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在研究了Activity的启动过程后，我觉得很有必要对Activity的launchMode进行分析一下，因为到目前为止，我发现网上对launchMode的讲解都是通过实例讲解，看完了总是似懂非懂的感觉，并没有根本上理解launchMode的原理。这里我会从源码的角度讲解launchMode。相信大家会和我一样，看了源码之后就会有一种豁然开朗的感觉。
Activity的启动模式一种有四种，分别如下：
1、standard
2、singleTop
3、singleTask
4、singleInstance
这里我们分两种情况讨论上述四种启动模式：app内和app之间
首先讨论app内
standard：不论当前任务栈中是否存在该Activity，都会新建一个Activity，如 任务栈为A B，要启动B 那么任务栈为 A B B
singleTop：如果当前要创建的Activity就在任务栈的顶端，那么不会创建新的Activity，仅仅调用Activity的onNewIntent，如果不在栈顶(或者栈中没有该Activity)，那么还是会创建新的Activity，如任务栈为A B 启动B 任务栈变为 A B 如果 启动A 那么任务栈为 A B A
singleTask：如果当前任务中存在要启动的Activity，那么就不会创建新的Activity，如果不存在就会创建新的Activity，如任务栈为 A B C，启动B ，那么任务栈就会变为A B
singleInstance：将一个Activity的launchMode设置为该值时，表明这个Activity独自占用一个任务队列，这个队列中不让在加入其他的Activity
不同的app之间：
既然在不同的app之间，那么就说明是两个任务栈，并且有一个处理前台，一个运行在后台
standard：当前台的Activity启动后台任务的Activity，不管这个Activity在后台任务栈中是否已经存在，都会创建一个新的Activity，并将它加入前台的任务栈中
singleTop：如果前台的Activity启动后台任务的Activity，并且这个Activity已经在后台任务的栈顶，和app内不同的是，这里还不能确定是否会创建新的Activity，这里还需要看启动Activity的Intent里面是否有Intent.FLAG_ACTIVITY_NEW_TASK，如果此标记，那么就会将后台的任务队列移动到前端，如 前端任务栈 A B 后端任务栈 E F C，现在前端需要启动一个C，如果有Intent.FLAG_ACTIVITY_NEW_TASK，那么后端任务栈就会移到前端(并调用C的onNewIntent)，前端栈退居后端。如果没有这个标记，那么仅仅就是在前端任务栈中创建一个新的Activity C。
singleTask：如果前台的Activity启动后台任务的Activity，不管这个Activity是否在栈顶，都会将后台的任务栈移到前台，前台任务栈移至后台。这里不需要考虑标记问题，因为被启动的Activity如果是singleTask，那么自动在启动Activity的intent中加入上述标记。
singleInstance：如果前台Activity启动后台任务的Activity，如果后台任务栈中已经有该Activity，那么就会调用该Activity的onNewIntent，并且后台任务还是在后台。如果后台任务栈中没有该Activity，那么会重新创建一个Acitivyt，并单独放入一个任务栈，其实在启动该Acitivity的Intent中也会加入上述标记
其实还有一种情况比较特殊，不过很少使用，就是在一个app之间，我们给某一个Activity配置了taskAffinity属性，这个属性会影响singleTask等属性，这个可以大家自己去分析。
下面我们就来看看和launchMode处理有关的代码吧
对launchMode处理的逻辑主要是放在了ActivityStack的startActivityUncheckedLocked方法中，这份方法的逻辑有些复杂，我们来一部分一部分的分析：
第一部分：
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if (sourceRecord == null) { // This activity is not being started from another... in this // case we -always- start a new task. if ((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) { Slog.w(TAG, "startActivity called from non-Activity context; forcing Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent); launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK; } } else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) { // The original activity who is starting us is running as a single // instance... this new activity it is starting must go on its // own task. launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK; } else if (r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) { // The activity being started is a single instance... it always // gets launched into its own task. launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK; } |
首先判断sourceRecord是否为Null(桌面启动一个Activity或者通过Context启动一个Activity时sourceRecord为null)，如果在启动的Intent中没有FLAG_ACTIVITY_NEW_TASK那么就会在该Intent中添加该标记。
如果sourceRecord的launchMode设置的是singleinstance，那么就会在Intent添加FLAG_ACTIVITY_NEW_TASK，因为对于singleinstance的Activity，是不会和别人共享一个队列的。
如果被启动的Activity的launchMode是singleinstance或者singletask，那么也会在Intent中添加FLAG_ACTIVITY_NEW_TASK标记(上面我们已经说过)。
在看第二部分代码
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if (r.resultTo != null && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) { // For whatever reason this activity is being launched into a new // task... yet the caller has requested a result back. Well, that // is pretty messed up, so instead immediately send back a cancel // and let the new task continue launched as normal without a // dependency on its originator. Slog.w(TAG, "Activity is launching as a new task, so cancelling activity result."); sendActivityResultLocked(-1, r.resultTo, r.resultWho, r.requestCode, Activity.RESULT_CANCELED, null); r.resultTo = null;} |
通过这段代码可以当Intent中包含了Intent.FLAG_ACTIVITY_NEW_TASK标记时，是不能使用startActivityforResult方法启动Activity的，也就是说如果一个Activity是sinleTask或者singleInstance时，都不能通过startActivityForResult方法调起(可以调起，但是无法传回值)
第三部分代码
