Activity的显示之ViewRootImpl初探

该文章是一个系列文章，是本人在Android开发的漫漫长途上的一点感想和记录，如果能给各位看官带来一丝启发或者帮助，那真是极好的。

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前言

在上一篇文章中我们主要分析了android.app.ActivityThread的main函数以及setContentView。另外我们还稍微分析了一下我们自己的源码，通过WindowManager添加View。我们知道调用setContentView把我们自己的xml布局添加到了DecorView ID为ID_ANDROID_CONTENT的布局后，最终还是会调用WindowManager.addView把DecorView加入PhoneWindow。到这里呢，我们把流程梳理一下。还是上图：

相信读者根据上图再结合前面所讲的内容应该对Activity的创建和显示有了初步的认识。那么本章我们来继续讲Activity的显示。该注意的是本系列并不意在带领读者去看清每一步具体的源码。在前面的文章中我也很少贴出源码。本系列文章意在让读者对Android系统有个更整体的把握。我所写的每一章知识都有可能在实际工作中用到。就如前面所讲解的Android下的进程问题以及Activity的生命周期以及本章要讲解的View的五大过程的基础ViewRootImpl。而理解View的五大过程（一般文章里都是三大过程）以及View的事件体系是更好的去自定义View的基础。

WindowManager 与 ViewRootImpl

WindowManager.addView()源码解析

public interface WindowManager extends ViewManager {
    //这里我们只列出了一部分函数，但是并没有addView、updateViewLayout、removeView这三个函数
    public Display getDefaultDisplay();
    public void removeViewImmediate(View view);
    ...
}

好吧，果然没有这么简单，WindowManager是个接口，而且在其方法中没有找到addView方法，那么我们只能看看ViewManager了

public interface ViewManager
{
    public void addView(View view, ViewGroup.LayoutParams params);
    public void updateViewLayout(View view, ViewGroup.LayoutParams params);
    public void removeView(View view);
}

还好是找到了，ViewManager没有再继承其他接口了。(要不然真不知道要找到什么时候去。)
既然WindowManager是个接口，那肯定要找它的实现类了。（在这里安利一个比较简单的方法，在Android Studio中）

public final class WindowManagerImpl implements WindowManager {
    private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
    private final Context mContext;
    private final Window mParentWindow;

    private IBinder mDefaultToken;

    public WindowManagerImpl(Context context) {
        this(context, null);
    }

    private WindowManagerImpl(Context context, Window parentWindow) {
        mContext = context;
        mParentWindow = parentWindow;
    }

    public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
        return new WindowManagerImpl(mContext, parentWindow);
    }

    public WindowManagerImpl createPresentationWindowManager(Context displayContext) {
        return new WindowManagerImpl(displayContext, mParentWindow);
    }

    /**
     * Sets the window token to assign when none is specified by the client or
     * available from the parent window.
     *
     * @param token The default token to assign.
     */
    public void setDefaultToken(IBinder token) {
        mDefaultToken = token;
    }

    @Override
    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
    }

    @Override
    public void updateViewLayout(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.updateViewLayout(view, params);
    }

    private void applyDefaultToken(@NonNull ViewGroup.LayoutParams params) {
        // Only use the default token if we don't have a parent window.
        if (mDefaultToken != null && mParentWindow == null) {
            if (!(params instanceof WindowManager.LayoutParams)) {
                throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
            }

            // Only use the default token if we don't already have a token.
            final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
            if (wparams.token == null) {
                wparams.token = mDefaultToken;
            }
        }
    }

    @Override
    public void removeView(View view) {
        mGlobal.removeView(view, false);
    }

    @Override
    public void removeViewImmediate(View view) {
        mGlobal.removeView(view, true);
    }

    @Override
    public void requestAppKeyboardShortcuts(
            final KeyboardShortcutsReceiver receiver, int deviceId) {
        IResultReceiver resultReceiver = new IResultReceiver.Stub() {
            @Override
            public void send(int resultCode, Bundle resultData) throws RemoteException {
                List<KeyboardShortcutGroup> result =
                        resultData.getParcelableArrayList(PARCEL_KEY_SHORTCUTS_ARRAY);
                receiver.onKeyboardShortcutsReceived(result);
            }
        };
        try {
            WindowManagerGlobal.getWindowManagerService()
                .requestAppKeyboardShortcuts(resultReceiver, deviceId);
        } catch (RemoteException e) {
        }
    }

    @Override
    public Display getDefaultDisplay() {
        return mContext.getDisplay();
    }
}

WindowManagerImpl的源码如上所示，我们可以看到WindowManagerImpl的addView方法，WindowManagerImpl把工作交给了WindowManagerGlobal

