	         return inflate(parser, root, attachToRoot);
    	     } finally {
    	         parser.close();
    	     }
         }
         
    	// 关键方法来了
         public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
         	....
            if (TAG_MERGE.equals(name)) {
                .. 其实内部最终也会调用 createViewFromTag
                rInflate(parser, root, inflaterContext, attrs, false);
            } else {
                //  关键方法 传入
                final View temp = createViewFromTag(root, name, inflaterContext, attrs);
            }
         }
    	
    	// 4 
        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) {
    	  //...可以看到
           if (mFactory2 != null) {
               view = mFactory2.onCreateView(parent, name, context, attrs);
           } else if (mFactory != null) {
               view = mFactory.onCreateView(name, context, attrs);
           } else {
               view = null;
           }
    }

> 总结前面分析

<table> 
 <thead> 
  <tr> 
   <th>关键方法</th> 
   <th>描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>LayoutInflater.from(mContext).inflate(resId, contentParent)</td> 
   <td>传入资源ID ，父容器，可空</td> 
  </tr> 
  <tr> 
   <td>XmlResourceParser parser = res.getLayout(resource);</td> 
   <td>通过资源ID 获取 xml解析后的类</td> 
  </tr> 
  <tr> 
   <td>mFactory2.onCreateView(parent, name, context, attrs)</td> 
   <td>而后调用 mFactory2.onCreateView 创建对应view</td> 
  </tr> 
 </tbody> 
</table>

mFactory2 在上面的 AppCompatDelegateImplV9 的 installViewFactory() 中已设置好了的,  
其实 mFactory2 就是 AppCompatDelegateImplV9.

### 查看 View的创建流程 ###

public class AppCompatDelegateImplV9{
        @Override
        public final View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
            // First let the Activity's Factory try and inflate the view
            final View view = callActivityOnCreateView(parent, name, context, attrs);
            if (view != null) {
                return view;
            }
    
            // If the Factory didn't handle it, let our createView() method try
            return createView(parent, name, context, attrs);
        }
    	@Override
        public View createView(View parent, final String name, @NonNull Context context,
                @NonNull AttributeSet attrs) {
            if (mAppCompatViewInflater == null) {
            		....
                    mAppCompatViewInflater = new AppCompatViewInflater();
                } else {
                   ....
            }
    		...
            return mAppCompatViewInflater.createView(....);
        }
    }

下面我们查看 系统控件的创建过程

public AppCompatViewInflater {
    	
    	/**
    	* 可以发现，如果是TextView，就直接返回V7包中的AppCompatTextView类，
    	* 所以这就是为什么当我们使   用AppCompatActivity的时候，TextView变成AppCompatTextView的原因。
    	*/
        @NonNull
        protected AppCompatTextView createTextView(Context context, AttributeSet attrs) {
            return new AppCompatTextView(context, attrs);
        }
    
    	// 创建 View
        final View createView(View parent, final String name, @NonNull Context context,
                @NonNull AttributeSet attrs, boolean inheritContext,
                boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
            final Context originalContext = context;
    		.... 
            switch (name) {
                case "TextView":
                    view = createTextView(context, attrs);
                    verifyNotNull(view, name);
                    break;
              
                default:
                    view = createView(context, name, attrs);
            }
            if (view == null && originalContext != context) {
                // 使用名字 inflate，主题将会无效
                view = createViewFromTag(context, name, attrs);
            }
            
            if (view != null) {
                // 如果我们尝试创建一个 view, check its android:onClick
                checkOnClickListener(view, attrs);
            }
    
            return view;
        }
    }

那么自定义控件，还有哪些需要 v7 包名的组件怎么创建的呢？

public AppCompatViewInflater {
    	
    	// 默认前缀包名列表
        private static final String[] sClassPrefixList = {
                "android.widget.",
                "android.view.",
                "android.webkit."
        };
        
        private View createViewFromTag(Context context, String name, AttributeSet attrs) {
            if (name.equals("view")) {
                name = attrs.getAttributeValue(null, "class");
            }
    
            try {
                mConstructorArgs[0] = context;
                mConstructorArgs[1] = attrs;
    			// 尝试从 默认前缀包名列表 中查找
                if (-1 == name.indexOf('.')) {
                    for (int i = 0; i < sClassPrefixList.length; i++) {
                        final View view = createViewByPrefix(context, name, sClassPrefixList[i]);
                        if (view != null) {
                            return view;
                        }
                    }
                    return null;
                } else {
                	// 加载自定义View，也就是完整名 比如 com.xm.xbutton
                    return createViewByPrefix(context, name, null);
                }
            } catch (Exception e) {
                return null;
            } finally {
                mConstructorArgs[0] = null;
                mConstructorArgs[1] = null;
            }
        }
    }

### 总结 ###

public interface Factory2 extends Factory {
        pulic View onCreateView(View parent, String name, Context context, AttributeSet attrs);
        }
    }

简单几句话讲解下 xml的创建过程

1.  AppCompatActivity onCreate 时调用 installViewFactory (实际为 AppCompatDelegateImplV9)
2.  AppCompatDelegateImplV9 实现接口 Factory2
3.  installViewFactory 方法将 LayoutInflater.setFactory2(AppCompatDelegateImplV9.this)
4.  将资源 id 解析为 XmlResourceParser ，并 createViewFromTag 的方法
5.  内部调用 Factory2.createView 创建View
6.  内部使用 AppCompatViewInflater.java 专门用来根据 xml中分别的名称，创建View

[https_blog.csdn.net_qq_30889373_article_details_80538211_Android_View_11]: https://blog.csdn.net/qq_30889373/article/details/80538211#Android_View_11
[70]: /images/20220510/21a52768c7874eadb8ef2a86b718ccfb.png
[https_github.com_ximsfei_Android-skin-support]: https://github.com/ximsfei/Android-skin-support