（八）理解Window和WindowManager

Android的所有的视图（Activity、Dialog、Toast等的View）都是通过Window来呈现的。
Window实际是View的直接管理者。

WindowManager.png

8.1 Window和WindowManager

如何使用一个WindowManager添加一个Window:
WindowManager.addView(View v , WindowManager.LayoutParams lp) 。示例源码如下：首先添加权限：

<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />

public class MainActivity extends Activity implements View.OnTouchListener {

    private Button mCreateWindowButton;

    private Button mFloatingButton;
    private WindowManager.LayoutParams mLayoutParams;
    private WindowManager mWindowManager;

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

    private void initView() {
        mCreateWindowButton = (Button) findViewById(R.id.button1);
        mWindowManager = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
    }

    public void onButtonClick(View v) {
        if (v == mCreateWindowButton) {
            mFloatingButton = new Button(this);
            mFloatingButton.setText("click me");

            mLayoutParams = new WindowManager.LayoutParams(
                    LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT, 0, 0,
                    PixelFormat.TRANSPARENT);
            mLayoutParams.flags = LayoutParams.FLAG_NOT_TOUCH_MODAL
                    | LayoutParams.FLAG_NOT_FOCUSABLE
                    | LayoutParams.FLAG_SHOW_WHEN_LOCKED;

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                mLayoutParams.type = WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY;
            }else {
                mLayoutParams.type = WindowManager.LayoutParams.TYPE_SYSTEM_ALERT;
            }

            mLayoutParams.gravity = Gravity.LEFT | Gravity.TOP;
            mLayoutParams.x = 100;
            mLayoutParams.y = 300;
            mFloatingButton.setOnTouchListener(this);
            mWindowManager.addView(mFloatingButton, mLayoutParams);
        }
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        int rawX = (int) event.getRawX();
        int rawY = (int) event.getRawY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                break;
            }
            case MotionEvent.ACTION_MOVE: {
                int x = (int) event.getX();
                int y = (int) event.getY();
                mLayoutParams.x = rawX;
                mLayoutParams.y = rawY;
                mWindowManager.updateViewLayout(mFloatingButton, mLayoutParams);
                break;
            }
            case MotionEvent.ACTION_UP: {
                break;
            }
            default:
                break;
        }

        return false;
    }

    @Override
    protected void onDestroy() {
        try {
            mWindowManager.removeView(mFloatingButton);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }
        super.onDestroy();
    }
}

效果图：

Window.jpg

8.2 Window的内部机制

Window是一个抽象的概念，每一个Window都对应着一个View和一个ViewRootImpl，Window和View通过ViewRootImpl建立联系。
WindowManager对Window主要有三大操作：添加、更新和删除。这三个方法主要是定义在ViewManager接口中：

public interface ViewManager {
    public void addView(View view, ViewGroup.LayoutParams params);//添加过程
    public void updateViewLayout(View view, ViewGroup.LayoutParams params);//更新过程
    public void removeView(View view);//删除过程
}

WindowManager也是一个接口，继承了ViewManager接口：

public interface WindowManager extends ViewManager {}

WindowManagerImpl 实现了WindowManager：源码如下：核心还是添加、更新和删除3个方法。

public final class WindowManagerImpl implements WindowManager{
        @Override
        public void addView(View view, ViewGroup.LayoutParams params){
            mGlobal.addView(view, params, mDisplay, mParentWindow);
        }
        
        @Override
        public void updateViewLayout(View view, ViewGroup.LayoutParams params){
            mGlobal.updateViewLayout(view, params);
        }
        
        @Override
        public void removeView(View view){
            mGlobal.removeView(view, false);
        }
}

WindowManagerImpl并没有直接实现Window的三大操作，而是交给了WindowManagerGlobal。WindowManagerGlobal以单例模式向外提供自己的实例：

private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();

8.2.1 Window的添加过程

1、检查参数是否合法，如果是子Window那么还需要调整一些布局参数
2、创建ViewRootImpl并将View添加到列表中
3、通过ViewRootImpl来更新界面并完成Window的添加过程

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

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
        if (parentWindow != null) {
            parentWindow.adjustLayoutParamsForSubWindow(wparams);
        } else {
            // If there's no parent, then hardware acceleration for this view is
            // set from the application's hardware acceleration setting.
            final Context context = view.getContext();
            if (context != null
                    && (context.getApplicationInfo().flags
                            & ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
                wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
            }
        }

        ViewRootImpl root;
        View panelParentView = null;

        synchronized (mLock) {
            // Start watching for system property changes.
            if (mSystemPropertyUpdater == null) {
                mSystemPropertyUpdater = new Runnable() {
                    @Override public void run() {
                        synchronized (mLock) {
                            for (int i = mRoots.size() - 1; i >= 0; --i) {
                                mRoots.get(i).loadSystemProperties();
                            }
                        }
                    }
                };
                SystemProperties.addChangeCallback(mSystemPropertyUpdater);
            }

            int index = findViewLocked(view, false);
            if (index >= 0) {
                if (mDyingViews.contains(view)) {
                    // Don't wait for MSG_DIE to make it's way through root's queue.
                    mRoots.get(index).doDie();
                } else {
                    throw new IllegalStateException("View " + view
                            + " has already been added to the window manager.");
                }
                // The previous removeView() had not completed executing. Now it has.
            }

            // If this is a panel window, then find the window it is being
            // attached to for future reference.
            if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
                    wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
                final int count = mViews.size();
                for (int i = 0; i < count; i++) {
                    if (mRoots.get(i).mWindow.asBinder() == wparams.token) {
                        panelParentView = mViews.get(i);
                    }
                }
            }

            root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);

            // do this last because it fires off messages to start doing things
            try {
                root.setView(view, wparams, panelParentView);
            } catch (RuntimeException e) {
                // BadTokenException or InvalidDisplayException, clean up.
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
                throw e;
            }
        }
    }

8.2.2 Window的删除过程

    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);
        }
    }

8.2.3 Window的更新过程

    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);
        }
    }

8.3 Window的创建过程

View是Android中得视图得呈现方式，但是View不能单独存在，它必须附着Window这个抽象得概念上马，因此有视图得地方就有Window。

8.3.1 Activity的Window的创建过程

步骤：
1、如果没有DecorView，那么就创建它；
2、将View添加到DecorView得mContentParent中；
3、回调Activity得onContentChanged方法通知Activity视图已经发生改变。

8.3.2 Dialog的Window的创建过程

步骤：
1、创建Window；
2、初始化DecorView并将Dialog得视图添加到DecorView中，
3、将DecorView添加到Window中显示。

8.3.3 Toast的Window的创建过程

Toast的Window的创建过程与Dialog的Window的创建过程类似，但是由于Toast具有定时取消的功能，所以系统采用了Handler。