1、Layoutinfalter.inflate 参数解读

view怎么创建出来的   xml文件解析-->Layoutinfalter.inflate()  通过反射构造2个参数的 Fragment   listview   recycleview   viewpager  popupwindow

1、分析源码 多看背后实现的思想与架构 找出共性 发散思维

2、setContentView

Activity

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.add_item_confirmation_activity);
}

public void setContentView(@LayoutRes int layoutResID) {
    //这里调用了Window的setContentView方法，而Window是一个抽象类，PhoneWindow是其唯一派生子类
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

PhoneWindow

@Override
public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        //创建DecorView，并添加到mContentParent上
        installDecor(); // 1 
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
         .removeAllViews();
    }

    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                getContext());
        transitionTo(newScene);
    } else {
        //将布局加载到mContentParent上
        mLayoutInflater.inflate(layoutResID, mContentParent);// 2
    }
}

LayoutInflater
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
    final Resources res = getContext().getResources();

    View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
    if (view != null) {
            return view;
    }
    //通过res.getLayout(resource)方法对布局资源进行了XML解析，将布局资源解析成XmlResourceParser对象
    XmlResourceParser parser = res.getLayout(resource);
    try {
            return inflate(parser, root, attachToRoot); //1
    } finally {
            parser.close();
    }
}

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;
        View result = root;

        try {
            // 循环查找顶节点
            advanceToRootNode(parser);
            
            final String name = parser.getName();

            if (DEBUG) {
                System.out.println("**************************");
                System.out.println("Creating root view: "
                        + name);
                System.out.println("**************************");
            }

            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, inflaterContext, attrs, false);
            } else {
                // Temp is the root view that was found in the xml
                // 先生成顶节点布局
                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
                    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);
                    }
                }

                if (DEBUG) {
                    System.out.println("-----> start inflating children");
                }
                
                 // 实例化所有子View，以temp顶节点为父布局，递归生成View树
                // Inflate all children under temp against its context.
                rInflateChildren(parser, temp, attrs, true);

                if (DEBUG) {
                    System.out.println("-----> done inflating children");
                }

                // 将生成的View树添加到root父布局上，
                // setContentView时这里的root是mContentParent
                // 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);
                }

                // 如果root根视图为空，则返回在XML中找到的顶视图
                // Decide whether to return the root that was passed in or the
                // top view found in xml.
                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(
                    getParserStateDescription(inflaterContext, attrs)
                    + ": " + e.getMessage(), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } finally {
            // Don't retain static reference on context.
            mConstructorArgs[0] = lastContext;
            mConstructorArgs[1] = null;

            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

        return result;
    }
}

首先，循环上面解析好的布局资源XmlResourceParser对象，取出布局的根节点，调用createViewFromTag方法先生成根节点View。然后再调用rInflateChildren方法递归的生成整个布局View树。最后将生成的View树添加到DecorView的root根布局上

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

    // Apply a theme wrapper, if allowed and one is specified.
    if (!ignoreThemeAttr) {
        final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
        final int themeResId = ta.getResourceId(0, 0);
        if (themeResId != 0) {
            context = new ContextThemeWrapper(context, themeResId);
        }
        ta.recycle();
    }

    try {
        View view = tryCreateView(parent, name, context, attrs);

        if (view == null) {
            final Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = context;
            try {
                    // 生成View对象
                if (-1 == name.indexOf('.')) {
                    // Android系统自带View
                    view = onCreateView(context, parent, name, attrs);
                } else {
                    // 自定义View
                    view = createView(context, name, null, attrs);
                }
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        }

        return view;
    }
}

最后会调用onCreateView或createView方法来生成View对象，这里的onCreateView方法中会为View的节点名添加"android.view."前缀，然后再调用createView方法生成View对象。做这一步操作是因为createView方法是通过反射生成的对象，所以需要完整的包名+类名，而Android系统自带的View都是放在android.view包下

@Nullable
public final View createView(@NonNull Context viewContext, @NonNull String name,
        @Nullable String prefix, @Nullable AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
    Objects.requireNonNull(viewContext);
    Objects.requireNonNull(name);
    
    // 获取静态全局变量中是否有缓存相应的构造方法
    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) {
        
            // 通过反射机制获取View的Class对象
            // Class not found in the cache, see if it's real, and try to add it
            clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                    mContext.getClassLoader()).asSubclass(View.class);

            if (mFilter != null && clazz != null) {
                boolean allowed = mFilter.onLoadClass(clazz);
                if (!allowed) {
                    failNotAllowed(name, prefix, viewContext, attrs);
                }
            }
            
            // 获取构造方法，用于View的实例化
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            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) {
                
                    // 通过反射机制获取View的Class对象
                    // New class -- remember whether it is allowed
                    clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                            mContext.getClassLoader()).asSubclass(View.class);

                    boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                    mFilterMap.put(name, allowed);
                    if (!allowed) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                } else if (allowedState.equals(Boolean.FALSE)) {
                    failNotAllowed(name, prefix, viewContext, attrs);
                }
            }
        }

        Object lastContext = mConstructorArgs[0];
        mConstructorArgs[0] = viewContext;
        Object[] args = mConstructorArgs;
        args[1] = attrs;

        try {
        
            // 实例化View对象
            final View view = constructor.newInstance(args);
            if (view instanceof ViewStub) {
            
                // 如果是ViewStub 就为其设置LayoutInflater 以便后续inflate
                // Use the same context when inflating ViewStub later.
                final ViewStub viewStub = (ViewStub) view;
                viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
            }
            return view;
        } finally {
            mConstructorArgs[0] = lastContext;
        }
    }
}

最后会调用2个参数的构造方法
@UnsupportedAppUsage
static final Class<?>[] mConstructorSignature = new Class[] {
        Context.class, AttributeSet.class};

createView中先是通过ClassLoader的反射机制，获取到对应的Class对象，然后获取到它的构造方法。最后通过构造方法实例化这个View对象并返回

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

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

    final int depth = parser.getDepth();
    int type;
    boolean pendingRequestFocus = false;

    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)) {
            pendingRequestFocus = true;
            consumeChildElements(parser);
        } else if (TAG_TAG.equals(name)) {
            parseViewTag(parser, parent, attrs);
        } else if (TAG_INCLUDE.equals(name)) {
            if (parser.getDepth() == 0) {
                throw new InflateException("<include /> cannot be the root element");
            }
            parseInclude(parser, context, parent, attrs);
        } else if (TAG_MERGE.equals(name)) {
            throw new InflateException("<merge /> must be the root element");
        } else {
            // 生成View对象
            final View view = createViewFromTag(parent, name, context, attrs);
            // 父布局
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            // 递归调用rInflateChildren，递归生成View树
            rInflateChildren(parser, view, attrs, true);
            viewGroup.addView(view, params);
        }
    }

    if (pendingRequestFocus) {
        parent.restoreDefaultFocus();
    }

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

首先先调用上面已经分析过的createViewFromTag方法生成当前的节点View对象，然后这里会把当前这个View作为父节点递归调用rInflateChildren方法，如果当前View有子节点，就会递归的生成子节点并调用addView方法层层的插入对应的父节点。最终就会生成一个完整的View树，整体的页面布局也就显现出来了