对Android近期任务列表（Recent Applications）的简单分析

转载自：

http://www.cnblogs.com/coding-way/archive/2013/06/05/3118732.html

这里的近期任务列表就是长按Home键出来的那个Dialog，里面放着近期打开过的应用，当然3.0以上系统的多任务切换键也是。

这个Dialog的实现在Android源码的/frameworks/base/policy/src/com/android/internal/policy/impl/RecentApplicationsDialog.java中。

接下来就对这个源码分析一下。

先把整个源码贴出来：

public class RecentApplicationsDialog extends Dialog implements OnClickListener {    // Elements for debugging support//  private static final String LOG_TAG = "RecentApplicationsDialog";    private static final boolean DBG_FORCE_EMPTY_LIST = false;    static private StatusBarManager sStatusBar;    private static final int NUM_BUTTONS = 8;    private static final int MAX_RECENT_TASKS = NUM_BUTTONS * 2;    // allow for some discards    final TextView[] mIcons = new TextView[NUM_BUTTONS];    View mNoAppsText;    IntentFilter mBroadcastIntentFilter = new IntentFilter(Intent.ACTION_CLOSE_SYSTEM_DIALOGS);    class RecentTag {        ActivityManager.RecentTaskInfo info;        Intent intent;    }    Handler mHandler = new Handler();    Runnable mCleanup = new Runnable() {        public void run() {            // dump extra memory we're hanging on to            for (TextView icon: mIcons) {                icon.setCompoundDrawables(null, null, null, null);                icon.setTag(null);            }        }    };    public RecentApplicationsDialog(Context context) {        super(context, com.android.internal.R.style.Theme_Dialog_RecentApplications);    }    /**     * We create the recent applications dialog just once, and it stays around (hidden)     * until activated by the user.     *     * @see PhoneWindowManager#showRecentAppsDialog     */    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Context context = getContext();        if (sStatusBar == null) {            sStatusBar = (StatusBarManager)context.getSystemService(Context.STATUS_BAR_SERVICE);        }        Window window = getWindow();        window.requestFeature(Window.FEATURE_NO_TITLE);        window.setType(WindowManager.LayoutParams.TYPE_SYSTEM_DIALOG);        window.setFlags(WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM,                WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM);        window.setTitle("Recents");        setContentView(com.android.internal.R.layout.recent_apps_dialog);        final WindowManager.LayoutParams params = window.getAttributes();        params.width = WindowManager.LayoutParams.MATCH_PARENT;        params.height = WindowManager.LayoutParams.MATCH_PARENT;        window.setAttributes(params);        window.setFlags(0, WindowManager.LayoutParams.FLAG_DIM_BEHIND);        //默认显示8个        mIcons[0] = (TextView)findViewById(com.android.internal.R.id.button0);        mIcons[1] = (TextView)findViewById(com.android.internal.R.id.button1);        mIcons[2] = (TextView)findViewById(com.android.internal.R.id.button2);        mIcons[3] = (TextView)findViewById(com.android.internal.R.id.button3);        mIcons[4] = (TextView)findViewById(com.android.internal.R.id.button4);        mIcons[5] = (TextView)findViewById(com.android.internal.R.id.button5);        mIcons[6] = (TextView)findViewById(com.android.internal.R.id.button6);        mIcons[7] = (TextView)findViewById(com.android.internal.R.id.button7);        mNoAppsText = findViewById(com.android.internal.R.id.no_applications_message);        //关键在哪，你懂得...        for (TextView b: mIcons) {            b.setOnClickListener(this);        }    }    @Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        if (keyCode == KeyEvent.KEYCODE_TAB) {            // Ignore all meta keys other than SHIFT.  The app switch key could be a            // fallback action chorded with ALT, META or even CTRL depending on the key map.            // DPad navigation is handled by the ViewRoot elsewhere.            final boolean backward = event.isShiftPressed();            final int numIcons = mIcons.length;            int numButtons = 0;            while (numButtons < numIcons && mIcons[numButtons].getVisibility() == View.VISIBLE) {                numButtons += 1;            }            if (numButtons != 0) {                int nextFocus = backward ? numButtons - 1 : 0;                for (int i = 0; i < numButtons; i++) {                    if (mIcons[i].hasFocus()) {                        if (backward) {                            nextFocus = (i + numButtons - 1) % numButtons;                        } else {                            nextFocus = (i + 1) % numButtons;                        }                        break;                    }                }                final int direction = backward ? View.FOCUS_BACKWARD : View.FOCUS_FORWARD;                if (mIcons[nextFocus].requestFocus(direction)) {                    mIcons[nextFocus].playSoundEffect(                            SoundEffectConstants.getContantForFocusDirection(direction));                }            }            // The dialog always handles the key to prevent the ViewRoot from            // performing the default navigation itself.            return true;        }        return super.onKeyDown(keyCode, event);    }    /**     * Dismiss the dialog and switch to the selected application.     */    public void dismissAndSwitch() {        final int numIcons = mIcons.length;        RecentTag tag = null;        for (int i = 0; i < numIcons; i++) {            if (mIcons[i].getVisibility() != View.VISIBLE) {                break;            }            if (i == 0 || mIcons[i].hasFocus()) {                tag = (RecentTag) mIcons[i].getTag();                if (mIcons[i].hasFocus()) {                    break;                }            }        }        if (tag != null) {            switchTo(tag);        }        dismiss();    }    /**     * Handler for user clicks.  If a button was clicked, launch the corresponding activity.     */    public void onClick(View v) {        for (TextView b: mIcons) {            if (b == v) {                RecentTag tag = (RecentTag)b.getTag();                switchTo(tag);                break;            }        }        dismiss();    }    //    private void switchTo(RecentTag tag) {        if (tag.info.id >= 0) {            // This is an active task; it should just go to the foreground.            final ActivityManager am = (ActivityManager)                    getContext().getSystemService(Context.ACTIVITY_SERVICE);            am.moveTaskToFront(tag.info.id, ActivityManager.MOVE_TASK_WITH_HOME);        } else if (tag.intent != null) {            tag.intent.addFlags(Intent.FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY                    | Intent.FLAG_ACTIVITY_TASK_ON_HOME);            try {                getContext().startActivity(tag.intent);            } catch (ActivityNotFoundException e) {                Log.w("Recent", "Unable to launch recent task", e);            }        }    }    /**     * Set up and show the recent activities dialog.     */    @Override    public void onStart() {        super.onStart();        reloadButtons();        if (sStatusBar != null) {            sStatusBar.disable(StatusBarManager.DISABLE_EXPAND);        }        // receive broadcasts        getContext().registerReceiver(mBroadcastReceiver, mBroadcastIntentFilter);        mHandler.removeCallbacks(mCleanup);    }    /**     * Dismiss the recent activities dialog.     */    @Override    public void onStop() {        super.onStop();        if (sStatusBar != null) {            sStatusBar.disable(StatusBarManager.DISABLE_NONE);        }        // stop receiving broadcasts        getContext().unregisterReceiver(mBroadcastReceiver);        mHandler.postDelayed(mCleanup, 100);     }    /**     * Reload the 6 buttons with recent activities     */    private void reloadButtons() {        final Context context = getContext();        final PackageManager pm = context.getPackageManager();        final ActivityManager am = (ActivityManager)                context.getSystemService(Context.ACTIVITY_SERVICE);        final List<ActivityManager.RecentTaskInfo> recentTasks =                am.getRecentTasks(MAX_RECENT_TASKS, ActivityManager.RECENT_IGNORE_UNAVAILABLE);        ActivityInfo homeInfo =             new Intent(Intent.ACTION_MAIN).addCategory(Intent.CATEGORY_HOME)                    .resolveActivityInfo(pm, 0);        IconUtilities iconUtilities = new IconUtilities(getContext());        // Performance note:  Our android performance guide says to prefer Iterator when        // using a List class, but because we know that getRecentTasks() always returns        // an ArrayList<>, we'll use a simple index instead.        int index = 0;        int numTasks = recentTasks.size();        for (int i = 0; i < numTasks && (index < NUM_BUTTONS); ++i) {            final ActivityManager.RecentTaskInfo info = recentTasks.get(i);            // for debug purposes only, disallow first result to create empty lists            if (DBG_FORCE_EMPTY_LIST && (i == 0)) continue;            Intent intent = new Intent(info.baseIntent);            if (info.origActivity != null) {                intent.setComponent(info.origActivity);            }            // Skip the current home activity.            if (homeInfo != null) {                if (homeInfo.packageName.equals(                        intent.getComponent().getPackageName())                        && homeInfo.name.equals(                                intent.getComponent().getClassName())) {                    continue;                }            }            intent.setFlags((intent.getFlags()&~Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)                    | Intent.FLAG_ACTIVITY_NEW_TASK);            final ResolveInfo resolveInfo = pm.resolveActivity(intent, 0);            if (resolveInfo != null) {                final ActivityInfo activityInfo = resolveInfo.activityInfo;                final String title = activityInfo.loadLabel(pm).toString();                Drawable icon = activityInfo.loadIcon(pm);                if (title != null && title.length() > 0 && icon != null) {                    final TextView tv = mIcons[index];                    tv.setText(title);                    icon = iconUtilities.createIconDrawable(icon);                    tv.setCompoundDrawables(null, icon, null, null);                    RecentTag tag = new RecentTag();                    tag.info = info;                    tag.intent = intent;                    tv.setTag(tag);                    tv.setVisibility(View.VISIBLE);                    tv.setPressed(false);                    tv.clearFocus();                    ++index;                }            }        }        // handle the case of "no icons to show"        mNoAppsText.setVisibility((index == 0) ? View.VISIBLE : View.GONE);        // hide the rest        for (; index < NUM_BUTTONS; ++index) {            mIcons[index].setVisibility(View.GONE);        }    }    /**     * This is the listener for the ACTION_CLOSE_SYSTEM_DIALOGS intent.  It's an indication that     * we should close ourselves immediately, in order to allow a higher-priority UI to take over     * (e.g. phone call received).     */    private BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() {        @Override        public void onReceive(Context context, Intent intent) {            String action = intent.getAction();            if (Intent.ACTION_CLOSE_SYSTEM_DIALOGS.equals(action)) {                String reason = intent.getStringExtra(PhoneWindowManager.SYSTEM_DIALOG_REASON_KEY);                if (! PhoneWindowManager.SYSTEM_DIALOG_REASON_RECENT_APPS.equals(reason)) {                    dismiss();                }            }        }    };}RecentApplicationsDialog.java完整源码

