从setContentView开始的源码阅读

朴灿烈づ我的快乐病毒、 2022-06-16 05:46 174阅读 0赞

## 从setContentView开始的源码阅读 ##

学习了一段时间的安卓自定义view,稍微总结一下学到的东西。最近也事情也比较烦，还是静下心来学习一波，不忘初心。  
对于安卓开发者来说，setContentView这个方法再熟悉不过了，每个Activity都会调用，大家都知道这个是设置当前activity界面的，里面具体做了什么呢？通过原来阅读来简单介绍一下。

一. DecorView的生成  
进入setContentView中，首先看到的  
![这里写图片描述][SouthEast]  
getWindow()自然就是PhoneWindow了，我们进入PhoneWindow  
![这里写图片描述][SouthEast 1]  
这里的关键代码在instalDecor()中，猜一下就是创建DecorView，DecorView又是什么呢？

//省略了数万行代码后
     private void installDecor() {
            mForceDecorInstall = false;
            if (mDecor == null) {
                mDecor = generateDecor(-1);
                mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
                mDecor.setIsRootNamespace(true);
                if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                    mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
                }
            } else {
                mDecor.setWindow(this);
            }
            if (mContentParent == null) {
                mContentParent = generateLayout(mDecor);
    
                // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
                mDecor.makeOptionalFitsSystemWindows();
    
                final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                        R.id.decor_content_parent);
    
                if (decorContentParent != null) {
                    mDecorContentParent = decorContentParent;
                    mDecorContentParent.setWindowCallback(getCallback());
                    if (mDecorContentParent.getTitle() == null) {
                        mDecorContentParent.setWindowTitle(mTitle);
                    }
    
                    final int localFeatures = getLocalFeatures();
                    for (int i = 0; i < FEATURE_MAX; i++) {
                        if ((localFeatures & (1 << i)) != 0) {
                            mDecorContentParent.initFeature(i);
                        }
                    }
    
    
                } else {
    
                }
    
    
    
            }
        }

这里看关键的mDecor = generateDecor(-1);  
这里只是生成了以DecorView，DecorView我们看源码就知道是继承FrameLayout的。  
接下来 mContentParent = generateLayout(mDecor);  
这里才是真正的绑定布局。

protected ViewGroup generateLayout(DecorView decor) {
    
    
            // Inflate the window decor.
    
            int layoutResource;
            int features = getLocalFeatures();
            // System.out.println("Features: 0x" + Integer.toHexString(features));
            if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
                layoutResource = R.layout.screen_swipe_dismiss;
            } else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
                if (mIsFloating) {
                    TypedValue res = new TypedValue();
                    getContext().getTheme().resolveAttribute(
                            R.attr.dialogTitleIconsDecorLayout, res, true);
                    layoutResource = res.resourceId;
                } else {
                    layoutResource = R.layout.screen_title_icons;
                }
                // XXX Remove this once action bar supports these features.
                removeFeature(FEATURE_ACTION_BAR);
                // System.out.println("Title Icons!");
            } else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
                    && (features & (1 << FEATURE_ACTION_BAR)) == 0) {
                // Special case for a window with only a progress bar (and title).
                // XXX Need to have a no-title version of embedded windows.
                layoutResource = R.layout.screen_progress;
                // System.out.println("Progress!");
            } else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
                // Special case for a window with a custom title.
                // If the window is floating, we need a dialog layout
                if (mIsFloating) {
                    TypedValue res = new TypedValue();
                    getContext().getTheme().resolveAttribute(
                            R.attr.dialogCustomTitleDecorLayout, res, true);
                    layoutResource = res.resourceId;
                } else {
                    layoutResource = R.layout.screen_custom_title;
                }
                // XXX Remove this once action bar supports these features.
                removeFeature(FEATURE_ACTION_BAR);
            } else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
                // If no other features and not embedded, only need a title.
                // If the window is floating, we need a dialog layout
                if (mIsFloating) {
                    TypedValue res = new TypedValue();
                    getContext().getTheme().resolveAttribute(
                            R.attr.dialogTitleDecorLayout, res, true);
                    layoutResource = res.resourceId;
                } else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
                    layoutResource = a.getResourceId(
                            R.styleable.Window_windowActionBarFullscreenDecorLayout,
                            R.layout.screen_action_bar);
                } else {
                    layoutResource = R.layout.screen_title;
                }
                // System.out.println("Title!");
            } else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
                layoutResource = R.layout.screen_simple_overlay_action_mode;
            } else {
                // Embedded, so no decoration is needed.
                layoutResource = R.layout.screen_simple;
                // System.out.println("Simple!");
            }
    
            mDecor.startChanging();
            mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
    
            ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
            if (contentParent == null) {
                throw new RuntimeException("Window couldn't find content container view");
            }
    
            if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
                ProgressBar progress = getCircularProgressBar(false);
                if (progress != null) {
                    progress.setIndeterminate(true);
                }
            }
    
            if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
                registerSwipeCallbacks();
            }
    
            // Remaining setup -- of background and title -- that only applies
            // to top-level windows.
            if (getContainer() == null) {
                final Drawable background;
                if (mBackgroundResource != 0) {
                    background = getContext().getDrawable(mBackgroundResource);
                } else {
                    background = mBackgroundDrawable;
                }
                mDecor.setWindowBackground(background);
    
                final Drawable frame;
                if (mFrameResource != 0) {
                    frame = getContext().getDrawable(mFrameResource);
                } else {
                    frame = null;
                }
                mDecor.setWindowFrame(frame);
    
                mDecor.setElevation(mElevation);
                mDecor.setClipToOutline(mClipToOutline);
    
                if (mTitle != null) {
                    setTitle(mTitle);
                }
    
                if (mTitleColor == 0) {
                    mTitleColor = mTextColor;
                }
                setTitleColor(mTitleColor);
            }
    
            mDecor.finishChanging();
    
            return contentParent;
        }

这里同样省略了很多代码，我们看一开始的注释就知道是来给DecorView绑定布局了，这里我们看一个最简单的布局R.layout.screen\_simple，

看完这个布局我们就知道了我们自己写的布局就是添加到content这个ID的FrameLayout里的，这里整个布局就是添加到DecorView中的，并且都是matchParent的，在这行代码可以找到答案 mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);  
那么我们整个视图就是这个样子的  
![这里写图片描述][SouthEast 2]

二. LayoutInflater分析  
这里我们继续回到setContentView中，mContentParent已经赋值了，就是我们的content,这里调用的 mLayoutInflater.inflate(layoutResID, mContentParent);  
当然就是把我们自己的布局添加到mContentParent中。分析一下内部实现。

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
            return inflate(resource, root, root != null);
        }

这里调用两个参数的方法，最后一个boolean自然是true了，我们继续跟，最后到了

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 {
                    // Look for the root node.
                    int type;
                    while ((type = parser.next()) != XmlPullParser.START_TAG &&
                            type != XmlPullParser.END_DOCUMENT) {
                        // Empty
                    }
    
                    if (type != XmlPullParser.START_TAG) {
                        throw new InflateException(parser.getPositionDescription()
                                + ": No start tag found!");
                    }
    
                    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
                        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");
                        }
    
                        // Inflate all children under temp against its context.
                        rInflateChildren(parser, temp, attrs, true);
    
                        if (DEBUG) {
                            System.out.println("-----> done inflating children");
                        }
    
                        // 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;
                        }
                    }
    
                } 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(parser.getPositionDescription()
                            + ": " + 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;
            }
        }

大概理解一下，这里通过递归不断解析xml来创建view,最后添加到mContentParent中。  
这里主要关注几个点，merge如果为没有添加到其他布局而作为直接根布局会抛出异常，include不能作为xml的根布局，平时我写的很多的的LayoutInflater.from(context).inflate(R.layout.xx,null);这里经常不能正确渲染出宽高，通过查看源码。

// Decide whether to return the root that was passed in or the
                        // top view found in xml.
                        if (root == null || !attachToRoot) {
                            result = temp;
                        }

这里的temp就是直接通过xml标签通过反射创建的view，没有设置LayoutParams。我们在ViewGroup中查看addView的源码，

public void addView(View child, int index) {
            if (child == null) {
                throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
            }
            LayoutParams params = child.getLayoutParams();
            if (params == null) {
                params = generateDefaultLayoutParams();
                if (params == null) {
                    throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
                }
            }
            addView(child, index, params);
        }

其中会执行 params = generateDefaultLayoutParams();这里生成的params就是对应的父容器的默认params,这里宽高的都是wrap\_content，所以这就是我们这样使用LayoutInflate渲染出来的view直接在xml根布局里修改宽高并没有什么作用的原因。

小结一下，setContentView的作用就是将指定的xml布局添加到decorView的content中，等待之后添加到window进行渲染。实际使用的还是LayoutInflater。

[SouthEast]: /images/20220616/e05101a7bebc4bb78352b101bda394d7.png
[SouthEast 1]: /images/20220616/231a740aaa1b4ec5aade5fc2009e9993.png
[SouthEast 2]: /images/20220616/bf43c739824a4da9af47f98c52345968.png