1.1 从Launcher启动
首先我们通过git命令获取到Launcher的最新源码
git clone -b master https://android.googlesource.com/platform/packages/apps/Launcher3

1.1.1进入到Launcher.java 找到

public void onClick(View v) {
      .....
     .......
        Object tag = v.getTag();
        if (tag instanceof ShortcutInfo) {
          //点击应用图标
            onClickAppShortcut(v);
        } else if (tag instanceof FolderInfo) {
            if (v instanceof FolderIcon) {
                onClickFolderIcon(v);
            }
        } 
      ...
      ...
}

1.1.2 进入OnClickAppShortcut(v)

/**
 * Event handler for an app shortcut click.
 *
 * @param v The view that was clicked. Must be a tagged with a {@link ShortcutInfo}.
 */
protected void onClickAppShortcut(final View v) {
    if (LOGD) Log.d(TAG, "onClickAppShortcut");
    Object tag = v.getTag();
    if (!(tag instanceof ShortcutInfo)) {
        throw new IllegalArgumentException("Input must be a Shortcut");
    }

    // Open shortcut
    final ShortcutInfo shortcut = (ShortcutInfo) tag;

    if (shortcut.isDisabled != 0) {
        if ((shortcut.isDisabled &
                ~ShortcutInfo.FLAG_DISABLED_SUSPENDED &
                ~ShortcutInfo.FLAG_DISABLED_QUIET_USER) == 0) {
            // If the app is only disabled because of the above flags, launch activity anyway.
            // Framework will tell the user why the app is suspended.
        } else {
            if (!TextUtils.isEmpty(shortcut.disabledMessage)) {
                // Use a message specific to this shortcut, if it has one.
                Toast.makeText(this, shortcut.disabledMessage, Toast.LENGTH_SHORT).show();
                return;
            }
            // Otherwise just use a generic error message.
            int error = R.string.activity_not_available;
            if ((shortcut.isDisabled & ShortcutInfo.FLAG_DISABLED_SAFEMODE) != 0) {
                error = R.string.safemode_shortcut_error;
            } else if ((shortcut.isDisabled & ShortcutInfo.FLAG_DISABLED_BY_PUBLISHER) != 0 ||
                    (shortcut.isDisabled & ShortcutInfo.FLAG_DISABLED_LOCKED_USER) != 0) {
                error = R.string.shortcut_not_available;
            }
            Toast.makeText(this, error, Toast.LENGTH_SHORT).show();
            return;
        }
    }

    // Check for abandoned promise
    if ((v instanceof BubbleTextView) && shortcut.hasPromiseIconUi()) {
        String packageName = shortcut.intent.getComponent() != null ?
                shortcut.intent.getComponent().getPackageName() : shortcut.intent.getPackage();
        if (!TextUtils.isEmpty(packageName)) {
            onClickPendingAppItem(v, packageName,
                    shortcut.hasStatusFlag(ShortcutInfo.FLAG_INSTALL_SESSION_ACTIVE));
            return;
        }
    }

    // Start activities
    startAppShortcutOrInfoActivity(v);
}

1.1.3进入 startAppShortcutOrInfoActivity（）

private void startAppShortcutOrInfoActivity(View v) {
    ItemInfo item = (ItemInfo) v.getTag();
    Intent intent;
    if (item instanceof PromiseAppInfo) {
        PromiseAppInfo promiseAppInfo = (PromiseAppInfo) item;
        intent = promiseAppInfo.getMarketIntent();
    } else {
        intent = item.getIntent();
    }
    if (intent == null) {
        throw new IllegalArgumentException("Input must have a valid intent");
    }
    boolean success = startActivitySafely(v, intent, item);
    getUserEventDispatcher().logAppLaunch(v, intent); // TODO for discovered apps b/35802115

    if (success && v instanceof BubbleTextView) {
        mWaitingForResume = (BubbleTextView) v;
        mWaitingForResume.setStayPressed(true);
    }
}

