Sometimes it is interesting to modify the application code to access hidden information for you (maybe well obfuscated passwords or flags). Then, it could be interesting to decompile the apk, modify the code and recompile it.
You can changeinstructions, change the value of some variables or add new instructions. I change the Smali code using VS Code, you then install the smalise extension and the editor will tell you if any instruction is incorrect.
Some examples can be found here:
If you prefer to use **[apksigner](https://developer.android.com/studio/command-line/apksigner) instead of jarsigner, you should sing the apk after applying the optimization with zipaling. BUT NOTICE THAT YOU ONLY HAVE TO SIGN THE APPLCIATION ONCE** WITH jarsigner (before zipalign) OR WITH aspsigner(after zipaling).
If you are going to use declared variables inside the function (declared v0,v1,v2...) put these lines between the .local <number> and the declarations of the variables (const v0, 0x1)
If you want to put the logging code in the middle of the code of a function:
Add 2 to the number of declared variables: Ex: from .locals 10 to .locals 12
The new variables should be the next numbers of the already declared variables (in this example should be v10 and v11, remember that it starts in v0).
Change the code of the logging function and use v10 and v11 instead of v5 and v1.
Remember to add 3 to the number of .locals at the begging of the function.
This code is prepared to be inserted in the middle of a function (change the number of the variables as necessary). It will take the value of this.o, transform it to String and them make a toast with its value.