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boolean addingToTask = false; boolean movedHome = false; TaskRecord reuseTask = null; if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 && (launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0) || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK || r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) { // If bring to front is requested, and no result is requested, and // we can find a task that was started with this same // component, then instead of launching bring that one to the front. if (r.resultTo == null) { // See if there is a task to bring to the front. If this is // a SINGLE_INSTANCE activity, there can be one and only one // instance of it in the history, and it is always in its own // unique task, so we do a special search. ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE ? findTaskLocked(intent, r.info) : findActivityLocked(intent, r.info); if (taskTop != null) { if (taskTop.task.intent == null) { // This task was started because of movement of // the activity based on affinity... now that we // are actually launching it, we can assign the // base intent. taskTop.task.setIntent(intent, r.info); } // If the target task is not in the front, then we need // to bring it to the front... except... well, with // SINGLE_TASK_LAUNCH it‘s not entirely clear. We‘d like // to have the same behavior as if a new instance was // being started, which means not bringing it to the front // if the caller is not itself in the front. ActivityRecord curTop = topRunningNonDelayedActivityLocked(notTop); if (curTop != null && curTop.task != taskTop.task) { r.intent.addFlags(Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT); boolean callerAtFront = sourceRecord == null || curTop.task == sourceRecord.task; if (callerAtFront) { // We really do want to push this one into the // user‘s face, right now. movedHome = true; moveHomeToFrontFromLaunchLocked(launchFlags); moveTaskToFrontLocked(taskTop.task, r, options); options = null; } } // If the caller has requested that the target task be // reset, then do so. if ((launchFlags&Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED) != 0) { taskTop = resetTaskIfNeededLocked(taskTop, r); } if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) { // We don‘t need to start a new activity, and // the client said not to do anything if that // is the case, so this is it! And for paranoia, make // sure we have correctly resumed the top activity. if (doResume) { resumeTopActivityLocked(null, options); } else { ActivityOptions.abort(options); } return ActivityManager.START_RETURN_INTENT_TO_CALLER; } if ((launchFlags & (Intent.FLAG_ACTIVITY_NEW_TASK|Intent.FLAG_ACTIVITY_CLEAR_TASK)) == (Intent.FLAG_ACTIVITY_NEW_TASK|Intent.FLAG_ACTIVITY_CLEAR_TASK)) { // The caller has requested to completely replace any // existing task with its new activity. Well that should // not be too hard... reuseTask = taskTop.task; performClearTaskLocked(taskTop.task.taskId); reuseTask.setIntent(r.intent, r.info); } else if ((launchFlags&Intent.FLAG_ACTIVITY_CLEAR_TOP) != 0 || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK || r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) { // In this situation we want to remove all activities // from the task up to the one being started. In most // cases this means we are resetting the task to its // initial state. ActivityRecord top = performClearTaskLocked( taskTop.task.taskId, r, launchFlags); if (top != null) { if (top.frontOfTask) { // Activity aliases may mean we use different // intents for the top activity, so make sure // the task now has the identity of the new // intent. top.task.setIntent(r.intent, r.info); } logStartActivity(EventLogTags.AM_NEW_INTENT, r, top.task); top.deliverNewIntentLocked(callingUid, r.intent); } else { // A special case: we need to // start the activity because it is not currently // running, and the caller has asked to clear the // current task to have this activity at the top. addingToTask = true; // Now pretend like this activity is being started // by the top of its task, so it is put in the // right place. sourceRecord = taskTop; } } else if (r.realActivity.equals(taskTop.task.realActivity)) { // In this case the top activity on the task is the // same as