WindowManagerGlobal

/**
WindowManagerGlobal 源码比较长，这里我们只列出了一部分
*/
public final class WindowManagerGlobal {
    private WindowManagerGlobal() {
    }

    public static void initialize() {
        getWindowManagerService();
    }

    public static WindowManagerGlobal getInstance() {
        synchronized (WindowManagerGlobal.class) {
            if (sDefaultWindowManager == null) {
                sDefaultWindowManager = new WindowManagerGlobal();
            }
            return sDefaultWindowManager;
        }
    }

    public static IWindowManager getWindowManagerService() {
        synchronized (WindowManagerGlobal.class) {
            if (sWindowManagerService == null) {
                sWindowManagerService = IWindowManager.Stub.asInterface(
                        ServiceManager.getService("window"));
                try {
                    if (sWindowManagerService != null) {
                        ValueAnimator.setDurationScale(
                                sWindowManagerService.getCurrentAnimatorScale());
                    }
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
            return sWindowManagerService;
        }
    }

    public static IWindowSession getWindowSession() {
        synchronized (WindowManagerGlobal.class) {
            if (sWindowSession == null) {
                try {
                    InputMethodManager imm = InputMethodManager.getInstance();
                    IWindowManager windowManager = getWindowManagerService();
                    sWindowSession = windowManager.openSession(
                            new IWindowSessionCallback.Stub() {
                                @Override
                                public void onAnimatorScaleChanged(float scale) {
                                    ValueAnimator.setDurationScale(scale);
                                }
                            },
                            imm.getClient(), imm.getInputContext());
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
            return sWindowSession;
        }
    }

    public static IWindowSession peekWindowSession() {
        synchronized (WindowManagerGlobal.class) {
            return sWindowSession;
        }
    }

    //addView方法
    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        ...  //参数检查

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
        if (parentWindow != null) {
            //① 如果当前窗口需要被添加为另一个窗口的附属窗口(子窗口)，则需要父窗口视自己的情况对当前窗口的布局参数进行调整
            parentWindow.adjustLayoutParamsForSubWindow(wparams);
        }

        ViewRootImpl root;
        View panelParentView = null;

       
        int index = findViewLocked(view, false);
          if (index >= 0) {
              if (mDyingViews.contains(view)) {
                  mRoots.get(index).doDie();
              } else {
              //同一个View不允许被添加2次
                  throw new IllegalStateException("View " + view
                          + " has already been added to the window manager.");
              }
          }
        //② 创建一个ViewRootImpl对象并保存在root变量中
         root = new ViewRootImpl(view.getContext(), display);

         view.setLayoutParams(wparams);
        //③ 保存作为窗口的控件、布局参数以及新建的ViewRootImpl
         mViews.add(view);
         mRoots.add(root);
         mParams.add(wparams);

         // do this last because it fires off messages to start doing things
         try {
             // ④ 将作为窗口的控件设置给ViewRootImpl.这个动作将导致ViewRootImpl向WMS添加新的窗口、申请Surface以及托管控件在Surface上的重绘工作。这才是真正意义上完成了窗口的添加工作。
             root.setView(view, wparams, panelParentView);
         } catch (RuntimeException e) {
             // BadTokenException or InvalidDisplayException, clean up.
             if (index >= 0) {
                 removeViewLocked(index, true);
             }
             throw e;
         }
        }
    }

    public void updateViewLayout(View view, ViewGroup.LayoutParams params) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }
        if (!(params instanceof WindowManager.LayoutParams)) {
            throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
        }

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;

        view.setLayoutParams(wparams);

        synchronized (mLock) {
            int index = findViewLocked(view, true);
            ViewRootImpl root = mRoots.get(index);
            mParams.remove(index);
            mParams.add(index, wparams);
            root.setLayoutParams(wparams, false);
        }
    }

    public void removeView(View view, boolean immediate) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }

        synchronized (mLock) {
            int index = findViewLocked(view, true);
            View curView = mRoots.get(index).getView();
            removeViewLocked(index, immediate);
            if (curView == view) {
                return;
            }

            throw new IllegalStateException("Calling with view " + view
                    + " but the ViewAncestor is attached to " + curView);
        }
    }

   
}

我们可以看到WindowManagerGlobal的私有构造函数以及getInstance()这个熟悉的静态方法名字。可以看出WindowManagerGlobal是个典型的单例。
WindowManagerGlobal 的addView方法并不复杂，其主要的关键点我们已经标注并写了注释。也就是说WindowManagerGlobal的职责如下：

1. 同意管理整个进程中所有窗口的信息。包括控件、布局参数以及ViewRootImpl这三个元素。（这一点从第③个注释可以看出）
2. WindowManagerGlobal将窗口的创建、销毁、布局更新等任务交给了ViewRootImpl完成。

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本篇总结

本篇文章分析了WindowManager的addView的过程，WindowManager是个接口，它的实现类是WindowManagerImpl类，而WindowManagerImpl又把相关逻辑交给了WindowManagerGlobal处理。WindowManagerGlobal是个单例类，它在进程中只存在一个实例，是它内部的addView方法最终创建了我们的核心类ViewRootImpl。ViewRootImpl实现了ViewParent接口，作为整个控件树的根部，它是控件树正常运作的动力所在，控件的测量、布局、绘制以及输入事件的派发处理窦世友ViewRootImpl出发。它是WindowManagerGlobal的实际工作者。

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下篇预告

在下一篇文章中我们将深入介绍ViewRootImpl的工作流程。测量、布局、以及绘制。

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参考博文

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此致，敬礼