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Android?Toast?cancel问题、源码分析和解决方案
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本文中部分内容摘自API ?部分代码来自android.widget.Toast类源代码
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解决方案中 ToastUtil代码 ?由http://www.linuxidc.com/Linux/2012-01/51925.htm 修改而来(基本没做什么修改 删除了一个方法 ?添加了一个方法)
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Toast介绍：
A?toast?is?a?view?containing?a?quick?little?message?for?the?user.?The?toast?class?helps?you?create?and?show?those.
When?the?view?is?shown?to?the?user,?appears?as?a?floating?view?over?the?application.?It?will?never?receive?focus.?The?user?will?probably?be?in?the?middle?of?typing?something?else.?The?idea?is?to?be?as?unobtrusive?as?possible,?while?still?showing?the?user?the?information?you?want?them?to?see.?Two?examples?are?the?volume?control,?and?the?brief?message?saying?that?your?settings?have?been?saved.
The?easiest?way?to?use?this?class?is?to?call?one?of?the?static?methods?that?constructs?everything?you?need?and?returns?a?new?Toast?object.
问题：
问题主要出现在通过按钮或事件触发Toast时，若多次点击按钮（或某事件多次触发），会触发一系列Toast，它们会依次缓慢出现在屏幕上，无法消除，令用户看起来很难受。
当然?Toast类中存在消除Toast的方法
public?void?cancel?()
Since:?API?Level?1
Close?the?view?if?it's?showing,?or?don't?show?it?if?it?isn't?showing?yet.?You?do?not?normally?have?to?call?this.?Normally?view?will?disappear?on?its?own?after?the?appropriate?duration.
但是事实证明，API中对cancel类的说明完全是胡扯。
比如我在程序中存放一个ArrayList<Toast>对象用来存放所有的Toast对象，按照API的说法，即使我按了无数次的按钮触发了无数次Toast，但是只要遍历ArrayList?将全部Toast??cancel掉不就一切都清净了？
遗憾的事，即使遍历ArrayList?对每一个Toast对象指向cancel方法，只有当前正在显示的Toast会被cancel掉，队列中的Toast的依然会依次展现。
查看下Toast类的源代码：
首先找到?cancel方法及相关调用链：
?public?void?cancel()?{
????????mTN.hide();
????????//?TODO?this?still?needs?to?cancel?the?inflight?notification?if?any
????}
??/**
???*?schedule?handleHide?into?the?right?thread
???*/
???public?void?hide()?{
????if?(localLOGV)?Log.v(TAG,?"HIDE:?"?+?this);
????mHandler.post(mHide);
?}
?final?Runnable?mHide?=?new?Runnable()?{
????????????public?void?run()?{
????????????????handleHide();
????????????}
????????};
?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;
????????????}
????????}
看到了么，源代码中显示，执行cancel方法?只会把正在显示的Toast移除，还未被显示的Toast，根本就通不过上面的if条件判断。
API中所说的?Close?the?view?if?it's?showing,?or?don't?show?it?if?it?isn't?showing?yet.
只有前半段是对的，后半段纯属胡扯。
解决方案：
由于Toast中控制显示?隐藏的部分private?class?TN?extends?ITransientNotification.Stub???是个私有内部类，所以继承Toast修改代码是不成了，也听说有高手用反射动态修改这部分的，不知解决情况究竟如何。
这里这个解决办法很简单，既然加入Toast队列的Toast我无法控制，我自己在外部实现一个Toast队列，自己控制它就是了。
简单的实现：全局只有1个Toast对象?由此类中的静态方法控制显示。
每次显示时，cancel掉原来的Toast对象。即，Toast队列中，只有0或1个Toast。每次触发Toast动作，都会立即清除上一个Toast.
package?XXXXXXXXXXXXXXXXXXXXXX;
import?android.content.Context;
import?android.os.Handler;
import?android.os.Looper;
import?android.widget.Toast;
public?class?ToastUtil?{
private?static?Handler?handler?=?new?Handler(Looper.getMainLooper());
private?static?Toast?toast?=?null;
private?static?Object?synObj?=?new?Object();
public?static?void?showMessage(final?Context?act,?final?String?msg)?{
showMessage(act,?msg,?Toast.LENGTH_SHORT);
}
public?static?void?showMessage(final?Context?act,?final?int?msg)?{
showMessage(act,?msg,?Toast.LENGTH_SHORT);
}
public?static?void?showMessage(final?Context?act,?final?int?msg,
final?int?len)?{
new?Thread(new?Runnable()?{
public?void?run()?{
handler.post(new?Runnable()?{
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@Override
public?void?run()?{
synchronized?(synObj)?{
if?(toast?!=?null)?{
toast.cancel();
toast.setText(msg);
toast.setDuration(len);
}?else?{
toast?=?Toast.makeText(act,?msg,?len);
}
toast.show();
}
}
});
}
}).start();
}
public?static?void?cancelCurrentToast()?{
if?(toast?!=?null)?{
toast.cancel();
}
}
}
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2月28日追加：
实测 在2.2?? 2.3中此方法工作良好。
在4.0系统中效果极差
多次触发Toast? 无法正常显示