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This article introduces the relevant knowledge of "what are the basic knowledge points of Android". In the actual case operation process, many people will encounter such difficulties. Next, let Xiaobian lead you to learn how to deal with these situations! I hope you can read carefully and learn something!

I. Activity

1. Activity's four startup modes and application scenarios

（1）standard

Standard mode, where a new Activity instance is generated in the active stack each time. Usually the activities we use are standard patterns.

（2）singleTop

Stack-top multiplexing: If an Activity instance already exists at the top of the stack, no new instance is created in the active stack. A common scenario is to set the notification jump Activity, because you definitely don't want to click the notification and create another Activity for you when the foreground Activity is already the Activity.

（3）singleTask

Intra-stack reuse: If an Activity instance already exists on the current stack, all other Activity instances above the current Activity instance will be removed from the stack. It is common to jump to the main interface.

（4）singleInstance

Single-instance mode, creating a new task stack in which the active instance resides alone.

II. Screen adaptation

1. Screen adaptation principle

Relationship between device pixel px and device independent pixel dp px = dp * density

III. Android messaging mechanism

1. Four concepts of Android messaging mechanism

ThreadLocal、MessageQueue、Looper、Handler

2. Process

1) Preparation

Create Handler, if it is created in a child thread, you also need to call Looper#prepare(), in the Handler constructor, it will bind Looper and MessageQueue.

2) Send a message

Create a message and send it using Handler.

3) Enter Message Queue

Because the Handler is bound to the message queue, Message is naturally placed in the message queue.

(4) Looper polling message queue

Looper is an endless loop, always observe whether there is a new message coming, then take out the bound Handler from Message, and finally call the processing logic in Handler, all of which occurs in the thread of Looper loop, which is also the reason why Handler can process tasks in the specified thread.

3. Looper has no reason to block the interface in the endless loop in the main thread

What causes the card to die is that executing time-consuming operations in the Ui thread causes the interface to drop frames, even ANR, Looper.loop() This operation itself does not cause this situation;Looper blocks the current thread when there is no message, releases CPU resources, and wakes up the main thread when there is a message; The App process needs an endless loop, and if the loop ends, the App process ends.

4. Introduction to IdleHandler

IdleHandler is a mechanism for handling idle tasks when the Hanlder is idle. Its execution scenario: MessageQueue has no message and the queue is empty;MessageQueue belongs to delayed messages and no message is currently executed.

IV. Bitmap

1. Bitmap memory calculation method

Given the pixel length and width of the image, factors that affect memory size include resource file location and pixel size.

2, Bitmap efficient loading

Bitmap efficient loading is also used in Glide, the idea is to get the required length and width, generally get the length and width of the control; set inJustDecodeBounds in BitmapFactory.Options to true, which can help us get the length and width of Bitmap without loading into memory; compare the required length and width with the length and width of Bitmap to get the compression ratio, and put it into the inSampleSize property in BitmapFactory.Options. Set inJustDecodeBounds in BitmapFactory.Options to false, load the image into memory, and then set it to the control.

"What are the basic knowledge points of Android" is introduced here. Thank you for reading. If you want to know more about industry-related knowledge, you can pay attention to the website. Xiaobian will output more high-quality practical articles for everyone!

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