the one being launched, so we take that // as a request to bring the task to the foreground. // If the top activity in the task is the root // activity, deliver this new intent to it if it // desires. if ((launchFlags&Intent.FLAG_ACTIVITY_SINGLE_TOP) != 0 && taskTop.realActivity.equals(r.realActivity)) { logStartActivity(EventLogTags.AM_NEW_INTENT, r, taskTop.task); if (taskTop.frontOfTask) { taskTop.task.setIntent(r.intent, r.info); } taskTop.deliverNewIntentLocked(callingUid, r.intent); } else if (!r.intent.filterEquals(taskTop.task.intent)) { // In this case we are launching the root activity // of the task, but with a different intent. We // should start a new instance on top. addingToTask = true; sourceRecord = taskTop; } } else if ((launchFlags&Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED) == 0) { // In this case an activity is being launched in to an // existing task, without resetting that task. This // is typically the situation of launching an activity // from a notification or shortcut. We want to place // the new activity on top of the current task. addingToTask = true; sourceRecord = taskTop; } else if (!taskTop.task.rootWasReset) { // In this case we are launching in to an existing task // that has not yet been started from its front door. // The current task has been brought to the front. // Ideally, we‘d probably like to place this new task // at the bottom of its stack, but that‘s a little hard // to do with the current organization of the code so // for now we‘ll just drop it. taskTop.task.setIntent(r.intent, r.info); } if (!addingToTask && reuseTask == null) { // We didn‘t do anything... but it was needed (a.k.a., client // don‘t use that intent!) And for paranoia, make // sure we have correctly resumed the top activity. if (doResume) { resumeTopActivityLocked(null, options); } else { ActivityOptions.abort(options); } return ActivityManager.START_TASK_TO_FRONT; } } } } |
这段代码的逻辑有些复杂，但是功能很简单，就是为要启动的Activity寻找或者创建一个任务栈，现在我使用singleTask为例，跟踪一下上述代码流程。
假设后台任务栈为 A B C，前台任务中要启动一个Activity B，我们就使用这个需求来跟踪上面的代码逻辑。
首先注意默认的addingToTask是false，reuseTask为null，由于是singleTask的，所以((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 && (launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)是成立的，即使这里不成立，后面的逻辑也是成立，所以必然进入该if语句
继续跟进代码，会发现调用了findTaskLocked方法返回一个ActivityRecord，你可以查看findTaskLocked的逻辑，你会知道这个ActiivtyRecord 就是对应的是Activity C，继续往下走，就会调用 moveTaskToFrontLocked将后台任务移动到前台，由于我们考虑的是launchMode是singleTask，所以将会进入
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else if ((launchFlags&Intent.FLAG_ACTIVITY_CLEAR_TOP) != 0 || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK || r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) |
进入上面的if语句之后，就会调用performClearTaskLocked返回一个ActivityRecord，你可以去研究一下performClearTaskLocked的逻辑，这个方法传入了一个ActivityRecord r，就是将任务栈中r之上的AcrivityRecord 清除掉，并返回ActivityRecord r，这里返回的ActivityRecord 对应的就是Activity B。接着调用Activity B的onNewIntent方法。执行了这个else if，其他else if是都不会执行的，所以就直接执行到了
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if (!addingToTask && reuseTask == null) { // We didn‘t do anything... but it was needed (a.k.a., client // don‘t use that intent!) And for paranoia, make // sure we have correctly resumed the top activity. if (doResume) { resumeTopActivityLocked(null, options); } else { ActivityOptions.abort(options); } return ActivityManager.START_TASK_TO_FRONT; } |
这里直接调用了resumeTopActivityLocked方法。到这里singleTask就分析完了，其他的启动模式大家可以按照这种思路进行分析。
第四部分代码：
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if (r.packageName != null) { // If the activity being launched is the same as the one currently // at the top, then we need to check if it should only be launched // once. ActivityRecord top = topRunningNonDelayedActivityLocked(notTop); if (top != null && r.resultTo == null) { if (top.realActivity.equals(r.realActivity) && top.userId == r.userId) { if (top.app != null && top.app.thread != null) { if ((launchFlags&Intent.FLAG_ACTIVITY_SINGLE_TOP) != 0 || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TOP || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) { logStartActivity(EventLogTags.AM_NEW_INTENT, top, top.task); // For