1.1.4 进入startActivitySafely

    public boolean startActivitySafely(View v, Intent intent, ItemInfo item) {
        if (mIsSafeModeEnabled && !Utilities.isSystemApp(this, intent)) {
            Toast.makeText(this, R.string.safemode_shortcut_error, Toast.LENGTH_SHORT).show();
            return false;
        }
        // Only launch using the new animation if the shortcut has not opted out (this is a
        // private contract between launcher and may be ignored in the future).
        boolean useLaunchAnimation = (v != null) &&
                !intent.hasExtra(INTENT_EXTRA_IGNORE_LAUNCH_ANIMATION);
        Bundle optsBundle = useLaunchAnimation ? getActivityLaunchOptions(v) : null;

        UserHandle user = item == null ? null : item.user;

        // Prepare intent
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        if (v != null) {
            intent.setSourceBounds(getViewBounds(v));
        }
        try {
            if (Utilities.ATLEAST_MARSHMALLOW
                    && (item instanceof ShortcutInfo)
                    && (item.itemType == Favorites.ITEM_TYPE_SHORTCUT
                     || item.itemType == Favorites.ITEM_TYPE_DEEP_SHORTCUT)
                    && !((ShortcutInfo) item).isPromise()) { //注释1
                // Shortcuts need some special checks due to legacy reasons.
                startShortcutIntentSafely(intent, optsBundle, item);
            } else if (user == null || user.equals(Process.myUserHandle())) {//注释2
                // Could be launching some bookkeeping activity
                startActivity(intent, optsBundle);
            } else { //注释3
                LauncherAppsCompat.getInstance(this).startActivityForProfile(
                        intent.getComponent(), user, intent.getSourceBounds(), optsBundle);
            }
            return true;
        } catch (ActivityNotFoundException|SecurityException e) {
            Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
            Log.e(TAG, "Unable to launch. tag=" + item + " intent=" + intent, e);
        }
        return false;
    }

分析
方法中有三个if else的判断
注释1
6.0以上的系统，快捷方式启动，需要做一些校验。所谓的快捷方式，是google提供的新特性，用于启动应用内的快捷操作，譬如是 发信息给某一个用户。
注释2
如果user为空 或就是launcher所在用户
startActivity() //启动Activity的地方
注释3 （哪种情况会走这里的逻辑，待确认）

1.2 APP内部启动
1.2.1 显式启动
直接指定我们要启动的class名字

    private void ExplicitStartActivity(){

        Intent intent = new Intent(this,ExplicitIntentActivity.class);
        startActivity(intent);

    }

1.2.2 隐式启动
设定intent的action,让系统去匹配查找

    private void ImplicitStartActivity(){

        Intent intent = new Intent("com.shine.activitystudy.ImplicitIntentActivity");

        startActivity(intent);

    }

1.3 从startActivity开始

可以看到 无论是从launcher启动还是app内部启动，最后都会调用到Activity.java的startActivity()方法。

1.3.1 startActivity分析

    public void startActivity(Intent intent, @Nullable Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1); //注释1
        }
    }

以options是否为null来调用startActivityForResult()
来看注释1的实现

    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
        startActivityForResult(intent, requestCode, null);
    }

也是调用到了这个方法。

 public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) 

下一节，我们就以

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
  @Nullable Bundle options) 

作为开始来进一步分析Activity是如何启动的。

参考文章
http://www.lovemingnuo.com/2018/03/16/App的启动流程（Android8.0）%20第一篇/
http://www.voidcn.com/article/p-krltxsbq-qh.html

快捷方式启动 参考
https://blog.csdn.net/qibin0506/article/details/52878690

android 8.0创建快捷方式
https://blog.csdn.net/rentee/article/details/77005547

google官方Intent介绍
https://developer.android.com/guide/components/intents-filters?hl=zh-cn