就像上個文章說的，觸控事件的傳遞機制是從外層到內層的過程。
我們想來看看這個頁面裡面的層級關係：

以下我們就用what-how-why三部曲的方式來分析View的繪製過程。
由於篇幅很大，所以分幾篇來解析這個過程。
這篇主要是自定義view/viewgroup，以及從Activity到DecorView的載入過程。

1.what：怎麼自定義一個View

1.1自定義View

自定義View的話，常見過程如下：

/**
 *   @author     DemanMath
 *   @date       2020-02-16
 *
 */
class CustomView : View {

    constructor(context: Context):super(context)
    constructor(context: Context,attributeSet: AttributeSet):super(context,attributeSet)
    constructor(context: Context,attributeSet: AttributeSet,def:Int):super(context,attributeSet,def)

    override fun onDraw(canvas: Canvas?) {
        super.onDraw(canvas)
        AppLog.i()
    }

    override fun onLayout(changed: Boolean, left: Int, top: Int, right: Int, bottom: Int) {
        super.onLayout(changed, left, top, right, bottom)
        AppLog.i()
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
        AppLog.i()
    }

}
 

三個構造方法+三個可以複寫的方法。
我們先看下這3個方法的順序：

2020-02-16 13:50:28.212 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.222 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.253 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.255 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.259 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onLayout:  [at (CustomView.kt:27)]
2020-02-16 13:50:28.403 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onDraw:  [at (CustomView.kt:22)]
 

1.2自定義ViewGroup

上程式碼

package com.joyfulmath.androidarchitecture.view

import android.content.Context
import android.util.AttributeSet
import android.view.ViewGroup
import com.joyfulmath.androidarchitecture.base.AppLog
import kotlin.math.PI
import kotlin.math.cos
import kotlin.math.min
import kotlin.math.sin

/**
 *   @author     DemanMath
 *   @date       2020-02-16
 *
 */
class FerrisWheel:ViewGroup {

    var count = 12
    var a = 2*PI/count
    var startA = PI/2

    constructor(context: Context):super(context){
        initViews()
    }
    constructor(context: Context,attributeSet: AttributeSet):super(context,attributeSet){
        initViews()
    }
    constructor(context: Context,attributeSet: AttributeSet,def:Int):super(context,attributeSet,def){
        initViews()
    }

    private fun initViews() {
        for(i in 0 until count){
            this.addView(CustomView(context))
        }
    }

    override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {
        var mViewWidth = measuredWidth
        var mViewHeight = measuredHeight
        AppLog.i("$mViewWidth,$mViewHeight")
        var cx = mViewWidth/2
        var cy = mViewHeight/2
        var r  = min(measuredWidth,measuredHeight)*0.5f -20
        AppLog.i("r:$r,cx:$cx")
        for(i in 0 until count){
            var view = getChildAt(i)
            var width = view.measuredWidth
            var height = view.measuredHeight
            var cx1 = r* sin(startA+a*i)
            var cy1 = -r* cos(startA+a*i)
            AppLog.i("width:$width,height:$height")
            AppLog.i("cx1:$cx1,cy1:$cy1")
            view.layout(cx+(cx1-width/2).toInt(),
                cy+(cy1-height/2).toInt(),
                cx+(cx1+width/2).toInt(),
                cy+(cy1+height/2).toInt())
        }
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        measureChildren(widthMeasureSpec,heightMeasureSpec)
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
    }
}
 

效果如下：

這裡override了layout方法。可見View的繪製跟他的父View只有一個關係，ViewGroup指定了子View的位置。
關於View/ViewGroup繪製的機制，在下一節討論。

2.How：View的繪製機制是什麼

從上一節看出：整個繪製流程三個過程，measure，layout，draw這三個過程。
下面我們從原始碼的角度來分析下是不是這個過程。

final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
        .......

        // TODO Push resumeArgs into the activity for consideration
        r = performResumeActivity(token, clearHide, reason);

        if (r != null) {
            final Activity a = r.activity;

            if (localLOGV) Slog.v(
                TAG, "Resume " + r + " started activity: " +
                a.mStartedActivity + ", hideForNow: " + r.hideForNow
                + ", finished: " + a.mFinished);

            final int forwardBit = isForward ?
                    WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;

