前言

“当Dialog弹出来时，Dialog下面的控件能否点击？”
“当然是不能点击啊”
“那么为什么不能点击呢？”
“额，emmmm”

---

相信不少人对于Dialog使用驾轻就熟了，对于Dialog下面的控件能否点击也是非常清楚，但是当被问之原理时，可能有部分童鞋就答不出来了。那么这里我们就来从源码层面上面讲解下这个原因吧。
首先我们先来看看Dialog是怎么使用的：

            Dialog dialog = new Dialog(TestActivity.this);
            dialog.setContentView(R.layout.dialog_test);
            dialog.show();

那么我们看到有三步，我们一步一步讲

    Dialog(@NonNull Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
        if (createContextThemeWrapper) {
            if (themeResId == ResourceId.ID_NULL) {
                final TypedValue outValue = new TypedValue();
                context.getTheme().resolveAttribute(R.attr.dialogTheme, outValue, true);
                themeResId = outValue.resourceId;
            }
            mContext = new ContextThemeWrapper(context, themeResId);
        } else {
            mContext = context;
        }

        mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        //新建PhoneWindow
        final Window w = new PhoneWindow(mContext);
        mWindow = w;
        //将回调注入到window当中
        w.setCallback(this);
        w.setOnWindowDismissedCallback(this);
        w.setOnWindowSwipeDismissedCallback(() -> {
            if (mCancelable) {
                cancel();
            }
        });
        w.setWindowManager(mWindowManager, null, null);
        w.setGravity(Gravity.CENTER);

        mListenersHandler = new ListenersHandler(this);
    }

从代码上面可以看出，这里是新建了一个PhoneWindow，Dialog和View之间都是通过Window来交互，这一点跟Activity和View之间的关系也是类似。

    /**
     * Set the screen content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the screen.
     * 
     * @param layoutResID Resource ID to be inflated.
     */
    public void setContentView(@LayoutRes int layoutResID) {
        mWindow.setContentView(layoutResID);
    }

代码就一行，这里我们可以看到Dialog其实把主要处理逻辑也都交给了window，这里面的window对象其实就是PhoneWindow，所以我们去看下PhoneWindow里面的操作：

    @Override
    public void setContentView(int layoutResID) {
        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
        // decor, when theme attributes and the like are crystalized. Do not check the feature
        // before this happens.
        if (mContentParent == null) {
            //初始化decorView
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }
        ...代码省略...
    }

    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);
                    }
                }

                mDecorContentParent.setUiOptions(mUiOptions);

                if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) != 0 ||
                        (mIconRes != 0 && !mDecorContentParent.hasIcon())) {
                    mDecorContentParent.setIcon(mIconRes);
                } else if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) == 0 &&
                        mIconRes == 0 && !mDecorContentParent.hasIcon()) {
                    mDecorContentParent.setIcon(
                            getContext().getPackageManager().getDefaultActivityIcon());
                    mResourcesSetFlags |= FLAG_RESOURCE_SET_ICON_FALLBACK;
                }
                if ((mResourcesSetFlags & FLAG_RESOURCE_SET_LOGO) != 0 ||
                        (mLogoRes != 0 && !mDecorContentParent.hasLogo())) {
                    mDecorContentParent.setLogo(mLogoRes);
                }

                // Invalidate if the panel menu hasn't been created before this.
                // Panel menu invalidation is deferred avoiding application onCreateOptionsMenu
                // being called in the middle of onCreate or similar.
                // A pending invalidation will typically be resolved before the posted message
                // would run normally in order to satisfy instance state restoration.
                PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
                if (!isDestroyed() && (st == null || st.menu == null) && !mIsStartingWindow) {
                    invalidatePanelMenu(FEATURE_ACTION_BAR);
                }
            } else {
                mTitleView = findViewById(R.id.title);
                if (mTitleView != null) {
                    if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
                        final View titleContainer = findViewById(R.id.title_container);
                        if (titleContainer != null) {
                            titleContainer.setVisibility(View.GONE);
                        } else {
                            mTitleView.setVisibility(View.GONE);
                        }
                        mContentParent.setForeground(null);
                    } else {
                        mTitleView.setText(mTitle);
                    }
                }
            }

            if (mDecor.getBackground() == null && mBackgroundFallbackResource != 0) {
                mDecor.setBackgroundFallback(mBackgroundFallbackResource);
            }

            // Only inflate or create a new TransitionManager if the caller hasn't
            // already set a custom one.
            if (hasFeature(FEATURE_ACTIVITY_TRANSITIONS)) {
                if (mTransitionManager == null) {
                    final int transitionRes = getWindowStyle().getResourceId(
                            R.styleable.Window_windowContentTransitionManager,
                            0);
                    if (transitionRes != 0) {
                        final TransitionInflater inflater = TransitionInflater.from(getContext());
                        mTransitionManager = inflater.inflateTransitionManager(transitionRes,
                                mContentParent);
                    } else {
                        mTransitionManager = new TransitionManager();
                    }
                }

