Android Toast 的通常用法为：
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Toast.makeText(context, "message", Toast.LENGTH_SHORT).show();
Toast.makeText 源码如下(android-14)：
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/**
* Make a standard toast that just contains a text view.
*
* @param context The context to use. Usually your {@link android.app.Application}
* or {@link android.app.Activity} object.
* @param text The text to show. Can be formatted text.
* @param duration How long to display the message. Either {@link #LENGTH_SHORT} or
* {@link #LENGTH_LONG}
*
*/
public static Toast makeText(Context context, CharSequence text, int duration) {
Toast result = new Toast(context);
LayoutInflater inflate = (LayoutInflater)
context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);
TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message);
tv.setText(text);
result.mNextView = v;
result.mDuration = duration;
return result;
}
可以看到 makeText 函数新建了个 Toast 对象，并设置变量 mNextView 以及 mDuration 。
mDuration 即 makeText 传入的 Toast 的持续时间， mNextView 的布局文件为 transient_notification ：
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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:background="@drawable/toast_frame">
<TextView
android:id="@android:id/message"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:textAppearance="@style/TextAppearance.Small"
android:textColor="@color/bright_foreground_dark"
android:shadowColor="#BB000000"
android:shadowRadius="2.75"
/>
</LinearLayout>
可以看到，Toast 的布局很简单：一个 LinearLayout 之中包含一个 id 为 @android:id/message 的 TextView。
再来看看 Toast 的构造函数：
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/**
* Construct an empty Toast object. You must call {@link #setView} before you
* can call {@link #show}.
*
* @param context The context to use. Usually your {@link android.app.Application}
* or {@link android.app.Activity} object.
*/
public Toast(Context context) {
mContext = context;
mTN = new TN();
mTN.mY = context.getResources().getDimensionPixelSize(
com.android.internal.R.dimen.toast_y_offset);
}
可以看到 Toast 的构造函数中创建了一个内部类 TN 的实例：
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private static class TN extends ITransientNotification.Stub {
final Runnable mShow = new Runnable() {
public void run() {
handleShow();
}
};
final Runnable mHide = new Runnable() {
public void run() {
handleHide();
// Don't do this in handleHide() because it is also invoked by handleShow()
mNextView = null;
}
};
private final WindowManager.LayoutParams mParams = new WindowManager.LayoutParams();
final Handler mHandler = new Handler();
int mGravity = Gravity.CENTER_HORIZONTAL | Gravity.BOTTOM;
int mX, mY;
float mHorizontalMargin;
float mVerticalMargin;
View mView;
View mNextView;
WindowManagerImpl mWM;
TN() {
// XXX This should be changed to use a Dialog, with a Theme.Toast
// defined that sets up the layout params appropriately.
final WindowManager.LayoutParams params = mParams;
params.height = WindowManager.LayoutParams.WRAP_CONTENT;
params.width = WindowManager.LayoutParams.WRAP_CONTENT;
params.flags = WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
| WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE
| WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON;
params.format = PixelFormat.TRANSLUCENT;
params.windowAnimations = com.android.internal.R.style.Animation_Toast;
params.type = WindowManager.LayoutParams.TYPE_TOAST;
params.setTitle("Toast");
}
/**
* schedule handleShow into the right thread
*/
public void show() {
if (localLOGV) Log.v(TAG, "SHOW: " + this);
mHandler.post(mShow);
}
/**
* schedule handleHide into the right thread
*/
public void hide() {
if (localLOGV) Log.v(TAG, "HIDE: " + this);
mHandler.post(mHide);
}
public void handleShow() {
if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
+ " mNextView=" + mNextView);
if (mView != mNextView) {
// remove the old view if necessary
handleHide();
mView = mNextView;
mWM = WindowManagerImpl.getDefault();
final int gravity = mGravity;
mParams.gravity = gravity;
if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
mParams.horizontalWeight = 1.0f;
}
if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
mParams.verticalWeight = 1.0f;
}
mParams.x = mX;
mParams.y = mY;
mParams.verticalMargin = mVerticalMargin;
mParams.horizontalMargin = mHorizontalMargin;
if (mView.getParent() != null) {
if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
mWM.removeView(mView);
}
if (localLOGV) Log.v(TAG, "ADD! " + mView + " in " + this);
mWM.addView(mView, mParams);
trySendAccessibilityEvent();
}
}
private void trySendAccessibilityEvent() {
AccessibilityManager accessibilityManager =
AccessibilityManager.getInstance(mView.getContext());
if (!accessibilityManager.isEnabled()) {
return;
}
// treat toasts as notifications since they are used to
// announce a transient piece of information to the user
AccessibilityEvent event = AccessibilityEvent.obtain(
AccessibilityEvent.TYPE_NOTIFICATION_STATE_CHANGED);
event.setClassName(getClass().getName());
event.setPackageName(mView.getContext().getPackageName());
mView.dispatchPopulateAccessibilityEvent(event);
accessibilityManager.sendAccessibilityEvent(event);
}
public void handleHide() {
if (localLOGV) Log.v(TAG, "HANDLE HIDE: " + this + " mView=" + mView);
if (mView != null) {
// note: checking parent() just to make sure the view has
// been added... i have seen cases where we get here when
// the view isn't yet added, so let's try not to crash.