            // If the window hasn't yet been added to the window manager,
            // and this guy didn't finish itself or start another activity,
            // then go ahead and add the window.
            boolean willBeVisible = !a.mStartedActivity;
            if (!willBeVisible) {
                try {
                    willBeVisible = ActivityManager.getService().willActivityBeVisible(
                            a.getActivityToken());
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
            if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
                if (r.mPreserveWindow) {
                    a.mWindowAdded = true;
                    r.mPreserveWindow = false;
                    // Normally the ViewRoot sets up callbacks with the Activity
                    // in addView->ViewRootImpl#setView. If we are instead reusing
                    // the decor view we have to notify the view root that the
                    // callbacks may have changed.
                    ViewRootImpl impl = decor.getViewRootImpl();
                    if (impl != null) {
                        impl.notifyChildRebuilt();
                    }
                }
                if (a.mVisibleFromClient) {
                    if (!a.mWindowAdded) {
                        a.mWindowAdded = true;
                        wm.addView(decor, l);
                    } else {
                        // The activity will get a callback for this {@link LayoutParams} change
                        // earlier. However, at that time the decor will not be set (this is set
                        // in this method), so no action will be taken. This call ensures the
                        // callback occurs with the decor set.
                        a.onWindowAttributesChanged(l);
                    }
                }

            // If the window has already been added, but during resume
            // we started another activity, then don't yet make the
            // window visible.
            } else if (!willBeVisible) {
                if (localLOGV) Slog.v(
                    TAG, "Launch " + r + " mStartedActivity set");
                r.hideForNow = true;
            }

            // Get rid of anything left hanging around.
            cleanUpPendingRemoveWindows(r, false /* force */);

            // The window is now visible if it has been added, we are not
            // simply finishing, and we are not starting another activity.
            if (!r.activity.mFinished && willBeVisible
                    && r.activity.mDecor != null && !r.hideForNow) {
                if (r.newConfig != null) {
                    performConfigurationChangedForActivity(r, r.newConfig);
                    if (DEBUG_CONFIGURATION) Slog.v(TAG, "Resuming activity "
                            + r.activityInfo.name + " with newConfig " + r.activity.mCurrentConfig);
                    r.newConfig = null;
                }
                if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward="
                        + isForward);
                WindowManager.LayoutParams l = r.window.getAttributes();
                if ((l.softInputMode
                        & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
                        != forwardBit) {
                    l.softInputMode = (l.softInputMode
                            & (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
                            | forwardBit;
                    if (r.activity.mVisibleFromClient) {
                        ViewManager wm = a.getWindowManager();
                        View decor = r.window.getDecorView();
                        wm.updateViewLayout(decor, l);
                    }
                }

                r.activity.mVisibleFromServer = true;
                mNumVisibleActivities++;
                if (r.activity.mVisibleFromClient) {
                    r.activity.makeVisible();
                }
            }

            if (!r.onlyLocalRequest) {
                r.nextIdle = mNewActivities;
                mNewActivities = r;
                if (localLOGV) Slog.v(
                    TAG, "Scheduling idle handler for " + r);
                Looper.myQueue().addIdleHandler(new Idler());
            }
            r.onlyLocalRequest = false;

            // Tell the activity manager we have resumed.
            if (reallyResume) {
                try {
                    ActivityManager.getService().activityResumed(token);
                } catch (RemoteException ex) {
                    throw ex.rethrowFromSystemServer();
                }
            }

        } else {
            // If an exception was thrown when trying to resume, then
            // just end this activity.
            try {
                ActivityManager.getService()
                    .finishActivity(token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

2.1 關鍵是頁面繪製流程

整個的過程就是一開始講的層級關係。
第一點：performResumeActivity 比wm.addView(decor, l)先執行。所以Activity是先獲取焦點，才繪製view。
performResumeActivity->r.activity.performResume()->mInstrumentation.callActivityOnResume(this)->activity.onResume()
在performResume最後可以看到onPostResume

final void performResume() {
        performRestart();
        ...
        // mResumed is set by the instrumentation
        mInstrumentation.callActivityOnResume(this);
      ...
        onPostResume();
       ...
    }
    
    protected void onPostResume() {
        final Window win = getWindow();
        if (win != null) win.makeActive();
        if (mActionBar != null) mActionBar.setShowHideAnimationEnabled(true);
        mCalled = true;
    }

window出現了，這個就是phonewindow。
下面我們去看docorview的過程。

//2020.02.18 phonewindow在這裡獲取
if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                //2020.02.18 docorview在這裡獲取
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
               ...
                if (a.mVisibleFromClient) {
                    if (!a.mWindowAdded) {
                        a.mWindowAdded = true;
                        wm.addView(decor, l);
                    } else {
                        // The activity will get a callback for this {@link LayoutParams} change
                        // earlier. However, at that time the decor will not be set (this is set
                        // in this method), so no action will be taken. This call ensures the
                        // callback occurs with the decor set.
                        a.onWindowAttributesChanged(l);
                    }
                }

我們來看下wm.addView(decor, l);這個的過程。wm的實現就是WindowManagerImpl

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

mGlobal是WindowManagerGlobal， addview的核心程式碼如下

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

            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);
            
            root.setView(view, wparams, panelParentView);
    

關於從ViewGroup開始的繪製流程，請看下篇。

更多內容：demanmath
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