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。
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。