if (mView.getParent() != null) {
if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
mWM.removeView(mView);
}
mView = null;
}
}
}
我们来分析下内部类 TN 的源码。
TN 继承于 ITransientNotification.Stub，而 ITransientNotification.aidl 位于 frameworks/base/core/java/android/app/ITransientNotification.aidl：
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package android.app;
/** @hide */
oneway interface ITransientNotification {
void show();
void hide();
}
ITransientNotification 中定义了两个方法： show()、hide()，它们在 TN 中的实现为：
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public void show() {
if (localLOGV) Log.v(TAG, "SHOW: " + this);
mHandler.post(mShow);
}
public void hide() {
if (localLOGV) Log.v(TAG, "HIDE: " + this);
mHandler.post(mHide);
}
这里的 show 和 hide 是通过 Handler 机制来调用具体逻辑的，对应的 Runnable 实现为：
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final Runnable mShow = new Runnable() {
public void run() {
handleShow();
}
};
final Runnable mHide = new Runnable() {
public void run() {
handleHide();
// Don't do this in handleHide() because it is also invoked by handleShow()
mNextView = null;
}
};
可以看到，show() 和 hide() 方法分别调用了 handleShow() 和 handleHide() 方法。
首先看下 handleShow()：
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public void handleShow() {
if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
+ " mNextView=" + mNextView);
if (mView != mNextView) {
// remove the old view if necessary
handleHide();
mView = mNextView;
mWM = WindowManagerImpl.getDefault();
final int gravity = mGravity;
mParams.gravity = gravity;
if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
mParams.horizontalWeight = 1.0f;
}
if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
mParams.verticalWeight = 1.0f;
}
mParams.x = mX;
mParams.y = mY;
mParams.verticalMargin = mVerticalMargin;
mParams.horizontalMargin = mHorizontalMargin;
if (mView.getParent() != null) {
if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
mWM.removeView(mView);
}
if (localLOGV) Log.v(TAG, "ADD! " + mView + " in " + this);
mWM.addView(mView, mParams);
trySendAccessibilityEvent();
}
}
handleShow() 方法就是具体显示 Toast 的代码。
首先移除旧的 View，然后设置 mView 对应的布局参数 mParams。最终调用 mWM 的 addView 方法将 mView 显示到手机屏幕。
mWM 是 WindowManagerImpl 的实例，而 WindowManagerImpl 类对外是不可见的，我们如果想实现自己的 Toast 自己控制 Toast的显示，可以通过如下方式获取 mWM：
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mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
handleHide() 方法就是具体移除 Toast 显示的代码，调用的是 mWM 的 removeView() 方法：
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public void handleHide() {
if (localLOGV) Log.v(TAG, "HANDLE HIDE: " + this + " mView=" + mView);
if (mView != null) {
// note: checking parent() just to make sure the view has
// been added... i have seen cases where we get here when
// the view isn't yet added, so let's try not to crash.
if (mView.getParent() != null) {
if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
mWM.removeView(mView);
}
mView = null;
}
}
我们在调用 Toast.makeText() 方法生成 Toast 实例后，需要调用 Toast.show() 方法来调用 Toast 的显示功能：
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/**
* Show the view for the specified duration.