paranoia, make sure we have correctly // resumed the top activity. if (doResume) { resumeTopActivityLocked(null); } ActivityOptions.abort(options); if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) { // We don‘t need to start a new activity, and // the client said not to do anything if that // is the case, so this is it! return ActivityManager.START_RETURN_INTENT_TO_CALLER; } top.deliverNewIntentLocked(callingUid, r.intent); return ActivityManager.START_DELIVERED_TO_TOP; } } } } } else { if (r.resultTo != null) { sendActivityResultLocked(-1, r.resultTo, r.resultWho, r.requestCode, Activity.RESULT_CANCELED, null); } ActivityOptions.abort(options); return ActivityManager.START_CLASS_NOT_FOUND; } boolean newTask = false; boolean keepCurTransition = false; // Should this be considered a new task? if (r.resultTo == null && !addingToTask && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) { if (reuseTask == null) { // todo: should do better management of integers. mService.mCurTask++; if (mService.mCurTask <= 0) { mService.mCurTask = 1; } r.setTask(new TaskRecord(mService.mCurTask, r.info, intent), null, true); if (DEBUG_TASKS) Slog.v(TAG, "Starting new activity " + r + " in new task " + r.task); } else { r.setTask(reuseTask, reuseTask, true); } newTask = true; if (!movedHome) { moveHomeToFrontFromLaunchLocked(launchFlags); } } else if (sourceRecord != null) { if (!addingToTask && (launchFlags&Intent.FLAG_ACTIVITY_CLEAR_TOP) != 0) { // In this case, we are adding the activity to an existing // task, but the caller has asked to clear that task if the // activity is already running. ActivityRecord top = performClearTaskLocked( sourceRecord.task.taskId, r, launchFlags); keepCurTransition = true; if (top != null) { logStartActivity(EventLogTags.AM_NEW_INTENT, r, top.task); top.deliverNewIntentLocked(callingUid, r.intent); // For paranoia, make sure we have correctly // resumed the top activity. if (doResume) { resumeTopActivityLocked(null); } ActivityOptions.abort(options); return ActivityManager.START_DELIVERED_TO_TOP; } } else if (!addingToTask && (launchFlags&Intent.FLAG_ACTIVITY_REORDER_TO_FRONT) != 0) { // In this case, we are launching an activity in our own task // that may already be running somewhere in the history, and // we want to shuffle it to the front of the stack if so. int where = findActivityInHistoryLocked(r, sourceRecord.task.taskId); if (where >= 0) { ActivityRecord top = moveActivityToFrontLocked(where); logStartActivity(EventLogTags.AM_NEW_INTENT, r, top.task); top.updateOptionsLocked(options); top.deliverNewIntentLocked(callingUid, r.intent); if (doResume) { resumeTopActivityLocked(null); } return ActivityManager.START_DELIVERED_TO_TOP; } } // An existing activity is starting this new activity, so we want // to keep the new one in the same task as the one that is starting // it. r.setTask(sourceRecord.task, sourceRecord.thumbHolder, false); if (DEBUG_TASKS) Slog.v(TAG, "Starting new activity " + r + " in existing task " + r.task); } else { // This not being started from an existing activity, and not part // of a new task... just put it in the top task, though these days // this case should never happen. final int N = mHistory.size(); ActivityRecord prev = N > 0 ? mHistory.get(N-1) : null; r.setTask(prev != null ? prev.task : new TaskRecord(mService.mCurTask, r.info, intent), null, true); if (DEBUG_TASKS) Slog.v(TAG, "Starting new activity " + r + " in new guessed " + r.task); } mService.grantUriPermissionFromIntentLocked(callingUid, r.packageName, intent, r.getUriPermissionsLocked()); if (newTask) { EventLog.writeEvent(EventLogTags.AM_CREATE_TASK, r.task.taskId); } logStartActivity(EventLogTags.AM_CREATE_ACTIVITY, r, r.task); startActivityLocked(r, newTask, doResume, keepCurTransition, options); return ActivityManager.START_SUCCESS; |
我刚刚使用singleTask模式分析时，并没有走到这里，大家可以另一种情况来分析，就可以走到这里。
后台任务栈 A B，前台任务栈 E F，在前台任务栈中要启动后台任务的一个Activity C，也就是说后台任务栈中不存在C的情况，或者在一个app内通过配置taskAffinity属性也可以走到这里。
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原文地址：http://www.cnblogs.com/jimmyfang/p/4787021.html