从源码可以看出，关键部分有三处。

 

一个很关键的内部类：

// 每个任务都包含一个Tag，这个Tag保存着这个App的一些非常有用的信息    class RecentTag {        ActivityManager.RecentTaskInfo info;        Intent intent;    }

这个RecentTag保存在每个近期任务的图标里，并且保存着这个任务的原始信息。

 

刚启动Dialog时对每个任务的初始化：

private void reloadButtons() {        final Context context = getContext();        final PackageManager pm = context.getPackageManager();        final ActivityManager am = (ActivityManager)                context.getSystemService(Context.ACTIVITY_SERVICE);                        //拿到最近使用的应用的信息列表        final List<ActivityManager.RecentTaskInfo> recentTasks =                am.getRecentTasks(MAX_RECENT_TASKS, ActivityManager.RECENT_IGNORE_UNAVAILABLE);        //自制一个home activity info，用来区分        ActivityInfo homeInfo =             new Intent(Intent.ACTION_MAIN).addCategory(Intent.CATEGORY_HOME)                    .resolveActivityInfo(pm, 0);        IconUtilities iconUtilities = new IconUtilities(getContext());        int index = 0;        int numTasks = recentTasks.size();        //开始初始化每个任务的信息        for (int i = 0; i < numTasks && (index < NUM_BUTTONS); ++i) {            final ActivityManager.RecentTaskInfo info = recentTasks.get(i);            //复制一个任务的原始Intent            Intent intent = new Intent(info.baseIntent);            if (info.origActivity != null) {                intent.setComponent(info.origActivity);            }            //跳过home activity            if (homeInfo != null) {                if (homeInfo.packageName.equals(                        intent.getComponent().getPackageName())                        && homeInfo.name.equals(                                intent.getComponent().getClassName())) {                    continue;                }            }            intent.setFlags((intent.getFlags()&~Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)                    | Intent.FLAG_ACTIVITY_NEW_TASK);            final ResolveInfo resolveInfo = pm.resolveActivity(intent, 0);            if (resolveInfo != null) {                final ActivityInfo activityInfo = resolveInfo.activityInfo;                final String title = activityInfo.loadLabel(pm).toString();                Drawable icon = activityInfo.loadIcon(pm);                if (title != null && title.length() > 0 && icon != null) {                    final TextView tv = mIcons[index];                    tv.setText(title);                    icon = iconUtilities.createIconDrawable(icon);                    tv.setCompoundDrawables(null, icon, null, null);                    //new一个Tag，保存这个任务的RecentTaskInfo和Intent                    RecentTag tag = new RecentTag();                    tag.info = info;                    tag.intent = intent;                    tv.setTag(tag);                    tv.setVisibility(View.VISIBLE);                    tv.setPressed(false);                    tv.clearFocus();                    ++index;                }            }        }       ...//无关紧要的代码    }

这里的过程是：先用ActivityManager获取RecentTasks并生成一个List，然后利用这个List为Dialog中的每个任务初始化，并生成对应的信息RecentTag。

需要注意的是，RecentTag中的Intent是从对应任务的原始Intent复制过来的，这意味着那个原始Intent的一些Extra参数将会一并复制过来，

我来举个例子：比如我的App支持从第三方启动，并且第三方要提供一个token，当然这个token就以Extra参数的形式放进Intent里，然后通过startActivity()启动我的App；然后我的App根据这个token来处理，注意这里，当我的App退出后，再从近期任务里启动这个App，之前的那个token还会传递给我的App，这里就会出现错误了，原因就是上面分析的。这就是为什么从第三方跳转的应用不会出现在近期任务的列表里（比如点击短信里的url启动一个浏览器，之后近期任务里只有短信app，不会出现浏览器app）。要想不出现在近期任务里，可以给Intent设置FLAG_ACTIVITY_NO_HISTORY标志。