*/
public void show() {
if (mNextView == null) {
throw new RuntimeException("setView must have been called");
}
INotificationManager service = getService();
String pkg = mContext.getPackageName();
TN tn = mTN;
tn.mNextView = mNextView;
try {
service.enqueueToast(pkg, tn, mDuration);
} catch (RemoteException e) {
// Empty
}
}
可以看到，Toast 的显示并不是立即调用的，而是通过 INotificationManager 实例 sService 的 enqueueToast() 方法，将 mTN 添加到 sService 队列中，由 sService 控制 Toast 的显示与移除。
下面看下 sService：
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private static INotificationManager sService;
static private INotificationManager getService() {
if (sService != null) {
return sService;
}
sService = INotificationManager.Stub.asInterface(ServiceManager.getService("notification"));
return sService;
}
这里 INofiticationManager 的具体实现类位于 frameworks/base/services/java/com/android/server/NotificationManagerService.java。
首先看下 NotificationManagerService.enqueueToast()：
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public void enqueueToast(String pkg, ITransientNotification callback, int duration)
{
if (DBG) Slog.i(TAG, "enqueueToast pkg=" + pkg + " callback=" + callback + " duration=" + duration);
if (pkg == null || callback == null) {
Slog.e(TAG, "Not doing toast. pkg=" + pkg + " callback=" + callback);
return ;
}
synchronized (mToastQueue) {
int callingPid = Binder.getCallingPid();
long callingId = Binder.clearCallingIdentity();
try {
ToastRecord record;
int index = indexOfToastLocked(pkg, callback);
// If it's already in the queue, we update it in place, we don't
// move it to the end of the queue.
if (index >= 0) {
record = mToastQueue.get(index);
record.update(duration);
} else {
// Limit the number of toasts that any given package except the android
// package can enqueue. Prevents DOS attacks and deals with leaks.
if (!"android".equals(pkg)) {
int count = 0;
final int N = mToastQueue.size();
for (int i=0; i<N; i++) {
final ToastRecord r = mToastQueue.get(i);
if (r.pkg.equals(pkg)) {
count++;
if (count >= MAX_PACKAGE_NOTIFICATIONS) {
Slog.e(TAG, "Package has already posted " + count
+ " toasts. Not showing more. Package=" + pkg);
return;
}
}
}
}
record = new ToastRecord(callingPid, pkg, callback, duration);
mToastQueue.add(record);
index = mToastQueue.size() - 1;
keepProcessAliveLocked(callingPid);
}
// If it's at index 0, it's the current toast. It doesn't matter if it's
// new or just been updated. Call back and tell it to show itself.
// If the callback fails, this will remove it from the list, so don't
// assume that it's valid after this.
if (index == 0) {
showNextToastLocked();
}
} finally {
Binder.restoreCallingIdentity(callingId);
}
}
}
mToastQueue 是一个 ToastRecord 对象队列：
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private static final class ToastRecord
{
final int pid;
final String pkg;
final ITransientNotification callback;
int duration;
ToastRecord(int pid, String pkg, ITransientNotification callback, int duration)
{
this.pid = pid;
this.pkg = pkg;
this.callback = callback;
this.duration = duration;
}
void update(int duration) {
this.duration = duration;
}
void dump(PrintWriter pw, String prefix) {
pw.println(prefix + this);
}
@Override
public final String toString()
{
return "ToastRecord{"
+ Integer.toHexString(System.identityHashCode(this))
+ " pkg=" + pkg
+ " callback=" + callback
+ " duration=" + duration;
}
}
ToastRecord 中：pid 为生成并调用显示 Toast 的进程的 id 号；pkg 为包名；callback 为 ITransientNotification 类型的回调，即前面提到的 mTN；duration 为 Toast 显示持续时间。
enqueueToast 方法中，首先根据 pkg 和 callback 来判断 Toast 是否已在队列中：
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private int indexOfToastLocked(String pkg, ITransientNotification callback)
{
IBinder cbak = callback.asBinder();
ArrayList<ToastRecord> list = mToastQueue;
int len = list.size();
for (int i=0; i<len; i++) {
ToastRecord r = list.get(i);
if (r.pkg.equals(pkg) && r.callback.asBinder() == cbak) {
return i;
}
}
return -1;
}
如果在队列中，调用 ToastRecord.update 方法更新 Toast 持续时间。
不在队列中，如果 Toast 不是系统 Toast 且队列超过最大数量，直接返回；否则，新建 ToastRecord 对象添加到 mToastQueue 队列中，并调用 keepProcessAliveLocked() 方法将对应应用进程切换到前台进程，防止进程被回收：
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private void keepProcessAliveLocked(int pid)
{
int toastCount = 0; // toasts from this pid
ArrayList<ToastRecord> list = mToastQueue;
int N = list.size();
for (int i=0; i<N; i++) {
ToastRecord r = list.get(i);
if (r.pid == pid) {
toastCount++;
}
}
try {
mAm.setProcessForeground(mForegroundToken, pid, toastCount > 0);
} catch (RemoteException e) {
// Shouldn't happen.
