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

最新推荐文章于 2024-06-18 10:54:43 发布

一阵石头雨

最新推荐文章于 2024-06-18 10:54:43 发布

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分类专栏：笔记文章标签： android AndroidManager getRecentTasks

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转载请注明出处，谢谢！ 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。

响应每个任务的点击事件：

private void switchTo(RecentTag tag) {
        if (tag.info.id >= 0) {
            // 这个Task没有退出，直接移动到前台
            final ActivityManager am = (ActivityManager)
                    getContext().getSystemService(Context.ACTIVITY_SERVICE);
            am.moveTaskToFront(tag.info.id, ActivityManager.MOVE_TASK_WITH_HOME);
        } else if (tag.intent != null) {
            //task退出了的话，id为-1，则使用RecentTag中的Intent重新启动
            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);
            }
        }
    }

如果该Task没有退出，只是切到后台，则切换到前台；如果已经退出，就要重新启动了。

这里的Intent就是之前说的，重复使用的旧Intent了，这里注意，系统添加了FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY和FLAG_ACTIVITY_TASK_ON_HOME标志，所以我们可以在App中通过判断Intent的flag是否包含这两个来判断是否是从近期任务里启动的。注意FLAG_ACTIVITY_TASK_ON_HOME标志是Api 11添加的，所以11一下的之判断FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY就行了。