Android 四大组件之——Acitivity(三) 深入了解Activity的启动流程

上图为整个Activity的启动流程

接下来我们大概分析

在我们的Android系统中，应用程序是由Launcher这个应用启动起来的。当我们安装好应用程序之后，就会在Launcher的界面上生成一个图标，我们点击图标时Launch就会启动我们的应用程序。

1.点击图标，launcher调用onClick方法

 /**
     * 
     * 
     * @param v      The view representing the clicked shortcut.  1.此处的view是指被点击的桌面图标
     */
    public void onClick(View v) {
        // Make sure that rogue clicks don't get through while allapps is
        // launching, or after the
        // view has detached (it's possible for this to happen if the view is
        // removed mid touch).
        if (v.getWindowToken() == null) {
            return;
        }

        if (!mWorkspace.isFinishedSwitchingState()) {
            return;
        }

        Object tag = v.getTag();
        if (tag instanceof ShortcutInfo) {
            // 打开快捷方式对应的intent
            final Intent intent = ((ShortcutInfo) tag).intent;
            int[] pos = new int[2];
            v.getLocationOnScreen(pos);
            intent.setSourceBounds(new Rect(pos[0], pos[1], pos[0]
                    + v.getWidth(), pos[1] + v.getHeight()));

            boolean success = startActivitySafely(v, intent, tag);//2.同时调用startActivitySafely
            if (success && v instanceof BubbleTextView) {
                mWaitingForResume = (BubbleTextView) v;
                mWaitingForResume.setStayPressed(true);
            }
        } else if (tag instanceof FolderInfo) {
            if (v instanceof FolderIcon) {
                // 打开文件夹
                FolderIcon fi = (FolderIcon) v;
                handleFolderClick(fi);
            }
        } else if (v == mAllAppsButton) {
            // 显示或者不显示“全部程序”界面
            if (isAllAppsVisible()) {
                showWorkspace(true);
            } else {
                onClickAllAppsButton(v);
            }
        }
    }

2.在launcher的onClick方法里调用了startActivitySafely()方法，见上述代码

boolean startActivitySafely(View v, Intent intent, Object tag) {
        boolean success = false;
        try {
            success = startActivity(v, intent, tag);  由此可见StartActivitySafely(),最终是调用startActivity()方法
        } catch (ActivityNotFoundException e) {
            Toast.makeText(this, R.string.activity_not_found,
                    Toast.LENGTH_SHORT).show();
            Log.e(TAG, "Unable to launch. tag=" + tag + " intent=" + intent, e);
        }
        return success;
    }

 boolean startActivity(View v, Intent intent, Object tag) {
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

        try {
            // 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);
            if (useLaunchAnimation) {
                ActivityOptions opts = ActivityOptions.makeScaleUpAnimation(v,
                        0, 0, v.getMeasuredWidth(), v.getMeasuredHeight());

                startActivity(intent, opts.toBundle());
            } else {
                startActivity(intent);
            }
            return true;
        } catch (SecurityException e) {
            Toast.makeText(this, R.string.activity_not_found,
                    Toast.LENGTH_SHORT).show();
            Log.e(TAG,
                    "Launcher does not have the permission to launch "
                            + intent
                            + ". Make sure to create a MAIN intent-filter for the corresponding activity "
                            + "or use the exported attribute for this activity. "
                            + "tag=" + tag + " intent=" + intent, e);
        }
        return false;
    }

3.在startActivity()方法中，intent 添加了flag FLAG_ACTIVITY_NEW_TASK , 此标志为创建新的任务栈，在创建目标

任务栈之前，首先会调用ActivityThread类启动launcher的进程，然后启动目标应用的任务栈，启动完成之后，目标应用的任务栈会通知

ActivityThread调用launcher的onPause方法。此时，ActivityThread启动新的应用进程（也就是目标应用的进程）。

4.目标应用的进程通过loadClass加载MainActivity，然后通过H(handler)来控制MainActivity的生命周期

Android 四大组件之——Acitivity(三) 深入了解Activity的启动流程

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