}
}
然后判断 Toast 序号是否为0，为0则调用 showNextToastLocked() 方法开始处理 Toast 队列的显示。首次添加 Toast 时，序号当然为0：
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private void showNextToastLocked() {
ToastRecord record = mToastQueue.get(0);
while (record != null) {
if (DBG) Slog.d(TAG, "Show pkg=" + record.pkg + " callback=" + record.callback);
try {
record.callback.show();
scheduleTimeoutLocked(record, false);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Object died trying to show notification " + record.callback
+ " in package " + record.pkg);
// remove it from the list and let the process die
int index = mToastQueue.indexOf(record);
if (index >= 0) {
mToastQueue.remove(index);
}
keepProcessAliveLocked(record.pid);
if (mToastQueue.size() > 0) {
record = mToastQueue.get(0);
} else {
record = null;
}
}
}
}
showNextToastLocked() 方法每次从队列列首取出一个 Toast 记录 record，回调 mTN 的 show() 方法将 Toast 显示到屏幕，然后调用 scheduleTimeoutLocked() 方法：
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private void scheduleTimeoutLocked(ToastRecord r, boolean immediate)
{
Message m = Message.obtain(mHandler, MESSAGE_TIMEOUT, r);
long delay = immediate ? 0 : (r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY);
mHandler.removeCallbacksAndMessages(r);
mHandler.sendMessageDelayed(m, delay);
}
scheduleTimeoutLocked() 方法使用 Handler 机制向 mHandler 的消息队列中添加延迟消息 m，其作用就是在 Toast 的持续时间结束后，从屏幕移除 Toast。
这里可以看到，消息 m 的延时时间根据 duration 的不同，只能为 LONG_DELAY 和 SHORT_DELAY 之一：
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private static final int LONG_DELAY = 3500; // 3.5 seconds
private static final int SHORT_DELAY = 2000; // 2 seconds
因此，Toast 的持续时间是无法随意定义的，只能长短二选一。
我们再来看看延时时间到达后，Toast 队列的后续处理：
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private final class WorkerHandler extends Handler
{
@Override
public void handleMessage(Message msg)
{
switch (msg.what)
{
case MESSAGE_TIMEOUT:
handleTimeout((ToastRecord)msg.obj);
break;
}
}
}
private void handleTimeout(ToastRecord record)
{
if (DBG) Slog.d(TAG, "Timeout pkg=" + record.pkg + " callback=" + record.callback);
synchronized (mToastQueue) {
int index = indexOfToastLocked(record.pkg, record.callback);
if (index >= 0) {
cancelToastLocked(index);
}
}
}
可以看到最终调用了 cancelToastLocked() 方法：
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private void cancelToastLocked(int index) {
ToastRecord record = mToastQueue.get(index);
try {
record.callback.hide();
} catch (RemoteException e) {
Slog.w(TAG, "Object died trying to hide notification " + record.callback
+ " in package " + record.pkg);
// don't worry about this, we're about to remove it from
// the list anyway
}
mToastQueue.remove(index);
keepProcessAliveLocked(record.pid);
if (mToastQueue.size() > 0) {
// Show the next one. If the callback fails, this will remove
// it from the list, so don't assume that the list hasn't changed
// after this point.
showNextToastLocked();
}
}
首先回调 mTN 的 hide() 方法将 Toast 从屏幕移除；然后从 mToastQueue 队列中移除 Toast；此时，如果 mToastQueue 队列非空，调用 showNextToastLocked() 方法处理下一个 Toast。
这样，Toast 显示、隐藏、队列处理的逻辑就都明了了。