                mEnterTransition = getTransition(mEnterTransition, null,
                        R.styleable.Window_windowEnterTransition);
                mReturnTransition = getTransition(mReturnTransition, USE_DEFAULT_TRANSITION,
                        R.styleable.Window_windowReturnTransition);
                mExitTransition = getTransition(mExitTransition, null,
                        R.styleable.Window_windowExitTransition);
                mReenterTransition = getTransition(mReenterTransition, USE_DEFAULT_TRANSITION,
                        R.styleable.Window_windowReenterTransition);
                mSharedElementEnterTransition = getTransition(mSharedElementEnterTransition, null,
                        R.styleable.Window_windowSharedElementEnterTransition);
                mSharedElementReturnTransition = getTransition(mSharedElementReturnTransition,
                        USE_DEFAULT_TRANSITION,
                        R.styleable.Window_windowSharedElementReturnTransition);
                mSharedElementExitTransition = getTransition(mSharedElementExitTransition, null,
                        R.styleable.Window_windowSharedElementExitTransition);
                mSharedElementReenterTransition = getTransition(mSharedElementReenterTransition,
                        USE_DEFAULT_TRANSITION,
                        R.styleable.Window_windowSharedElementReenterTransition);
                if (mAllowEnterTransitionOverlap == null) {
                    mAllowEnterTransitionOverlap = getWindowStyle().getBoolean(
                            R.styleable.Window_windowAllowEnterTransitionOverlap, true);
                }
                if (mAllowReturnTransitionOverlap == null) {
                    mAllowReturnTransitionOverlap = getWindowStyle().getBoolean(
                            R.styleable.Window_windowAllowReturnTransitionOverlap, true);
                }
                if (mBackgroundFadeDurationMillis < 0) {
                    mBackgroundFadeDurationMillis = getWindowStyle().getInteger(
                            R.styleable.Window_windowTransitionBackgroundFadeDuration,
                            DEFAULT_BACKGROUND_FADE_DURATION_MS);
                }
                if (mSharedElementsUseOverlay == null) {
                    mSharedElementsUseOverlay = getWindowStyle().getBoolean(
                            R.styleable.Window_windowSharedElementsUseOverlay, true);
                }
            }
        }
    }

重点关注installDecor()。window都会先初始化一个DecorView，然后把我们setContentView进来的View给添加到这个对应的DecorView里面去（Activity里面的Window操作亦是如此）。所以这里先初始化了一个DecorView。然后将window注入到decorView当中去。

    /**
     * Start the dialog and display it on screen.  The window is placed in the
     * application layer and opaque.  Note that you should not override this
     * method to do initialization when the dialog is shown, instead implement
     * that in {@link #onStart}.
     */
    public void show() {
        ...代码省略...

        mWindowManager.addView(mDecor, l);
      ...代码省略
    }

这里通过windowManager将decorView添加到window上面去，此处WindowManager是WindowManagerImpl,然后又执行WindowManagerGlobal的addView方法。

    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
             ...代码省略...

            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;
            }
        }
    }

关键一步在这里，我们可以看到new了一个ViewRootImpl的对象，并且将DecorView注入进去，赋值给mView。
通过原来Android触控机制竟是这样的？这篇文章，这才找到了源头，所有的点击事件通过硬件设备检测，经过底层会调用到ViewRootImpl的ViewPostImeInputStage里面来。但是因为ViewRootImpl存在着多个，到底是哪个会接收到回调呢，十分钟了解Android触摸事件原理（InputManagerService)告诉我们会根据Z轴的高度，获取最近一个窗口，然后执行对应ViewRootImpl里面ViewPostImeInputStage的监听方法->执行mView.dispatchPointerEvent(event)。
此处的mView便是Dialog所在的DecorView，然后才开始由decorView进行事件分发,我们先看View的dispatchPointerEvent方法：

    /**
     * Dispatch a pointer event.
     * <p>
     * Dispatches touch related pointer events to {@link #onTouchEvent(MotionEvent)} and all
     * other events to {@link #onGenericMotionEvent(MotionEvent)}.  This separation of concerns
     * reinforces the invariant that {@link #onTouchEvent(MotionEvent)} is really about touches
     * and should not be expected to handle other pointing device features.
     * </p>
     *
     * @param event The motion event to be dispatched.
     * @return True if the event was handled by the view, false otherwise.
     * @hide
     */
    public final boolean dispatchPointerEvent(MotionEvent event) {
        if (event.isTouchEvent()) {
            return dispatchTouchEvent(event);
        } else {
            return dispatchGenericMotionEvent(event);
        }
    }

这里会执行decorView的dispatchTouchEvent：

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        final Window.Callback cb = mWindow.getCallback();
        return cb != null && !mWindow.isDestroyed() && mFeatureId < 0
                ? cb.dispatchTouchEvent(ev) : super.dispatchTouchEvent(ev);
    }

因为当前decorView注入的窗口是在创建dialog时新建的，另外dialog本身就是实现了Window.Callback接口，第一步当中已经将dialog注入到window当中了，因此此处的cb对象就是dialog对象，这时就直接调用了dialog的dispatchTouchEvent方法。接下来就是熟知的事件分发流程了。此处便不再多做介绍。

---

结论：

Activity的dispatchTouchEvent也是从对应的decorView的dispatchTouchEvent中分发出来的，而Activity所处的decorView跟Dialog所处的decorView并不属于同一个，所以Activity自然接收不到任何点击事件。