原文: http://tryenough.com/android-startActivity
在使用中的不同
1.在Activity中跳轉到其他的Activity時,兩種使用方法是一樣的:
this.startActivity(intent);
context.startActivity(intent);
2.從非 Activity (例如從其他Context中)啓動Activity則必須給intent設置Flag:FLAG_ACTIVITY_NEW_TASK:
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK) ;
mContext.startActivity(intent);
探究一下爲什麼會有這方面的差異
原文: http://tryenough.com/android-startActivity
首先看下Activity和context的繼承關係:
要知道Activity和context中的StartActivity有什麼區別,我們看下它們分別是怎麼實現startActivity函數的:
原文: http://tryenough.com/android-startActivity
1.Context是抽象類,它的startActivity函數是抽象方法:
public abstract void startActivity(@RequiresPermission Intent intent);
2.ContextWrapper類只是調用了Context的實現:
@Override
public void startActivity(Intent intent) {
mBase.startActivity(intent);
}
3.ContextThemeWrapper中沒有實現此方法
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);
}
}
由此可見,在Activity中無論是使用哪一種startActivity方法都會調用到Activity自身的方法,所以是一樣的。
原文: http://tryenough.com/android-startActivity
然而在其他的Context子類,例如ContextImpl.java中的實現,會檢查有沒有設置Flag:FLAG_ACTIVITY_NEW_TASK,否則會報錯:
@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);
}
原文: http://tryenough.com/android-startActivity
這也就是爲什麼Activity的startActivity和Context的startActivity會有上面那些使用上的不同的原因了。