• Android开发教程:Android从xml加载到View对象过程解析
  • Linda 发表于 2016/3/9 12:29:00 | 分类标签: Android开发 View对象
  •  我们从Activity的setContentView()入手,开始源码解析,
        //Activity.setContentView
        public void setContentView(int layoutResID) {
            getWindow().setContentView(layoutResID);
            initActionBar();
        }
    
    
        //PhoneWindow.setContentView
        public void setContentView(int layoutResID) {
            if (mContentParent == null) {
                installDecor();
            } else {
                mContentParent.removeAllViews();
            }
            mLayoutInflater.inflate(layoutResID, mContentParent);
            final Callback cb = getCallback();
            if (cb != null && !isDestroyed()) {
                cb.onContentChanged();
            }
        }

    发现是使用mLayoutInflater创建View的,所以我们去LayoutInflater.inflate()里面看下,

        public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
            if (DEBUG) System.out.println("INFLATING from resource: " + resource);
            XmlResourceParser parser = getContext().getResources().getLayout(resource);
            try {
                return inflate(parser, root, attachToRoot);
            } finally {
                parser.close();
            }
        }

    先根据resource id 获取到XmlResourceParseer,意如其名,就是xml的解析器,继续往下,进入到inflate的核心方法,有些长,我们只分析关键部分:

    public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
            ......
    
                    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, attrs, false);
                    } else {
                        // Temp is the root view that was found in the xml
                        View temp;
                        if (TAG_1995.equals(name)) {
                            temp = new BlinkLayout(mContext, attrs);
                        } else {
                            temp = createViewFromTag(root, name, attrs);
                        }
            ......
                } catch (XmlPullParserException e) {
                    InflateException ex = new InflateException(e.getMessage());
                    ex.initCause(e);
                    throw ex;
                } catch (IOException e) {
                    InflateException ex = new InflateException(
                            parser.getPositionDescription()
                            + ": " + e.getMessage());
                    ex.initCause(e);
                    throw ex;
                } finally {
                    // Don't retain static reference on context.
                    mConstructorArgs[0] = lastContext;
                    mConstructorArgs[1] = null;
                }
    
                return result;
            }
        }

    如果tag的名字不是TAG_1995(名字是个梗),就调用函数createViewFromTag()创建View,进去看看,

        View createViewFromTag(View parent, String name, AttributeSet attrs) {
            if (name.equals("view")) {
                name = attrs.getAttributeValue(null, "class");
            }
    
            ......
    
                View view;
                if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs);
                else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs);
                else view = null;
    
                if (view == null && mPrivateFactory != null) {
                    view = mPrivateFactory.onCreateView(parent, name, mContext, attrs);
                }
                
                if (view == null) {
                    if (-1 == name.indexOf('.')) {
                        view = onCreateView(parent, name, attrs);
                    } else {
                        view = createView(name, null, attrs);
                    }
                }
    
                if (DEBUG) System.out.println("Created view is: " + view);
                return view;
        ......
        }

    首先尝试用3个Fractory创建View,如果成功就直接返回了。注意,我们可以利用这个机制,创建自己的Factory来控制View的创建过程。

    如果没有Factory或创建失败,那么走默认逻辑。

    先判断name中是否有'.'字符,如果没有,则认为使用android自己的View,此时会在name的前面加上包名"android.view.";如果有这个'.',则认为使用的自定义View,这时无需添加任何前缀,认为name已经包含全包名了。

    最终,使用这个全包名的name来创建实例,

        private static final HashMap<String, Constructor<? extends View>> sConstructorMap =
                new HashMap<String, Constructor<? extends View>>();
    
        protected View onCreateView(String name, AttributeSet attrs)
                throws ClassNotFoundException {
            return createView(name, "android.view.", attrs);
        }
    
        public final View createView(String name, String prefix, AttributeSet attrs)
                throws ClassNotFoundException, InflateException {
            Constructor<? extends View> constructor = sConstructorMap.get(name);
            Class<? extends View> clazz = null;
               ......
    
                if (constructor == null) {
                    // Class not found in the cache, see if it's real, and try to add it
                    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);
                    sConstructorMap.put(name, constructor);
                } else {
                    // If we have a filter, apply it to cached constructor
                    if (mFilter != null) {
                        // Have we seen this name before?
                        Boolean allowedState = mFilterMap.get(name);
                        if (allowedState == null) {
                            // New class -- remember whether it is allowed
                            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;
                return constructor.newInstance(args);
                ......
        }

    从源码中看到,在创建实例前,会先从一个静态Map中获取缓存,

    Constructor<? extends View> constructor = sConstructorMap.get(name);

    缓存的是Constructor对象,目的是用于创建实例,这里要注意sConstructorMap是静态的,并且通过Constructor创建的实例,是使用和Constructor对象同一个ClassLoader来创建的,换句话说,在同一个进程中,同一个自定义View对象,是无法用不同ClassLoader加载的,如果想解决这个问题,就不要让系统使用createView()接口创建View,做法就是自定义Factory或Factory2来自行创建View。

    继续往下看,如果缓存里没有,则创建View的Class对象clazz,并缓存到sConstructorMap中,

                if (constructor == null) {
                    // Class not found in the cache, see if it's real, and try to add it
                    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);
                    sConstructorMap.put(name, constructor);
                }

    然后就是newInstance了,至此这个View便从xml中变成了java对象,我们继续返回到inflate函数中,看看这个View返回之后做了什么,

                    ......
                        // Temp is the root view that was found in the xml
                        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) {
                            // Create layout params that match root, if supplied
                            params = root.generateLayoutParams(attrs);
                            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
                        rInflate(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;

    从createViewFromTag返回后,会调用个rInflate(),其中parent参数就是刚才创建出的View,应该能猜到里面做了什么,

        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)) {
                    if (parser.getDepth() == 0) {
                        throw new InflateException("<include /> cannot be the root element");
                    }
                    parseInclude(parser, parent, attrs);
                } else if (TAG_MERGE.equals(name)) {
                    throw new InflateException("<merge /> 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();
        }

    没错,就是递归的使用createViewFromTag()创建子View,并通过ViewGroup.addView添加到parent view中。

    之后,这个View树上的所有View都创建完毕。然后会根据inflate()的参数(root和attachToRoot)判断是否将新创建的View添加到root view中,

                if (root != null && attachToRoot) {
                    root.addView(temp, params);
                }

    然后,inflate()就将View返回了。

    整个分析到此结束。

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