原文: http://tryenough.com/android-startActivity
在使用中的不同
1.在Activity中跳转到其他的Activity时,两种使用方法是一样的:
this.startActivity(intent);context.startActivity(intent);复制代码
2.从非 Activity (例如从其他Context中)启动Activity则必须给intent设置Flag:FLAG_ACTIVITY_NEW_TASK:
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK) ; mContext.startActivity(intent);复制代码
探究一下为什么会有这方面的差异
原文: http://tryenough.com/android-startActivity
首先看下Activity和context的继承关系:
要知道Activity和context中的StartActivity有什么区别,我们看下它们分别是怎么实现startActivity函数的:
原文: http://tryenough.com/android-startActivity
1.Context是抽象类,它的startActivity函数是抽象方法:
public abstract void startActivity(@RequiresPermission Intent intent);复制代码
2.ContextWrapper类只是调用了Context的实现:
@Overridepublic void startActivity(Intent intent) { mBase.startActivity(intent);}复制代码
3.ContextThemeWrapper中没有实现此方法
4.Activity中:
@Override public void startActivity(Intent intent, @Nullable Bundle options) { if (options != null) { startActivityForResult(intent, -1, options); } else { // Note we want to go through this call for compatibility with // applications that may have overridden the method. startActivityForResult(intent, -1); } }复制代码
由此可见,在Activity中无论是使用哪一种startActivity方法都会调用到Activity自身的方法,所以是一样的。
原文: http://tryenough.com/android-startActivity
然而在其他的Context子类,例如ContextImpl.java中的实现,会检查有没有设置Flag:FLAG_ACTIVITY_NEW_TASK,否则会报错:
@Override public void startActivity(Intent intent, Bundle options) { warnIfCallingFromSystemProcess(); // Calling start activity from outside an activity without FLAG_ACTIVITY_NEW_TASK is // generally not allowed, except if the caller specifies the task id the activity should // be launched in. A bug was existed between N and O-MR1 which allowed this to work. We // maintain this for backwards compatibility. final int targetSdkVersion = getApplicationInfo().targetSdkVersion; if ((intent.getFlags() & Intent.FLAG_ACTIVITY_NEW_TASK) == 0 && (targetSdkVersion < Build.VERSION_CODES.N || targetSdkVersion >= Build.VERSION_CODES.P) && (options == null || ActivityOptions.fromBundle(options).getLaunchTaskId() == -1)) { throw new AndroidRuntimeException( "Calling startActivity() from outside of an Activity " + " context requires the FLAG_ACTIVITY_NEW_TASK flag." + " Is this really what you want?"); } mMainThread.getInstrumentation().execStartActivity( getOuterContext(), mMainThread.getApplicationThread(), null, (Activity) null, intent, -1, options); }复制代码
原文: http://tryenough.com/android-startActivity
这也就是为什么Activity的startActivity和Context的startActivity会有上面那些使用上的不同的原因了。